Large Cap Blend Portfolio Investment Risks - Large Cap Blend Portfolio	Dec. 31, 2025
Active Management Risk [Member]
Prospectus [Line Items]
Risk [Text Block]	•Active Management Risk – The adviser’s investment strategies and techniques may not perform as expected which could cause the Portfolio to underperform other mutual funds or lose money.
ADR Risk [Member]
Prospectus [Line Items]
Risk [Text Block]	•ADR Risk – ADRs are receipts representing ownership of shares of a foreign issuer held by a U.S. bank or similar financial institution that entitle the holder to dividends and capital gains on the underlying foreign shares. ADRs are alternatives to directly purchasing the underlying foreign securities in their national markets and currencies. They are subject to many of the risks associated with direct investments in foreign securities, such as currency risk, political and economic risk and market risk, because their values depend on the performance of the non-dollar denominated underlying foreign securities. The Portfolio is also subject to fees and the credit risk of the financial institution holding the ADRs. ADRs may be “sponsored,” meaning that they are implemented by a financial institution in collaboration with the issuing foreign company, or “unsponsored,” meaning that the financial institution created the instrument without the sponsorship or direct involvement of the foreign company. Differing registration requirements apply to each type of ADR. ADRs may transact on exchanges or on over-the-counter markets (“OTC”). Conducting transactions in OTC markets may result in higher costs, a lack of pricing transparency and lower liquidity when compared with exchange-based transactions. Risks associated with different ADR types will vary, based upon differences in registration, reporting and disclosure requirements that apply to such ADRs and the characteristics of the market in which transactions for the particular ADR are conducted.
Derivatives Risk [Member]
Prospectus [Line Items]
Risk [Text Block]	•Derivatives Risk – The value of a derivative generally depends upon, or is derived from, an underlying asset, reference rate or index. The primary risks associated with the Portfolio’s use of derivatives are the risk that changes in the value of the derivatives may not correlate as intended with the underlying asset, rate or index, the risk of adverse price movements in the market, the risk that the counterparty to a derivatives transaction fails to make the required payment, and the risk of missed opportunities in other investments. Certain derivatives involve leverage, which could cause the Portfolio to lose more than the principal amount invested. Other risks include management, market, and liquidity risks. The Portfolio’s purchase of futures contracts may involve risks related to imperfect correlation between the prices of such instruments and the price of the underlying asset, as well as leverage, liquidity and volatility risks.
Equity Securities Risk [Member]
Prospectus [Line Items]
Risk [Text Block]	•Equity Securities Risk – The value of equity securities, such as common stocks, could decline if the financial condition of the companies the Portfolio is invested in declines or if overall market and economic conditions deteriorate. Equity securities generally have greater price volatility than fixed income securities.
ESG Risk [Member]
Prospectus [Line Items]
Risk [Text Block]	•ESG Risk – Incorporating the consideration of ESG factors in the investment process may affect the Portfolio’s exposure to certain issuers or industries and may not work as intended. The Portfolio may underperform other funds that do not consider ESG factors or that use different ESG factors to evaluate a security. Information used to evaluate ESG factors may not be readily available, complete, or accurate, and may vary across providers and issuers, as ESG factors are not uniformly defined or assessed. There is no guarantee that evaluating ESG considerations for individual securities will positively contribute to the Portfolio’s performance results.
Foreign Investing Risk [Member]
Prospectus [Line Items]
Risk [Text Block]	•Foreign Investing Risk – Investing in foreign securities may subject the Portfolio to more rapid and extreme changes in value or more losses than a fund that invests exclusively in U.S. securities. This risk is due to potentially smaller markets, differing reporting, accounting and auditing standards, and nationalization, expropriation or confiscatory taxation, currency blockage, political and economic conditions, or diplomatic developments. Foreign securities may be adversely affected by decreases in foreign currency values relative to the U.S. dollar and may be less liquid, more volatile, and harder to value than U.S. securities.
Investment Style Risk [Member]
Prospectus [Line Items]
Risk [Text Block]	•Investment Style Risk – A portfolio managed using a particular style of investing, such as growth or value or a combination of both, may underperform when the market does not favor the particular style used by the Portfolio. Different investment styles tend to shift in and out of favor, depending on market conditions and investor sentiment.
Large Cap Company Risk [Member]
Prospectus [Line Items]
