v3.26.1
Select Bond Portfolio Investment Risks - Select 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.
Debt Obligations of Foreign Governments Risk [Member]  
Prospectus [Line Items]  
Risk [Text Block] •Debt Obligations of Foreign Governments Risk – The issuer of the foreign debt or the governmental authorities that control the repayment of such debt may be unable or unwilling to repay principal or interest when due, and the Portfolio may have limited recourse in the event of a default. The market prices of debt obligations of governments and their agencies, and the Portfolio’s net asset value, may be more volatile than prices of U.S. debt obligations.
Derivatives Risk [Member]  
Prospectus [Line Items]  
Risk [Text Block] •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.
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 less liquid, more volatile, and harder to value than U.S. securities.
High Portfolio Turnover Risk [Member]  
Prospectus [Line Items]  
Risk [Text Block] •High Portfolio Turnover Risk – Active and frequent trading may cause higher brokerage expenses and other transaction costs, which may adversely affect the Portfolio’s performance.
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. 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 – Fixed income investments, including Rule 144A securities, may be difficult to purchase or sell at an advantageous time or price, if at all, during periods of reduced marketability for the investment or due to the size of the transaction. These risks may be magnified during periods of economic turmoil or in an extended economic downturn.
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.
Mortgage-and Asset-Backed Securities Risk [Member]  
Prospectus [Line Items]  
Risk [Text Block] •Mortgage- and Asset-Backed Securities Risk – The risks of investing in mortgage-related and other asset-backed securities, including mortgage dollar rolls, include interest rate risk, credit risk, liquidity risk, prepayment risk and extension risk. Mortgage-related and other asset-backed securities represent interests in pools of mortgages or other assets and often involve risks that are different or possibly more acute than risks associated with other types of debt instruments. The value of some mortgage- or asset-backed securities may be particularly sensitive to changes in prevailing interest rates. To-be-announced mortgage-backed securities increase interest rate risks because the underlying mortgages may be less favorable than anticipated by the Portfolio. Other asset-backed securities are subject to risks similar to those associated with mortgage-backed securities, as well as risks associated with the nature and servicing of the assets underlying the securities. Asset-backed securities may not have the benefit of a security interest in collateral comparable to that of mortgage assets, resulting in additional credit risk. The use of mortgage dollar rolls involves potential risks of loss that are different from those related to the mortgage securities underlying the transactions, including counterparty risk, market risk, and financial risk (including the risk that the value of the principal and interest payments associated with the mortgage instrument sold to a counterparty exceeds the compensation paid to the Portfolio by the counterparty).
Prepayment and Extension Risk [Member]  
Prospectus [Line Items]  
Risk [Text Block] •Prepayment and Extension Risk – Prepayment risk is the risk that principal on a debt obligation will be paid earlier than scheduled or expected, which could reduce yield and market value of the security and shorten the Portfolio’s average effective maturity. The rate of prepayments tends to increase as interest rates fall. Extension risk is the risk that, as interest rates rise, repayments on a debt obligation may occur more slowly than anticipated by the market and the obligation may remain outstanding longer.
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.
U.S. Government Securities Risk [Member]  
Prospectus [Line Items]  
Risk [Text Block] •U.S. Government Securities Risk – Not all obligations of the U.S. government, its agencies and instrumentalities are backed by the full faith and credit of the U.S. Treasury. Some obligations are backed only by the credit of the issuing agency or instrumentality, and in some cases there may be some risk of default by the issuer. Any guarantee by the U.S. government or its agencies or instrumentalities of a security held by the Portfolio does not apply to the market value of such security or to shares of the Portfolio itself.
When-Issued and Delayed Delivery Transactions Risk [Member]  
Prospectus [Line Items]  
Risk [Text Block] •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.
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.