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
Index 500 Stock Portfolio  
Risk Return Abstract rr_RiskReturnAbstract  
Risk/Return [Heading] rr_RiskReturnHeading Index 500 Stock 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 to achieve investment results that approximate the performance of the Standard & Poor’s 500 Composite Stock Price Index (“S&P 500® Index”).
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 2.21% of the average value of its portfolio.
Portfolio Turnover, Rate rr_PortfolioTurnoverRate 2.21%
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 The Portfolio employs a “passive management,” or indexing, investment approach designed to track the performance of the S&P 500® Index. The S&P 500® Index is composed of the stocks of primarily large capitalization companies that represent a broad spectrum of the U.S. economy and a substantial part of the U.S. stock market’s total capitalization. As of March 31, 2025, the market capitalization range of the S&P 500® Index was approximately $5.30 billion to $3.34 trillion. Normally, the Portfolio invests at least 80% of net assets (plus any borrowings for investment purposes) in stocks included in the underlying index. Further, the Portfolio attempts to achieve its objective by investing all, or substantially all, of its assets in the stocks that make up the S&P 500® Index, holding each stock in approximately the same proportion as its weighting in the Index. This is known as a full replication strategy. The Portfolio may also invest in equity index futures and exchange traded funds for cash management and liquidity purposes and to help achieve full replication.Standard & Poor’s constructs the Index by first identifying major industry categories and then allocating a representative sample of the larger and more liquid stocks in those industries to the index. S&P weights each stock according to its float-adjusted market value. For example, the 50 largest companies in the index may account for over 50% of its value.Because the Portfolio is managed with a goal of fully replicating the underlying S&P 500® Index, the approach employed by the Portfolio with respect to reconstitution and rebalancing aligns with the process followed generally by the S&P 500® Index. Changes to the underlying company constituents of the S&P 500® Index are made on an as-needed basis and are usually announced several days before they are scheduled to be implemented. The S&P 500® Index typically makes weightings adjustments based on changes in the amount of a constituent company’s shares outstanding on a quarterly basis. The constituent and share-based weightings changes made by the S&P 500® Index will be made in a parallel fashion by the Portfolio on substantially the same timeline. Additionally, the Portfolio utilizes cash equitization instruments, and rebalancing occurs as necessary to maintain balances within established target ranges for these instruments. Cash equitization as employed by the Portfolio is a strategy pursuant to which the Portfolio will acquire certain derivative instruments, such as equity index futures, whereby the futures will be utilized to replicate index returns and enable the Portfolio to be fully invested, while still maintaining cash balances as needed by the Portfolio. This strategy works to limit the extent to which uninvested cash will weigh negatively on overall Portfolio returns when markets go up.The Index 500 Stock Portfolio’s ability to match the performance of the S&P 500® Index will be affected to some extent by the size and timing of cash flows into and out of the Index 500 Stock Portfolio. The Portfolio will be managed with a view to reducing such effects.
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 represents the overall securities market (Broad-Based Index). Prior to February 1, 2021, the Portfolio was managed by Mason Street Advisors, LLC. Performance shown may have been different if the current strategy, and the current sub-adviser, had been in place during the periods shown. 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 represents the overall securities market (Broad-Based Index).</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 </span><span style="color:#000000;font-family:Times New Roman;font-size:10.02pt;margin-left:0%;">not an 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 </span><span style="color:#000000;font-family:Times New Roman;font-size:10.02pt;margin-left:0%;">variable annuity contract or variable life insurance policy separate account that invests in the Portfolio and returns would be lower if </span><span style="color:#000000;font-family:Times New Roman;font-size:10.02pt;">those fees and expenses were reflected.</span>
Bar Chart Closing [Text Block] rr_BarChartClosingTextBlock Quarter/YearReturnBest Quarter2nd quarter, 202020.47%Worst Quarter1st quarter, 2020-19.62%
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>
Index 500 Stock 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.
Index 500 Stock Portfolio | Derivatives Risk [Member]  
Risk Return Abstract rr_RiskReturnAbstract  
Risk [Text Block] rr_RiskTextBlock 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 and the risk of adverse price movements in the market. Certain derivatives involve leverage, which could cause the Portfolio to lose more than the principal amount invested. Other risks include counterparty 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.
Index 500 Stock 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 the stocks in which the Portfolio invests, 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.