Risk [Text Block]	•Large Cap Company Risk – Investing in large cap stocks could cause the Portfolio to underperform in markets favoring faster growing companies. Large cap stocks tend to be more mature with fewer opportunities to grow and may not have the same growth potential as stocks with smaller capitalizations.
Large Position Risk [Member]
Prospectus [Line Items]
Risk [Text Block]	•Large Position Risk – Based upon the implementation of the adviser’s investment strategy, although the Portfolio is considered “diversified” under applicable law, the Portfolio may hold and maintain large positions in a single or small number of securities. From time to time, such holdings may, on an aggregate basis, constitute a considerable percentage of Portfolio assets. As a result, the Portfolio’s performance could be significantly affected by changes in the value of a single security or small number of securities, and holding such positions may increase the overall volatility of Portfolio performance.
Market Risk [Member]
Prospectus [Line Items]
Risk [Text Block]	•Market Risk – The risk that the market price of securities owned by the Portfolio may go up or down, sometimes rapidly or unpredictably. The value of a security may decline due to changes in general market conditions, economic trends or events that are not specifically related to the issuer of the security, or factors that affect a particular issuer or issuers, exchange, country, group of countries, region, market, industry, group of industries, sector or asset class. Global economies and financial markets are increasingly interconnected, which magnifies the potential that conditions in one country or region might adversely impact issuers in, or foreign exchange rates with, a different country or region. Geopolitical and other events, including war, terrorism, economic uncertainty, trade disputes, tariffs, public health crises (such as epidemics and pandemics), and related events have led, and in the future may lead, to increased market volatility, which may disrupt U.S. and world economies and markets and may have significant adverse direct or indirect effects on the Portfolio and its investments.
Mid Cap Company Risk [Member]
Prospectus [Line Items]
Risk [Text Block]	•Mid Cap Company Risk – Investing in mid cap stocks may cause greater risk of loss and price fluctuation than investing in stocks of larger cap companies due to a more limited track record, narrower product markets, more limited resources and less liquid trading markets. These stocks may be more volatile and more difficult to buy and sell than stocks with larger capitalizations.
REITs Risk [Member]
Prospectus [Line Items]
Risk [Text Block]	•REITs Risk – Investments in REITs are subject to the types of risks associated with investing in the real estate industry, including, among other risks: adverse developments affecting the real estate industry; declines in real property values; changes in interest rates; defaults by mortgagors or other borrowers and tenants; lack of availability of mortgage funds or financing; extended vacancies of properties, especially during economic downturns; casualty or condemnation losses; property taxes and operating losses; and governmental actions, such as changes to tax laws, zoning regulations or environmental regulations. REITs are dependent upon the quality of their management, may have limited financial resources and heavy cash flow dependency, may not be diversified geographically or by property type, and may be subject to self-liquidation.
Sector Focus Risk [Member]
Prospectus [Line Items]
Risk [Text Block]	•Sector Focus Risk – To the extent the Portfolio invests a relatively high percentage of its assets in a particular sector, it will have greater exposure to the risks associated with that sector, including the risk that the securities of companies within the sector will underperform due to adverse economic conditions, regulatory or legislative changes, or increased competition affecting the sector. To the extent the Portfolio underweight in other sectors, the Portfolio risks missing out on advances in those sectors. Information Technology Sector Risk – Companies in the information technology sector face risks associated with intense competition, both domestically and internationally, as well as product obsolescence due to rapid technology developments, frequent new product introduction, unpredictable changes in growth rates, competition for the services of qualified personnel, and changing consumer preferences.
Underlying Portfolio Risk [Member]
Prospectus [Line Items]
Risk [Text Block]	•Underlying Portfolio Risk – The Portfolio may serve as an investment option, or “Underlying Portfolio,” for other portfolios of Northwestern Mutual Series Fund, Inc. that are managed as “fund of funds.” As a result, from time to time, the Portfolio may experience relatively large investments or redemptions from those other portfolios and could be required to invest cash or sell securities at a time when it is not advantageous to do so.
Risk Lose Money [Member]
Prospectus [Line Items]
Risk [Text Block]	Portfolio shares will rise and fall in value and there is a risk you could lose money by investing in the Portfolio. There can be no assurance that the Portfolio will achieve its objective.

