v3.25.1
Label Element Value
Risk Return Abstract rr_RiskReturnAbstract  
Registrant Name dei_EntityRegistrantName NORTHWESTERN MUTUAL SERIES FUND, INC.
Prospectus Date rr_ProspectusDate May 01, 2025
Small Cap Value Portfolio  
Risk Return Abstract rr_RiskReturnAbstract  
Risk/Return [Heading] rr_RiskReturnHeading Small Cap Value Portfolio – Summary
Objective [Heading] rr_ObjectiveHeading <span style="color:#000000;font-family:Arial;font-size:10.02pt;font-weight:bold;">INVESTMENT OBJECTIVE</span>
Objective, Primary [Text Block] rr_ObjectivePrimaryTextBlock The investment objective of the Portfolio is long-term growth of capital.
Expense [Heading] rr_ExpenseHeading <span style="color:#000000;font-family:Arial;font-size:10.02pt;font-weight:bold;">FEES AND EXPENSES OF THE PORTFOLIO</span>
Expense Narrative [Text Block] rr_ExpenseNarrativeTextBlock The table below describes the fees and expenses that you may pay when you buy, hold, and sell interests in a separate account that invests in shares of the Portfolio as a result of your purchase of a variable annuity contract or variable life insurance policy. The fees and expenses shown in the table and Example do not reflect fees and expenses separately charged by variable annuity contracts or variable life insurance policies. If the fees and expenses separately charged by variable annuity contracts and variable life insurance policies were included, the fees and expenses shown in the table and the Example would be higher.
Shareholder Fees Caption [Text] rr_ShareholderFeesCaption <span style="color:#000000;font-family:Times New Roman;font-size:10.02pt;font-style:italic;font-weight:bold;margin-left:0.0pt;">Shareholder Fees</span><span style="color:#000000;font-family:Times New Roman;font-size:10.02pt;margin-left:0.0pt;">(fees paid directly from your investment)</span>
Operating Expenses Caption [Text] rr_OperatingExpensesCaption <span style="color:#000000;font-family:Times New Roman;font-size:10.02pt;font-style:italic;font-weight:bold;margin-left:0.0pt;">Annual Portfolio Operating Expenses</span><span style="color:#000000;font-family:Times New Roman;font-size:10.02pt;margin-left:0.0pt;">(expenses that you pay each year as a percentage </span><span style="color:#000000;font-family:Times New Roman;font-size:10.02pt;margin-left:0.0pt;">of the value of your investment)</span>
Fee Waiver or Reimbursement over Assets, Date of Termination rr_FeeWaiverOrReimbursementOverAssetsDateOfTermination <span style="color:#000000;font-family:Times New Roman;font-size:10.02pt;">April 30, 2026</span>
Portfolio Turnover [Heading] rr_PortfolioTurnoverHeading <span style="color:#000000;font-family:Times New Roman;font-size:10pt;font-style:italic;font-weight:bold;">Portfolio Turnover</span>
Portfolio Turnover [Text Block] rr_PortfolioTurnoverTextBlock The Portfolio pays transaction costs, such as commissions, when it buys and sells securities (or “turns over” its portfolio). A higher portfolio turnover may indicate higher transaction costs. These costs, which are not reflected in Annual Portfolio Operating Expenses or in the Example, affect the Portfolio’s performance. During the most recent fiscal year, the Portfolio’s portfolio turnover rate was 31.98% of the average value of its portfolio.
