High Yield Bond Portfolio Investment Risks - High Yield Bond 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 and the adviser's quality determinations with respect to securities that are unrated by the major credit rating agencies may be inaccurate, which could cause the Portfolio to underperform other mutual funds or lose money.
Credit Risk [Member]
Prospectus [Line Items]
Risk [Text Block]	•Credit Risk – The Portfolio could lose money if the issuer or guarantor of a fixed income security is unwilling or unable to meet its financial obligations. In addition, changes in an issuer’s credit rating or the market’s perception of an issuer’s creditworthiness may also affect the value of the Portfolio’s investment in that issuer. Changes in credit spreads or improvements in an issuer’s credit quality may increase the risk that an issuer calls outstanding securities prior to their maturity.
Emerging Markets Risk [Member]
Prospectus [Line Items]
Risk [Text Block]	•Emerging Markets Risk – Investing in emerging market securities increases foreign investing risk, and may subject the Portfolio to more rapid and extreme changes in the value of its holdings compared with investments made in U.S. securities or in foreign, developed countries. Investments in emerging markets may be subject to political, economic, legal, market, and currency risks. Emerging market securities trade in smaller markets which may experience significant price and market volatility, fluctuations in currency values, interest rates and commodity prices, higher transaction costs, and the increased likelihood of the occurrence of trading difficulties, such as delays in executing, clearing and settling Portfolio transactions or in receiving payment of dividends. Special risks associated with investments in emerging market issuers may include a lack of publicly available information, a lack of uniform disclosure, accounting, financial reporting, and recordkeeping standards, and more limited investor protection provisions when compared with developed economies. Emerging market risks also may include unpredictable and changing political, economic and tax policies, the imposition of capital controls and/or foreign investment limitations by a country, nationalization of businesses, and the imposition of sanctions or restrictions in certain investments by other countries, such as the United States.
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, or in the case of hedged positions, that the U.S. dollar will decline in value relative to the currency being hedged, and may be less liquid, more volatile, and harder to value than U.S. securities.
High Yield Debt Risk [Member]
Prospectus [Line Items]
Risk [Text Block]	•High Yield Debt Risk – High yield debt securities (so called “junk bonds”) in which the Portfolio invests have greater interest rate and credit risk, may be more difficult to sell or sell at a reasonable price, have greater risk of loss than higher rated securities, and are predominantly speculative with respect to an issuer’s ability to pay interest and repay principal. In addition, high yield debt securities may be particularly sensitive to changes in the securities markets.
Inflation Risk [Member]
Prospectus [Line Items]
Risk [Text Block]	•Inflation Risk – Your investment in the Portfolio may not provide enough income to keep pace with inflation.
Interest Rate Risk [Member]
Prospectus [Line Items]
Risk [Text Block]	•Interest Rate Risk – Prices of fixed income instruments generally rise and fall in response to changes in market interest rates. In a rising interest rate environment, the value of the Portfolio’s fixed income investments is likely to decline. A significant rise in interest rates over a short period of time could cause significant losses in the market value of the Portfolio’s fixed income instruments. Duration measures the price sensitivity of a fixed income instrument to changes in interest rates. A portfolio with a longer average portfolio duration will be more sensitive to changes in interest rates than a portfolio with a shorter average portfolio duration. For example, the market value of a fixed income portfolio with an average duration of five years generally would be expected to fall approximately 5% if interest rates rose by one percentage point. Declining interest rates may increase the risk that an issuer calls outstanding securities prior to their maturity.
Liquidity Risk [Member]
Prospectus [Line Items]
Risk [Text Block]	•Liquidity Risk – High yield debt securities may be difficult to purchase or sell at an advantageous time or price, if at all. These risks may be magnified during periods of economic turmoil or in an extended economic downturn or when investing in emerging markets.
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.
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.

High Yield Bond Portfolio Investment Risks - High Yield Bond Portfolio

Dec. 31, 2025

•Active Management Risk – The adviser’s investment strategies and techniques may not perform as expected and the adviser's quality determinations with respect to securities that are unrated by the major credit rating agencies may be inaccurate, which could cause the Portfolio to underperform other mutual funds or lose money.

Prospectus [Line Items]

Risk [Text Block]

•Credit Risk – The Portfolio could lose money if the issuer or guarantor of a fixed income security is unwilling or unable to meet its financial obligations. In addition, changes in an issuer’s credit rating or the market’s perception of an issuer’s creditworthiness may also affect the value of the Portfolio’s investment in that issuer. Changes in credit spreads or improvements in an issuer’s credit quality may increase the risk that an issuer calls outstanding securities prior to their maturity.

Emerging Markets Risk [Member]

Prospectus [Line Items]

Risk [Text Block]

•Emerging Markets Risk – Investing in emerging market securities increases foreign investing risk, and may subject the Portfolio to more rapid and extreme changes in the value of its holdings compared with investments made in U.S. securities or in foreign, developed countries. Investments in emerging markets may be subject to political, economic, legal, market, and currency risks. Emerging market securities trade in smaller markets which may experience significant price and market volatility, fluctuations in currency values, interest rates and commodity prices, higher transaction costs, and the increased likelihood of the occurrence of trading difficulties, such as delays in executing, clearing and settling Portfolio transactions or in receiving payment of dividends. Special risks associated with investments in emerging market issuers may include a lack of publicly available information, a lack of uniform disclosure, accounting, financial reporting, and recordkeeping standards, and more limited investor protection provisions when compared with developed economies. Emerging market risks also may include unpredictable and changing political, economic and tax policies, the imposition of capital controls and/or foreign investment limitations by a country, nationalization of businesses, and the imposition of sanctions or restrictions in certain investments by other countries, such as the United States.

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, or in the case of hedged positions, that the U.S. dollar will decline in value relative to the currency being hedged, and may be less liquid, more volatile, and harder to value than U.S. securities.

High Yield Debt Risk [Member]

Prospectus [Line Items]

Risk [Text Block]

•High Yield Debt Risk – High yield debt securities (so called “junk bonds”) in which the Portfolio invests have greater interest rate and credit risk, may be more difficult to sell or sell at a reasonable price, have greater risk of loss than higher rated securities, and are predominantly speculative with respect to an issuer’s ability to pay interest and repay principal. In addition, high yield debt securities may be particularly sensitive to changes in the securities markets.

Inflation Risk [Member]

Prospectus [Line Items]

Risk [Text Block]

•Inflation Risk – Your investment in the Portfolio may not provide enough income to keep pace with inflation.

Interest Rate Risk [Member]

Prospectus [Line Items]

Risk [Text Block]

•Interest Rate Risk – Prices of fixed income instruments generally rise and fall in response to changes in market interest rates. In a rising interest rate environment, the value of the Portfolio’s fixed income investments is likely to decline. A significant rise in interest rates over a short period of time could cause significant losses in the market value of the Portfolio’s fixed income instruments. Duration measures the price sensitivity of a fixed income instrument to changes in interest rates. A portfolio with a longer average portfolio duration will be more sensitive to changes in interest rates than a portfolio with a shorter average portfolio duration. For example, the market value of a fixed income portfolio with an average duration of five years generally would be expected to fall approximately 5% if interest rates rose by one percentage point. Declining interest rates may increase the risk that an issuer calls outstanding securities prior to their maturity.

Liquidity Risk [Member]

Prospectus [Line Items]

Risk [Text Block]

•Liquidity Risk – High yield debt securities may be difficult to purchase or sell at an advantageous time or price, if at all. These risks may be magnified during periods of economic turmoil or in an extended economic downturn or when investing in emerging markets.

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.

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.