Large Cap Blend Portfolio Investment Risks - Large Cap Blend Portfolio

Dec. 31, 2025

•Active Management Risk – The adviser’s investment strategies and techniques may not perform as expected which could cause the Portfolio to underperform other mutual funds or lose money.

Prospectus [Line Items]

Risk [Text Block]

•ADR Risk – ADRs are receipts representing ownership of shares of a foreign issuer held by a U.S. bank or similar financial institution that entitle the holder to dividends and capital gains on the underlying foreign shares. ADRs are alternatives to directly purchasing the underlying foreign securities in their national markets and currencies. They are subject to many of the risks associated with direct investments in foreign securities, such as currency risk, political and economic risk and market risk, because their values depend on the performance of the non-dollar denominated underlying foreign securities. The Portfolio is also subject to fees and the credit risk of the financial institution holding the ADRs. ADRs may be “sponsored,” meaning that they are implemented by a financial institution in collaboration with the issuing foreign company, or “unsponsored,” meaning that the financial institution created the instrument without the sponsorship or direct involvement of the foreign company. Differing registration requirements apply to each type of ADR. ADRs may transact on exchanges or on over-the-counter markets (“OTC”). Conducting transactions in OTC markets may result in higher costs, a lack of pricing transparency and lower liquidity when compared with exchange-based transactions. Risks associated with different ADR types will vary, based upon differences in registration, reporting and disclosure requirements that apply to such ADRs and the characteristics of the market in which transactions for the particular ADR are conducted.

Derivatives Risk [Member]

Prospectus [Line Items]

Risk [Text Block]

•Derivatives Risk – The value of a derivative generally depends upon, or is derived from, an underlying asset, reference rate or index. The primary risks associated with the Portfolio’s use of derivatives are the risk that changes in the value of the derivatives may not correlate as intended with the underlying asset, rate or index, the risk of adverse price movements in the market, the risk that the counterparty to a derivatives transaction fails to make the required payment, and the risk of missed opportunities in other investments. Certain derivatives involve leverage, which could cause the Portfolio to lose more than the principal amount invested. Other risks include management, market, and liquidity risks. The Portfolio’s purchase of futures contracts may involve risks related to imperfect correlation between the prices of such instruments and the price of the underlying asset, as well as leverage, liquidity and volatility risks.

Equity Securities Risk [Member]

Prospectus [Line Items]

Risk [Text Block]

•Equity Securities Risk – The value of equity securities, such as common stocks, could decline if the financial condition of the companies the Portfolio is invested in declines or if overall market and economic conditions deteriorate. Equity securities generally have greater price volatility than fixed income securities.

ESG Risk [Member]

Prospectus [Line Items]

Risk [Text Block]

•ESG Risk – Incorporating the consideration of ESG factors in the investment process may affect the Portfolio’s exposure to certain issuers or industries and may not work as intended. The Portfolio may underperform other funds that do not consider ESG factors or that use different ESG factors to evaluate a security. Information used to evaluate ESG factors may not be readily available, complete, or accurate, and may vary across providers and issuers, as ESG factors are not uniformly defined or assessed. There is no guarantee that evaluating ESG considerations for individual securities will positively contribute to the Portfolio’s performance results.

Foreign Investing Risk [Member]

Prospectus [Line Items]

Risk [Text Block]

•Foreign Investing Risk – Investing in foreign securities may subject the Portfolio to more rapid and extreme changes in value or more losses than a fund that invests exclusively in U.S. securities. This risk is due to potentially smaller markets, differing reporting, accounting and auditing standards, and nationalization, expropriation or confiscatory taxation, currency blockage, political and economic conditions, or diplomatic developments. Foreign securities may be adversely affected by decreases in foreign currency values relative to the U.S. dollar and may be less liquid, more volatile, and harder to value than U.S. securities.

Investment Style Risk [Member]

Prospectus [Line Items]

Risk [Text Block]

•Investment Style Risk – A portfolio managed using a particular style of investing, such as growth or value or a combination of both, may underperform when the market does not favor the particular style used by the Portfolio. Different investment styles tend to shift in and out of favor, depending on market conditions and investor sentiment.