Portfolio Turnover, Rate rr_PortfolioTurnoverRate 31.98%
Expense Example [Heading] rr_ExpenseExampleHeading <span style="color:#000000;font-family:Times New Roman;font-size:10pt;font-style:italic;font-weight:bold;">Example</span>
Expense Example Narrative [Text Block] rr_ExpenseExampleNarrativeTextBlock This Example is intended to help you compare the cost of investing in the Portfolio with the cost of investing in other mutual funds. The Example assumes that you invest $10,000 in the Portfolio for the time periods indicated and then redeem or hold all of your shares at the end of those periods. The Example also assumes that your investment has a 5% return each year and that the Portfolio’s operating expenses remain the same. The Example reflects adjustments made to the Portfolio’s operating expenses due to the fee waiver agreement with the investment adviser for the first year only. Although your actual costs may be higher or lower, based on these assumptions your costs would be:
Strategy [Heading] rr_StrategyHeading <span style="color:#000000;font-family:Arial;font-size:10.02pt;font-weight:bold;">PRINCIPAL INVESTMENT STRATEGIES</span>
Strategy Narrative [Text Block] rr_StrategyNarrativeTextBlock Normally, the Portfolio invests at least 80% of net assets (plus any borrowings for investment purposes) in securities of small-capitalization value companies. For purposes of the Portfolio, small-capitalization companies are those companies with market capitalizations that do not exceed the maximum market capitalization of any security in the Russell 2000® Index (approximately $14.96 billion as of March 31, 2025). The market capitalizations of companies in the Portfolio and the Index changes over time and securities of companies whose market capitalizations no longer fall within this range after purchase may continue to be held by the Portfolio. The Portfolio may, on occasion, purchase companies with a market capitalization above the range. Securities falling outside of the market capitalization range noted above will be included in the overall calculation of assets but not counted as fulfilling the 80% minimum. The Portfolio may also invest in the equity securities of micro cap companies (defined as companies with stock market capitalizations less than $500 million at the time of investment).Value companies, as defined by the adviser, are those included in a third-party value index or those that exhibit certain financial characteristics (e.g., price-to-earnings ratio, price-to-book ratio, enterprise value-to-sales ratio, or cash flow trends) determined by the adviser to indicate the company is undervalued by the market. For this purpose, a third-party value index is an index developed, calculated, and maintained by a third-party that measures the performance of value stocks. The adviser generally selects for inclusion in the Portfolio companies whose current stock prices do not appear to adequately reflect their underlying value as measured by assets, earnings, cash flow or business franchises. While the Portfolio does not seek to focus its investments in any particular economic sector, the Portfolio may at times have a relatively high percentage of its assets invested in a particular sector as a result of the adviser’s stock selection process. The adviser integrates environmental, social, and governance (ESG) factors into its investment research process for certain investments. While ESG matters vary widely, the adviser generally considers ESG factors such as climate change, resource depletion, labor standards, diversity, human rights issues, and governance structure and practices. For certain types of investments, including, but not limited to, cash, currency positions, and particular types of derivatives, an ESG analysis may not be relevant or possible due to lack of data. Where ESG considerations are integrated into the investment research process, the adviser focuses on the ESG factors that it considers most likely to have a material impact on the performance of the holdings in the Portfolio. The adviser may conclude that other attributes of an investment outweigh ESG considerations when making investment decisions for the Portfolio.In pursuing its investment objective, the adviser has the discretion to deviate from its normal investment criteria, as described above, and purchase securities the adviser believes could provide an opportunity for substantial appreciation. These special situations might arise when the adviser believes a security could increase in value for a variety of reasons, including a change in management, a reorganization, a spin-off of a business line, a special dividend, or some other extraordinary corporate event, a new product introduction or innovation, or a favorable competitive environment.While most assets will be invested in U.S. common stocks, other securities may also be purchased, including American Depositary Receipts (ADRs) and foreign securities (up to 20% of net assets), real estate investment trust (REITs) and securities of other investment companies, including open-end funds, closed-end funds, exchange traded funds (ETFs) and business development companies (BDCs), in keeping with the Portfolio’s objectives.The Portfolio may sell securities for a variety of reasons, such as to secure gains, limit losses or redeploy assets into more promising opportunities.
Risk [Heading] rr_RiskHeading <span style="color:#000000;font-family:Arial;font-size:10.02pt;font-weight:bold;">PRINCIPAL RISKS</span>
Bar Chart and Performance Table [Heading] rr_BarChartAndPerformanceTableHeading <span style="color:#000000;font-family:Arial;font-size:10.02pt;font-weight:bold;">PERFORMANCE</span>
Performance Narrative [Text Block] rr_PerformanceNarrativeTextBlock The following bar chart illustrates the risks of investing in the Portfolio by showing how the performance of the Portfolio has varied from year to year. The table to the right of the bar chart shows the Portfolio’s average annual total return over certain time periods  and compares the Portfolio’s returns with those of an index that closely aligns with the Portfolio's investment strategy (Strategy Index). The table also shows the Portfolio's returns against an index that represents the overall securities market (Broad-Based Index), which the Portfolio has added to comply with new regulatory requirements.  Returns are based on past results and are not an indication of future performance. Neither the bar chart nor the table reflects the fees and expenses separately charged by the variable annuity contract or variable life insurance policy separate account that invests in the Portfolio and returns would be lower if those fees and expenses were reflected.
