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Risk Return Abstract rr_RiskReturnAbstract  
Registrant Name dei_EntityRegistrantName NORTHWESTERN MUTUAL SERIES FUND INC
Prospectus Date rr_ProspectusDate May 01, 2024
Mid Cap Growth Stock Portfolio  
Risk Return Abstract rr_RiskReturnAbstract  
Risk/Return [Heading] rr_RiskReturnHeading Mid Cap Growth 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 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 and hold 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, 2025</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 39.37% of the average value of its portfolio.
Portfolio Turnover, Rate rr_PortfolioTurnoverRate 39.37%
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. 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 stocks of mid-sized companies. The Portfolio considers a company to be a mid-capitalization company if it has a market capitalization within the collective range of the Russell MidCap® Index and the S&P MidCap 400® Index. As of March 31, 2024, this range was approximately $351.89 million to $89.01 billion. The market capitalization range of these indices changes over time. Securities of companies whose market capitalizations no longer fall within this collective range after purchase may continue to be held by the Portfolio.The Portfolio invests primarily in common stocks of mid cap companies selected on the basis of their potential for capital appreciation. The Portfolio focuses on companies that are determined to be of high quality. The key characteristics of high quality companies include a leadership position within an industry, a strong balance sheet, a high return on equity, and/or a strong management team. The adviser uses fundamental analysis to evaluate a security for purchase or sale by the Portfolio. Fundamental analysis of a company involves the assessment of such factors as its business environment, management quality, balance sheet, income statement, anticipated earnings, revenues and dividends, and environmental, social, and/or governance (ESG) characteristics. Examples of ESG factors that may be considered include, but are not limited to, employee engagement, talent attraction and retention, incentive structures, and corporate strategy. Consideration of ESG factors may not be determinative in the adviser’s investment process and the adviser may conclude that other attributes of an investment outweigh ESG considerations when making investment decisions for the Portfolio.The Portfolio seeks to reduce overall risk by diversifying across sectors, industry groups and companies. The Portfolio’s sector exposure relative to its benchmark is driven by an investment process which relies on fundamental company analysis and individual stock selection. As a result, based upon market or economic conditions, the Portfolio may at times have a relatively high percentage of its assets invested in a particular sector of the market.The Portfolio invests primarily in U.S. common stocks. The Portfolio may also invest up to 20% of net assets in American Depositary Receipts (ADRs) and other securities of foreign issuers, including non-U.S. dollar denominated securities. The Portfolio may invest in real estate investment trusts (REITs).The Portfolio typically trims positions as valuation appears incrementally less attractive, and may sell a stock when the adviser’s investment thesis is no longer valid, typically due to an erosion of company fundamentals relative to expectations or when valuation is no longer attractive. The Portfolio may, but is not required to, exit a position if the company’s capitalization grows beyond the mid cap range.
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 accompanying the bar chart shows the Portfolio’s average annual total return over certain time periods and compares the Portfolio’s returns with those of a broad measure of market performance.  Prior to October 27, 2017, the sub-adviser to the Portfolio was different. 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 </span><span style="color:#000000;font-family:Times New Roman;font-size:10.02pt;">from year to year. The table accompanying the bar chart shows the Portfolio’s average annual total return over certain time periods and compares the Portfolio’s returns with those of a broad measure of market performance. </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 indication of future </span><span style="color:#000000;font-family:Times New Roman;font-size:10.02pt;">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 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.</span>
Bar Chart Closing [Text Block] rr_BarChartClosingTextBlock Quarter/YearReturnBest Quarter4th quarter, 202026.37%Worst Quarter1st quarter, 2020-24.21%
Performance Table Heading rr_PerformanceTableHeading <span style="color:#000000;font-family:Times New Roman;font-size:10.02pt;font-weight:bold;margin-left:0%;">Average Annual Total Return</span><span style="color:#000000;font-family:Times New Roman;font-size:10.02pt;font-weight:bold;">(for periods ended December 31, 2023)</span>
Mid Cap Growth 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.
Mid Cap Growth Stock 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.
Mid Cap Growth Stock 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.
Mid Cap Growth 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 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.
Mid Cap Growth Stock 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.
Mid Cap Growth Stock Portfolio | Foreign Currency Risk [Member]  
Risk Return Abstract rr_RiskReturnAbstract  
Risk [Text Block] rr_RiskTextBlock •Foreign Currency Risk – The risk that foreign (non-U.S. dollar) currency denominated securities may be adversely affected by decreases in foreign currency values relative to the U.S. dollar. Investments in securities subject to foreign currency risk may have more rapid and extreme changes in value or more losses than investments in U.S. dollar denominated securities.
Mid Cap Growth Stock 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.
Mid Cap Growth Stock Portfolio | Investment Style Risk [Member]  
Risk Return Abstract rr_RiskReturnAbstract  
Risk [Text Block] rr_RiskTextBlock •Investment Style Risk – A portfolio managed using a growth 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.
Mid Cap Growth 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.
Mid Cap Growth Stock Portfolio | Mid Cap Company Risk [Member]  
Risk Return Abstract rr_RiskReturnAbstract  
Risk [Text Block] rr_RiskTextBlock •Mid Cap Company Risk – Investing in mid 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.
Mid Cap Growth Stock 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.
Mid Cap Growth Stock 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.
Mid Cap Growth 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.
Mid Cap Growth Stock Portfolio | Mid Cap Growth Stock Portfolio  
Risk Return Abstract rr_RiskReturnAbstract  
Shareholder Fees(fees paid directly from your investment) rr_ShareholderFeeOther
Management Fee rr_ManagementFeesOverAssets 0.52%
Distribution and Service (12b-1) Fees rr_DistributionAndService12b1FeesOverAssets none
Other Expenses rr_OtherExpensesOverAssets 0.02%
Total Annual Portfolio Operating Expenses rr_ExpensesOverAssets 0.54%
Fee Waiver rr_FeeWaiverOrReimbursementOverAssets none [1]
Total Annual Portfolio Operating Expenses After Fee Waiver rr_NetExpensesOverAssets 0.54% [1]
1 Year rr_ExpenseExampleYear01 $ 55
3 Years rr_ExpenseExampleYear03 173
5 Years rr_ExpenseExampleYear05 302
10 Years rr_ExpenseExampleYear10 $ 677
2014 rr_AnnualReturn2014 8.49%
2015 rr_AnnualReturn2015 0.71%
2016 rr_AnnualReturn2016 0.83%
2017 rr_AnnualReturn2017 20.29%
2018 rr_AnnualReturn2018 (7.38%)
2019 rr_AnnualReturn2019 33.01%
2020 rr_AnnualReturn2020 25.41%
2021 rr_AnnualReturn2021 10.18%
2022 rr_AnnualReturn2022 (23.77%)
2023 rr_AnnualReturn2023 14.96%
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 26.37%
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 (24.21%)
1 Yr rr_AverageAnnualReturnYear01 14.96%
5 Yr rr_AverageAnnualReturnYear05 10.00%
10 Yr rr_AverageAnnualReturnYear10 7.05%
Mid Cap Growth Stock Portfolio | Russell MidCap® Growth Index(reflects no deduction for fees, expenses or taxes)  
Risk Return Abstract rr_RiskReturnAbstract  
1 Yr rr_AverageAnnualReturnYear01 25.87%
5 Yr rr_AverageAnnualReturnYear05 13.81%
10 Yr rr_AverageAnnualReturnYear10 10.57%
[1] The Portfolio's investment adviser has entered into a written agreement to waive a portion of its management fee. This fee waiver agreement may be terminated by the adviser at any time after April 30, 2025.