Index 500 Stock Portfolio | Exchange Traded Funds Risk [Member]  
Risk Return Abstract rr_RiskReturnAbstract  
Risk [Text Block] rr_RiskTextBlock Exchange Traded Funds Risk – Investing in exchange traded funds (ETFs) may expose the Portfolio to greater risk of loss and price fluctuation than investing directly in a comparable portfolio of stocks comprising the index due to lack of liquidity, the additional expenses incurred as a shareholder in another investment company, and tracking error. ETFs are also subject to the risk that their market prices may trade at a premium or discount to their net asset value, which means the Portfolio will overpay for an ETF’s assets if it is trading at a premium and will get less than the value of the ETF’s assets when selling if it is trading at a discount. An active market for an ETF may not be developed or maintained. Trading of an ETF’s shares may be halted by the exchange, in which case the Portfolio would be unable to sell its ETF shares unless and until trading is resumed. To the extent that authorized participants do not place sufficient creation and redemption orders, an ETF's shares may trade at a premium or discount or may face a trading halt or delisting.
Index 500 Stock Portfolio | Indexing Strategy Risk [Member]  
Risk Return Abstract rr_RiskReturnAbstract  
Risk [Text Block] rr_RiskTextBlock Indexing Strategy Risk –A Portfolio using a passive management strategy is not “actively” managed, and therefore does not engage in shifting portfolio assets to take advantage of market opportunity, and does not attempt to manage market volatility, use defensive strategies or reduce the effects of any long-term periods of poor stock performance. Since the Portfolio implements a full replication strategy with respect to the index which it tracks, to the extent the index has a significant allocation to an industry or group of industries, a particular sector, or to a single issuer or small number of issuers, the Portfolio will exhibit a significant investment level in that industry, group of industries, sector, issuer or small number of issuers. Portfolio performance may be adversely affected by a significant investment in an industry, group of industries, sector, issuer or small number of issuers, and may be more susceptible to adverse economic, market, political or regulatory developments affecting the industry, group of industries sector or issuer(s) subject to a significant level of investment.
Index 500 Stock Portfolio | Large Cap Company Risk [Member]  
Risk Return Abstract rr_RiskReturnAbstract  
Risk [Text Block] rr_RiskTextBlock 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.
Index 500 Stock 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.
Index 500 Stock Portfolio | Tracking Error Risk [Member]  
Risk Return Abstract rr_RiskReturnAbstract  
Risk [Text Block] rr_RiskTextBlock Tracking Error Risk – The Portfolio may be subject to tracking error, which is the divergence of the Portfolio’s performance from that of the underlying index. Tracking error may occur due to a number of factors, including differences between the securities held in the Portfolio and those included in the underlying index, and based upon the fact that the Portfolio incurs fees and expenses, while the underlying index does not. Tracking error risk may be heightened during times of increased market volatility or other unusual market conditions.
Index 500 Stock 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.
Index 500 Stock Portfolio | Index 500 Stock Portfolio  
Risk Return Abstract rr_RiskReturnAbstract  
Shareholder Fees(fees paid directly from your investment) rr_ShareholderFeeOther
Management Fee rr_ManagementFeesOverAssets 0.20%
Distribution and Service (12b-1) Fees rr_DistributionAndService12b1FeesOverAssets none
Other Expenses rr_OtherExpensesOverAssets 0.01%
Total Annual Portfolio Operating Expenses rr_ExpensesOverAssets 0.21%
Fee Waiver rr_FeeWaiverOrReimbursementOverAssets (0.02%) [1]
Total Annual Portfolio Operating Expenses After Fee Waiver rr_NetExpensesOverAssets 0.19% [1]
1 Year rr_ExpenseExampleYear01 $ 19
3 Years rr_ExpenseExampleYear03 66
5 Years rr_ExpenseExampleYear05 116
10 Years rr_ExpenseExampleYear10 $ 266
2022 rr_AnnualReturn2022 (18.28%)
2023 rr_AnnualReturn2023 26.04%
2024 rr_AnnualReturn2024 24.75%
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 Jun. 30, 2020
Highest Quarterly Return rr_BarChartHighestQuarterlyReturn 20.47%
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 (19.62%)
1 Yr rr_AverageAnnualReturnYear01 24.75%
5 Yr rr_AverageAnnualReturnYear05 14.30%
10 Yr rr_AverageAnnualReturnYear10 12.86%
Index 500 Stock Portfolio | S&P 500® Index(reflects no deduction for fees, expenses or taxes)  
Risk Return Abstract rr_RiskReturnAbstract  
1 Yr rr_AverageAnnualReturnYear01 25.02%
5 Yr rr_AverageAnnualReturnYear05 14.53%
10 Yr rr_AverageAnnualReturnYear10 13.10%
[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.