Performance Information Illustrates Variability of Returns [Text] rr_PerformanceInformationIllustratesVariabilityOfReturns <span style="color:#000000;font-family:Times New Roman;font-size:10.02pt;margin-left:0%;">The following bar chart illustrates the risks of investing in the Portfolio by showing how the performance of the Portfolio has varied from year to year. The table to the right of the bar chart shows the Portfolio’s average annual total return over certain time periods  and compares the Portfolio’s returns with those of an index that closely aligns with the Portfolio's investment strategy (Strategy Index). The table also shows the Portfolio's returns against an index that represents the overall securities market (Broad-Based Index), which the Portfolio has added to comply with new regulatory requirements. </span>
Performance Additional Market Index [Text] rr_PerformanceAdditionalMarketIndex <span style="color:#000000;font-family:Times New Roman;font-size:9.02pt;">The Portfolio has added this broad-based index in response to new regulatory requirements.</span>
Performance Past Does Not Indicate Future [Text] rr_PerformancePastDoesNotIndicateFuture <span style="color:#000000;font-family:Times New Roman;font-size:10.02pt;"> Returns are based on past results and are not an </span><span style="color:#000000;font-family:Times New Roman;font-size:10.02pt;margin-left:0%;">indication of future performance.</span>
Bar Chart Does Not Reflect Sales Loads [Text] rr_BarChartDoesNotReflectSalesLoads <span style="color:#000000;font-family:Times New Roman;font-size:10.02pt;"> Neither the bar chart nor the table reflects the fees and expenses separately charged by the variable </span><span style="color:#000000;font-family:Times New Roman;font-size:10.02pt;margin-left:0%;">annuity contract or variable life insurance policy separate account that invests in the Portfolio and returns would be lower if those </span><span style="color:#000000;font-family:Times New Roman;font-size:10.02pt;">fees and expenses were reflected.</span>
Bar Chart Closing [Text Block] rr_BarChartClosingTextBlock Quarter/YearReturnBest Quarter4th quarter, 202028.62%Worst Quarter1st quarter, 2020-31.21%
Performance Table Heading rr_PerformanceTableHeading <span style="color:#000000;font-family:Times New Roman;font-size:10.02pt;font-weight:bold;">Average Annual Total Return </span> <br/><span style="color:#000000;font-family:Times New Roman;font-size:10.02pt;font-weight:bold;">(for periods ended December 31, 2024)</span>
Small Cap Value Portfolio | Risk Lose Money [Member]  
Risk Return Abstract rr_RiskReturnAbstract  
Risk [Text Block] rr_RiskTextBlock 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 | Active Management Risk [Member]  
Risk Return Abstract rr_RiskReturnAbstract  
Risk [Text Block] rr_RiskTextBlock 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.
Small Cap Value Portfolio | ADR Risk [Member]  
Risk Return Abstract rr_RiskReturnAbstract  
Risk [Text Block] rr_RiskTextBlock 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.
Small Cap Value Portfolio | Equity Securities Risk [Member]  
Risk Return Abstract rr_RiskReturnAbstract  
Risk [Text Block] rr_RiskTextBlock 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.
Small Cap Value Portfolio | ESG Risk [Member]  
Risk Return Abstract rr_RiskReturnAbstract  
Risk [Text Block] rr_RiskTextBlock ESG Risk – Incorporating the consideration of ESG factors in the investment process may result in the exclusion of certain securities for non-investment reasons and therefore the Portfolio may forego some market opportunities available to funds that do not consider ESG factors. The evaluation of ESG factors 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.
Small Cap Value Portfolio | Foreign Investing Risk [Member]  
Risk Return Abstract rr_RiskReturnAbstract  
Risk [Text Block] rr_RiskTextBlock 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.
Small Cap Value Portfolio | Investment Style Risk [Member]  
Risk Return Abstract rr_RiskReturnAbstract  
Risk [Text Block] rr_RiskTextBlock 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.
Small Cap Value Portfolio | Market Risk [Member]  
Risk Return Abstract rr_RiskReturnAbstract  
Risk [Text Block] rr_RiskTextBlock 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, 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.
Small Cap Value Portfolio | Micro Cap Company Risk [Member]  
Risk Return Abstract rr_RiskReturnAbstract  
Risk [Text Block] rr_RiskTextBlock 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.
Small Cap Value Portfolio | Other Investment Companies Risk [Member]  
Risk Return Abstract rr_RiskReturnAbstract  
Risk [Text Block] rr_RiskTextBlock 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.
Small Cap Value Portfolio | REITs Risk [Member]  
Risk Return Abstract rr_RiskReturnAbstract  
Risk [Text Block] rr_RiskTextBlock 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.