Large Cap Company Risk [Member]

Prospectus [Line Items]

Risk [Text Block]

•Large Cap Company Risk – Investing in large cap stocks could cause the Portfolio to underperform in markets favoring faster growing companies. Large cap stocks tend to be more mature with fewer opportunities to grow and may not have the same growth potential as stocks with smaller capitalizations.

Large Position Risk [Member]

Prospectus [Line Items]

Risk [Text Block]

•Large Position Risk – Based upon the implementation of the adviser’s investment strategy, although the Portfolio is considered “diversified” under applicable law, the Portfolio may hold and maintain large positions in a single or small number of securities. From time to time, such holdings may, on an aggregate basis, constitute a considerable percentage of Portfolio assets. As a result, the Portfolio’s performance could be significantly affected by changes in the value of a single security or small number of securities, and holding such positions may increase the overall volatility of Portfolio performance.

Market Risk [Member]

Prospectus [Line Items]

Risk [Text Block]

•Market Risk – The risk that the market price of securities owned by the Portfolio may go up or down, sometimes rapidly or unpredictably. The value of a security may decline due to changes in general market conditions, economic trends or events that are not specifically related to the issuer of the security, or factors that affect a particular issuer or issuers, exchange, country, group of countries, region, market, industry, group of industries, sector or asset class. Global economies and financial markets are increasingly interconnected, which magnifies the potential that conditions in one country or region might adversely impact issuers in, or foreign exchange rates with, a different country or region. Geopolitical and other events, including war, terrorism, economic uncertainty, trade disputes, tariffs, public health crises (such as epidemics and pandemics), and related events have led, and in the future may lead, to increased market volatility, which may disrupt U.S. and world economies and markets and may have significant adverse direct or indirect effects on the Portfolio and its investments.

Mid Cap Company Risk [Member]

Prospectus [Line Items]

Risk [Text Block]

•Mid Cap Company Risk – Investing in mid cap stocks may cause greater risk of loss and price fluctuation than investing in stocks of larger cap companies due to a more limited track record, narrower product markets, more limited resources and less liquid trading markets. These stocks may be more volatile and more difficult to buy and sell than stocks with larger capitalizations.

REITs Risk [Member]

Prospectus [Line Items]

Risk [Text Block]

•REITs Risk – Investments in REITs are subject to the types of risks associated with investing in the real estate industry, including, among other risks: adverse developments affecting the real estate industry; declines in real property values; changes in interest rates; defaults by mortgagors or other borrowers and tenants; lack of availability of mortgage funds or financing; extended vacancies of properties, especially during economic downturns; casualty or condemnation losses; property taxes and operating losses; and governmental actions, such as changes to tax laws, zoning regulations or environmental regulations. REITs are dependent upon the quality of their management, may have limited financial resources and heavy cash flow dependency, may not be diversified geographically or by property type, and may be subject to self-liquidation.

Sector Focus Risk [Member]

Prospectus [Line Items]

Risk [Text Block]

•Sector Focus Risk – To the extent the Portfolio invests a relatively high percentage of its assets in a particular sector, it will have greater exposure to the risks associated with that sector, including the risk that the securities of companies within the sector will underperform due to adverse economic conditions, regulatory or legislative changes, or increased competition affecting the sector. To the extent the Portfolio underweight in other sectors, the Portfolio risks missing out on advances in those sectors. Information Technology Sector Risk – Companies in the information technology sector face risks associated with intense competition, both domestically and internationally, as well as product obsolescence due to rapid technology developments, frequent new product introduction, unpredictable changes in growth rates, competition for the services of qualified personnel, and changing consumer preferences.

Underlying Portfolio Risk [Member]

Prospectus [Line Items]

Risk [Text Block]

•Underlying Portfolio Risk – The Portfolio may serve as an investment option, or “Underlying Portfolio,” for other portfolios of Northwestern Mutual Series Fund, Inc. that are managed as “fund of funds.” As a result, from time to time, the Portfolio may experience relatively large investments or redemptions from those other portfolios and could be required to invest cash or sell securities at a time when it is not advantageous to do so.

Risk Lose Money [Member]

Prospectus [Line Items]

Risk [Text Block]

Portfolio shares will rise and fall in value and there is a risk you could lose money by investing in the Portfolio. There can be no assurance that the Portfolio will achieve its objective.