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
Government Money Market Portfolio  
Risk Return Abstract rr_RiskReturnAbstract  
Risk/Return [Heading] rr_RiskReturnHeading Government Money Market 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 realize maximum current income to the extent consistent with liquidity and stability 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>
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 As a government money market portfolio, the Portfolio invests at least 99.5% of its total assets in cash, U.S. Treasury bills, notes and other obligations issued or guaranteed as to principal and interest by the U.S. Government, its agencies or instrumentalities, and repurchase agreements secured by such obligations or cash. The Portfolio may invest 100% of its total assets in such repurchase agreements. Under normal circumstances, the Portfolio will invest at least 80% of net assets in obligations issued or guaranteed as to principal and interest by the U.S. Government, its agencies or instrumentalities, and repurchase agreements secured by such obligations. The Portfolio invests in a portfolio of securities maturing in 397 days or less (with certain exceptions) that will have a dollar-weighted average maturity of 60 days or less and a dollar-weighted average life of 120 days or less. The Portfolio may invest in variable and floating rate instruments, and transact in securities on a when-issued, delayed delivery or forward commitment basis. The Portfolio seeks to maintain a net asset value of $1.00 per share.
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 .Returns are based on past results and are not an indication of future performance. Prior to May 1, 2016, the Portfolio operated as a prime money market fund and invested in certain types of securities that the Portfolio is no longer permitted to hold. Consequently, the performance information below may have been different if the current investment limitations had been in effect during the period prior to the Portfolio’s conversion to a government money market fund. 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 .</span>
Performance Past Does Not Indicate Future [Text] rr_PerformancePastDoesNotIndicateFuture <span style="color:#000000;font-family:Times New Roman;font-size:10.02pt;margin-left:0%;">Returns are based on past results and are 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 </span><span style="color:#000000;font-family:Times New Roman;font-size:10.02pt;margin-left:0%;">and expenses separately charged by the variable annuity contract or variable life insurance policy separate account that invests in the </span><span style="color:#000000;font-family:Times New Roman;font-size:10.02pt;">Portfolio and returns would be lower if those fees and expenses were reflected.</span>
Bar Chart Closing [Text Block] rr_BarChartClosingTextBlock Quarter/YearReturnBest Quarter4th quarter, 20231.30%Worst Quarter1st quarter, 20220.00%
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>
Performance Table Narrative rr_PerformanceTableNarrativeTextBlock For the seven-day period ended March 31, 2025, the Government Money Market Portfolio’s yield was4.05%.
Money Market Seven Day Yield, Caption [Text] rr_MoneyMarketSevenDayYieldCaption <span style="color:#000000;font-family:Times New Roman;font-size:10.02pt;margin-left:0%;">For the seven-day period ended March 31, 2025, the </span><span style="color:#000000;font-family:Times New Roman;font-size:10.02pt;">Government Money Market Portfolio’s yield was</span>
Money Market Seven Day Yield rr_MoneyMarketSevenDayYield 4.05%
Government Money Market Portfolio | Risk Lose Money [Member]  
Risk Return Abstract rr_RiskReturnAbstract  
Risk [Text Block] rr_RiskTextBlock You could lose money by investing in the Government Money Market Portfolio.
Government Money Market Portfolio | Risk Money Market Fund May Not Preserve Dollar [Member]  
Risk Return Abstract rr_RiskReturnAbstract  
Risk [Text Block] rr_RiskTextBlock Although the Portfolio seeks to preserve the value of your investment at $1.00 per share, it cannot guarantee it will do so.
Government Money Market Portfolio | Risk Money Market Fund Sponsor May Not Provide Support [Member]  
Risk Return Abstract rr_RiskReturnAbstract  
Risk [Text Block] rr_RiskTextBlock The Portfolio’s sponsor has no legal obligation to reimburse the Portfolio for losses and you should not expect that the sponsor will provide financial support to the Portfolio at any time, including during periods of market stress.
Government Money Market Portfolio | Risk Not Insured [Member]  
Risk Return Abstract rr_RiskReturnAbstract  
Risk [Text Block] rr_RiskTextBlock An investment in the Portfolio is not a bank account and is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency.
Government Money Market Portfolio | Active Management Risk [Member]  
Risk Return Abstract rr_RiskReturnAbstract  
Risk [Text Block] rr_RiskTextBlock Active Management Risk – The securities selected for the Portfolio may underperform the markets, relevant indices, or securities selected by other funds with similar investment objectives and investment strategies.
Government Money Market Portfolio | Credit Risk [Member]  
Risk Return Abstract rr_RiskReturnAbstract  
Risk [Text Block] rr_RiskTextBlock Credit Risk – The Portfolio could lose money if the issuer or guarantor of a fixed income security or the counterparty to a repurchase agreement 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.
Government Money Market Portfolio | Income Risk [Member]  
Risk Return Abstract rr_RiskReturnAbstract  
Risk [Text Block] rr_RiskTextBlock Income Risk – The risk that the Portfolio’s yield will vary as short-term securities in its portfolio mature and the proceeds are reinvested in securities with different interest rates.
Government Money Market Portfolio | Interest Rate Risk [Member]  
Risk Return Abstract rr_RiskReturnAbstract  
Risk [Text Block] rr_RiskTextBlock 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 low or negative interest rate environment poses additional risks to the Portfolio. Low yields on the Portfolio’s holdings may have an adverse impact on the Portfolio’s ability to provide a positive yield to its shareholders or pay expenses out of Portfolio assets. Additionally, securities issued or guaranteed by the U.S. government, its agencies and instrumentalities have historically involved little risk of loss of principal if held to maturity. However, due to fluctuations in interest rates, the market value of such securities may vary during the period you are invested in the Portfolio. 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.