Small Cap Value Portfolio | Sector Focus Risk [Member]  
Risk Return Abstract rr_RiskReturnAbstract  
Risk [Text Block] rr_RiskTextBlock 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 Value Portfolio | Small Cap Company Risk [Member]  
Risk Return Abstract rr_RiskReturnAbstract  
Risk [Text Block] rr_RiskTextBlock 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.
Small Cap Value Portfolio | Special Situation Risk [Member]  
Risk Return Abstract rr_RiskReturnAbstract  
Risk [Text Block] rr_RiskTextBlock Special Situation Risk – In special situations, the adviser may deviate from the Portfolio’s normal investment criteria when purchasing a security. In these special situations, there is the risk that the change or event anticipated by the adviser when purchasing a company might not occur or attract the expected attention, which could have a negative impact on the price of the company's securities. Investing in special situations may involve heightened volatility in the value of the securities purchased and may cause greater risk of loss.
Small Cap Value Portfolio | Underlying Portfolio Risk [Member]  
Risk Return Abstract rr_RiskReturnAbstract  
Risk [Text Block] rr_RiskTextBlock 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.
Small Cap Value Portfolio | Small Cap Value Portfolio  
Risk Return Abstract rr_RiskReturnAbstract  
Shareholder Fees(fees paid directly from your investment) rr_ShareholderFeeOther
Management Fee rr_ManagementFeesOverAssets 0.85%
Distribution and Service (12b-1) Fees rr_DistributionAndService12b1FeesOverAssets none
Other Expenses rr_OtherExpensesOverAssets 0.03%
Total Annual Portfolio Operating Expenses rr_ExpensesOverAssets 0.88%
Fee Waiver rr_FeeWaiverOrReimbursementOverAssets (0.01%) [1]
Total Annual Portfolio Operating Expenses After Fee Waiver rr_NetExpensesOverAssets 0.87% [1]
1 Year rr_ExpenseExampleYear01 $ 89
3 Years rr_ExpenseExampleYear03 280
5 Years rr_ExpenseExampleYear05 487
10 Years rr_ExpenseExampleYear10 $ 1,083
2015 rr_AnnualReturn2015 (5.45%)
2016 rr_AnnualReturn2016 32.39%
2017 rr_AnnualReturn2017 11.65%
2018 rr_AnnualReturn2018 (12.73%)
2019 rr_AnnualReturn2019 25.89%
2020 rr_AnnualReturn2020 9.29%
2021 rr_AnnualReturn2021 23.00%
2022 rr_AnnualReturn2022 (18.53%)
2023 rr_AnnualReturn2023 13.85%
2024 rr_AnnualReturn2024 10.36%
Highest Quarterly Return, Label rr_HighestQuarterlyReturnLabel <span style="color:#000000;font-family:Times New Roman;font-size:9.02pt;margin-left:0.0pt;">Best Quarter</span>
Highest Quarterly Return, Date rr_BarChartHighestQuarterlyReturnDate Dec. 31, 2020
Highest Quarterly Return rr_BarChartHighestQuarterlyReturn 28.62%
Lowest Quarterly Return, Label rr_LowestQuarterlyReturnLabel <span style="color:#000000;font-family:Times New Roman;font-size:9.02pt;margin-left:0.0pt;">Worst Quarter</span>
Lowest Quarterly Return, Date rr_BarChartLowestQuarterlyReturnDate Mar. 31, 2020
Lowest Quarterly Return rr_BarChartLowestQuarterlyReturn (31.21%)
1 Yr rr_AverageAnnualReturnYear01 10.36%
5 Yr rr_AverageAnnualReturnYear05 6.59%
10 Yr rr_AverageAnnualReturnYear10 7.77%
Small Cap Value Portfolio | S&P 500® Index(reflects no deduction for fees, expenses or taxes)  
Risk Return Abstract rr_RiskReturnAbstract  
1 Yr rr_AverageAnnualReturnYear01 25.02% [2]
5 Yr rr_AverageAnnualReturnYear05 14.53% [2]
10 Yr rr_AverageAnnualReturnYear10 13.10% [2]
Small Cap Value Portfolio | Russell 2000® Value Index(reflects no deduction for fees, expenses or taxes)  
Risk Return Abstract rr_RiskReturnAbstract  
1 Yr rr_AverageAnnualReturnYear01 8.06%
5 Yr rr_AverageAnnualReturnYear05 7.29%
10 Yr rr_AverageAnnualReturnYear10 7.14%
[1] The Portfolio's investment adviser has contractually agreed to waive a portion of its management fee. This contractual agreement will continue through at least April 30, 2026 and may not be terminated prior to that date without action by the Board of Directors.
[2] The Portfolio has added this broad-based index in response to new regulatory requirements.