Small Cap Value Portfolio Investment Risks - Small Cap Value 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.
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.
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. The Portfolio’s investments in emerging markets heighten these risks due to a lack of established legal, political, business and social frameworks to support securities markets.
Investment Style Risk [Member]
Prospectus [Line Items]
Risk [Text Block]	•Investment Style Risk – A portfolio managed using a value style of investing, such as the Portfolio, 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.
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.
Micro Cap Company Risk [Member]
Prospectus [Line Items]
Risk [Text Block]	•Micro Cap Company Risk – Investing in micro cap stocks may cause the Portfolio to experience more rapid and extreme changes in value than a fund that invests solely in small, mid and large cap stocks due to a more limited track record, narrower product markets, more limited resources, higher risk of failure, and less liquid trading markets.
Other Investment Companies Risk [Member]
Prospectus [Line Items]
Risk [Text Block]	•Other Investment Companies Risk – The Portfolio will indirectly bear its pro rata portion of the expenses of the investment companies in which it invests, including advisory fees, in addition to the direct expenses of the Portfolio. The expenses associated with some business development companies may be significant. Investments in other investment companies are subject to market and selection risks, and generally entail the same risks as the underlying securities held by them. ETFs, closed-end funds and BDCs are also subject to the risk that their market prices may trade at a premium or a discount to their net asset value, which means the Portfolio will overpay for a fund’s assets if it is trading at a premium and will get less than the value of the fund’s assets when selling if it is trading at a discount. An active trading market for an ETF, closed-end fund or BDC may not be developed or maintained. In the event of a trading halt by the exchange, the Portfolio would be unable to sell its ETF, closed-end or BDC shares unless and until trading is resumed. BDCs invest in small and medium-sized private companies that may not have access to public equity markets. As a result, a BDC’s portfolio may be less liquid, may be more adversely affected by poor economic or market conditions, and may be adversely affected by risks associated with industries and sectors in which portfolio companies may concentrate.
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 is underweight in other sectors, the Portfolio risks missing out on advances in those sectors.
Small Cap Company Risk [Member]
Prospectus [Line Items]
Risk [Text Block]	•Small Cap Company Risk – Investing in small 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.
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.

Small Cap Value Portfolio Investment Risks - Small Cap Value 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.

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.

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. The Portfolio’s investments in emerging markets heighten these risks due to a lack of established legal, political, business and social frameworks to support securities markets.

Investment Style Risk [Member]

Prospectus [Line Items]

Risk [Text Block]

•Investment Style Risk – A portfolio managed using a value style of investing, such as the Portfolio, 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.

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.

Micro Cap Company Risk [Member]

Prospectus [Line Items]

Risk [Text Block]

•Micro Cap Company Risk – Investing in micro cap stocks may cause the Portfolio to experience more rapid and extreme changes in value than a fund that invests solely in small, mid and large cap stocks due to a more limited track record, narrower product markets, more limited resources, higher risk of failure, and less liquid trading markets.

Other Investment Companies Risk [Member]

Prospectus [Line Items]

Risk [Text Block]

•Other Investment Companies Risk – The Portfolio will indirectly bear its pro rata portion of the expenses of the investment companies in which it invests, including advisory fees, in addition to the direct expenses of the Portfolio. The expenses associated with some business development companies may be significant. Investments in other investment companies are subject to market and selection risks, and generally entail the same risks as the underlying securities held by them. ETFs, closed-end funds and BDCs are also subject to the risk that their market prices may trade at a premium or a discount to their net asset value, which means the Portfolio will overpay for a fund’s assets if it is trading at a premium and will get less than the value of the fund’s assets when selling if it is trading at a discount. An active trading market for an ETF, closed-end fund or BDC may not be developed or maintained. In the event of a trading halt by the exchange, the Portfolio would be unable to sell its ETF, closed-end or BDC shares unless and until trading is resumed. BDCs invest in small and medium-sized private companies that may not have access to public equity markets. As a result, a BDC’s portfolio may be less liquid, may be more adversely affected by poor economic or market conditions, and may be adversely affected by risks associated with industries and sectors in which portfolio companies may concentrate.

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 is underweight in other sectors, the Portfolio risks missing out on advances in those sectors.

Small Cap Company Risk [Member]

Prospectus [Line Items]

Risk [Text Block]

•Small Cap Company Risk – Investing in small 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.

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.