Government Money Market Portfolio | Liquidity Risk [Member]  
Risk Return Abstract rr_RiskReturnAbstract  
Risk [Text Block] rr_RiskTextBlock Liquidity Risk – Investments for the Portfolio 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.  The liquidity requirements applicable to government money market funds are designed to help mitigate the potential impact of these risks.
Government Money Market 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.
Government Money Market Portfolio | Repurchase Agreements Risk [Member]  
Risk Return Abstract rr_RiskReturnAbstract  
Risk [Text Block] rr_RiskTextBlock Repurchase Agreements Risk – If the other party to a repurchase agreement defaults on its obligation under the agreement, the Portfolio may suffer delays and incur costs or lose money in exercising its rights under the agreement. These risks may be heightened if the other party is located outside of the U.S. If the seller fails to repurchase the security and the market value of the security declines, the Portfolio may lose money.
Government Money Market Portfolio | Stable Net Asset Value Risk [Member]  
Risk Return Abstract rr_RiskReturnAbstract  
Risk [Text Block] rr_RiskTextBlock Stable Net Asset Value Risk – The Portfolio may not be able to maintain a stable net asset value (“NAV”) of $1.00 per share at all times. If the Portfolio fails to maintain a stable NAV (or there is a perceived threat of such failure), the Portfolio, along with other money market funds, could be subject to increased redemption activity.
Government Money Market 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.
Government Money Market Portfolio | Treasury Obligations Risk [Member]  
Risk Return Abstract rr_RiskReturnAbstract  
Risk [Text Block] rr_RiskTextBlock Treasury Obligations Risk – Direct obligations of the U.S. Treasury have historically involved little risk of loss of principal if held to maturity. However, due to fluctuations in interest rates, the market value of such securities may vary during the period shareholders own shares of the Fund. In addition, notwithstanding that U.S. Treasury obligations are backed by the full faith and credit of the United States, circumstances could arise that could prevent the timely payment of interest of principal, such as reaching the legislative “debt ceiling.” Such non-payment could result in losses to the Fund and substantial negative consequences for the U.S. economy and the global financial system.
Government Money Market Portfolio | U.S. Government Obligations Risk [Member]  
Risk Return Abstract rr_RiskReturnAbstract  
Risk [Text Block] rr_RiskTextBlock U.S. Government Obligations Risk – Certain securities in which the Fund may invest, including securities issued by certain U.S. Government agencies and U.S. Government sponsored enterprises, are not guaranteed by the U.S. Government or supported by the full faith and credit of the United States. In addition, circumstances could arise that could prevent the timely payment of interest or principal on U.S. Government obligations, such as reaching the legislative “debt ceiling.” Such non- payment could result in losses to the Fund and substantial negative consequences for the U.S. economy and the global financial system.
Government Money Market Portfolio | Variable and Floating Rate Instrument Risk [Member]  
Risk Return Abstract rr_RiskReturnAbstract  
Risk [Text Block] rr_RiskTextBlock Variable and Floating Rate Instrument Risk – Variable and floating rate securities provide for a periodic adjustment in the interest rate paid on the obligations. The absence of an active market for these securities could make it difficult for the Portfolio to dispose of them if the issuer defaults.
Government Money Market Portfolio | When-Issued and Delayed Delivery Transactions Risk [Member]  
Risk Return Abstract rr_RiskReturnAbstract  
Risk [Text Block] rr_RiskTextBlock When-Issued and Delayed Delivery Transactions Risk – When issued and delayed delivery securities involve the risk that the security will lose value prior to its delivery. There is also the risk that the security will not be issued or that the other party to the transaction will not meet its obligation. If this occurs, the Portfolio loses both the investment opportunity for the assets it set aside to pay for the security and any gain the security’s price.
Government Money Market Portfolio | Government Money Market Portfolio  
Risk Return Abstract rr_RiskReturnAbstract  
Shareholder Fees(fees paid directly from your investment) rr_ShareholderFeeOther
Management Fee rr_ManagementFeesOverAssets 0.30%
Distribution and Service (12b-1) Fees rr_DistributionAndService12b1FeesOverAssets none
Other Expenses rr_OtherExpensesOverAssets 0.03%
Total Annual Portfolio Operating Expenses rr_ExpensesOverAssets 0.33%
Fee Waiver rr_FeeWaiverOrReimbursementOverAssets none [1]
Total Annual Portfolio Operating Expenses After Fee Waiver rr_NetExpensesOverAssets 0.33% [1]
1 Year rr_ExpenseExampleYear01 $ 34
3 Years rr_ExpenseExampleYear03 106
5 Years rr_ExpenseExampleYear05 185
10 Years rr_ExpenseExampleYear10 $ 418
2015 rr_AnnualReturn2015 0.01%
2016 rr_AnnualReturn2016 0.13%
2017 rr_AnnualReturn2017 0.60%
2018 rr_AnnualReturn2018 1.54%
2019 rr_AnnualReturn2019 1.94%
2020 rr_AnnualReturn2020 0.31%
2021 rr_AnnualReturn2021 0.01%
2022 rr_AnnualReturn2022 1.36%
2023 rr_AnnualReturn2023 4.83%
2024 rr_AnnualReturn2024 4.99%
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, 2023
Highest Quarterly Return rr_BarChartHighestQuarterlyReturn 1.30%
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, 2022
Lowest Quarterly Return rr_BarChartLowestQuarterlyReturn none
1 Yr rr_AverageAnnualReturnYear01 4.99%
5 Yr rr_AverageAnnualReturnYear05 2.28%
10 Yr rr_AverageAnnualReturnYear10 1.56%
[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.