S000015790 [Member] Investment Risks - BOND INDEX FUND |
Mar. 31, 2026 |
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| TRACKING RISK [Member] | |||||
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| Risk [Text Block] | TRACKING RISK is the risk that the Fund’s performance may vary from the performance of the index it tracks as a result of share purchases and redemptions, transaction costs, expenses and other factors. Market disruptions, regulatory restrictions or other abnormal market conditions could have an adverse effect on the Fund’s ability to adjust its exposure to required levels in order to track its Index or cause delays in the Index’s rebalancing schedule. During any such delay, it is possible that the Index, and, in turn, the Fund will deviate from the Index’s stated methodology and therefore experience returns different than those that would have been achieved under a normal rebalancing or reconstitution schedule.
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| INDEX RISK [Member] | |||||
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| Risk [Text Block] | INDEX RISK is the risk that that the Fund would not necessarily buy or sell a security unless that security is added or removed, respectively, from the Index, even if that security generally is underperforming, because unlike many investment companies, the Fund does not utilize an investing strategy that seeks returns in excess of the Index. Additionally, the Fund rebalances its portfolio in accordance with the Index, and, therefore, any changes to the Index’s rebalance schedule will result in corresponding changes to the Fund’s rebalance schedule.
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| SAMPLING RISK [Member] | |||||
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| Risk [Text Block] | SAMPLING RISK is the risk that the Fund’s use of a representative sampling approach may result in increased tracking error because the securities selected for the Fund in the aggregate may vary from the investment profile of the Index. Additionally, the use of a representative sampling approach may result in the Fund holding a smaller number of securities than the Index, and, as a result, an adverse development to an issuer of securities that the Fund holds could result in a greater decline in NAV than would be the case if the Fund held all of the securities in the Index.
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| INTEREST RATE RISKS [Member] | |||||
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| Risk [Text Block] | INTEREST RATE RISK is the risk that during periods of rising interest rates, the market value of the Fund’s securities will tend to be lower than prevailing market rates and in periods of falling interest rates, the market value of the Fund’s securities will tend to be higher. The Fund’s yield will vary as short-term securities in its portfolio mature and the proceeds are reinvested in securities with different interest rates. In general, securities with longer maturities or durations are more sensitive to interest rate changes. A general rise in interest rates may cause investors to move out of fixed income securities on a large scale, which could adversely affect the price and liquidity of fixed income securities and could also result in increased redemptions for the Fund. During periods when inflation rates are high or rising, or during periods of low interest rates, the Fund may be subject to a greater risk of rising interest rates. Interest rate changes can be sudden and unpredictable and may have unpredictable effects on the markets and the Fund’s investments, may result in heightened market volatility, may impact the liquidity of fixed-income securities and of the Fund, and may detract from Fund performance.
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| US GOVERNMENT SECURITIES RISK [Member] | |||||
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| Risk [Text Block] | U.S. GOVERNMENT SECURITIES RISK is the risk that the U.S. government will not provide financial support to its agencies, instrumentalities or sponsored enterprises if it is not obligated to do so by law. Certain U.S. government securities purchased by the Fund are neither issued nor guaranteed by the U.S. Treasury and, therefore, may not be backed by the full faith and credit of the United States. The maximum potential liability of the issuers of some U.S. government securities may greatly exceed their current resources, including any legal right to support from the U.S. Treasury. It is possible that the issuers of such securities will not have the funds to meet their payment obligations in the future.
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| CREDIT OR DEFAULT RISK [Member] | |||||
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| Risk [Text Block] | CREDIT (OR DEFAULT) RISK is the risk that the inability or unwillingness of an issuer or guarantor of a fixed-income security, or a counterparty to a repurchase or other transaction, to meet its principal or interest payments or other financial obligations in a timely manner will adversely affect the value of the Fund’s investments and its returns. Changes in an issuer’s financial strength, the market’s perception of an issuer’s creditworthiness, or in the credit rating of the issuer or the security may also affect the value of the Fund’s investment in that issuer.
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| ASSET BACKED AND MORTGAGE BACKED SECURITIES RISK [Member] | |||||
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| Risk [Text Block] | ASSET-BACKED AND MORTGAGE-BACKED SECURITIES RISK Asset-backed and mortgage-backed securities may be less liquid than other bonds, and may be more sensitive than other bonds to the market’s perception of issuers and creditworthiness of payees, particularly in declining general economic conditions when concern regarding mortgagees’ ability to pay (e.g., the ability of homeowners, commercial mortgagees, consumers with student loans, automobile loans or credit card debtholders to make payments on the underlying loan pools) rises, which may result in the Fund experiencing difficulty selling or valuing these securities. In addition, these securities may not be backed by the full faith and credit of the U.S. government, have experienced extraordinary weakness and volatility at various times in recent years, and may decline quickly in the event of a substantial economic or market downturn. Those asset-backed and mortgage-backed securities that are guaranteed as to the timely payment of interest and principal by a government entity, are not guaranteed as to market price, which will fluctuate. Small movements in interest rates (both increases and decreases) may quickly and significantly reduce the value of certain asset-backed and mortgage-backed securities. An unexpectedly high rate of defaults on the mortgages held by a mortgage pool may adversely affect the value of mortgage-backed securities and could result in losses to the Fund. Privately issued mortgage-backed securities and asset-backed securities may be less liquid than other types of securities and the Fund may be unable to sell these securities at the time or price it desires. During periods of market stress or high redemptions, the Fund may be forced to sell these securities at significantly reduced prices, resulting in losses. Liquid privately issued mortgage-backed securities and asset-backed securities can become illiquid during periods of market stress. Privately issued mortgage-related securities are not subject to the same underwriting requirements as those with government or government-sponsored entity guarantees and, therefore, mortgage loans underlying privately issued mortgage-related securities may have less favorable collateral, credit risk, liquidity risk or other underwriting characteristics, and wider variances in interest rate, term, size, purpose and borrower characteristics.
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| DEBT EXTENSION RISK [Member] | |||||
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| Risk [Text Block] | DEBT EXTENSION RISK is the risk that when interest rates rise an issuer will exercise its right to pay principal on certain debt securities held by the Fund later than expected. This will cause the value of the security, and the Fund’s NAV, to decrease, and the Fund may lose opportunities to invest in higher yielding securities.
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| LIQUIDITY RISK [Member] | |||||
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| Risk [Text Block] | LIQUIDITY RISK is the risk that certain securities may be less liquid than others, which may make them difficult or impossible to sell at the time and the price that the Fund would like and the Fund may have to lower the price, sell other securities instead or forgo an investment opportunity, adversely affecting the value of the Fund’s investments and its returns. In addition, less liquid securities may be more difficult to value and markets may become less liquid when there are fewer interested buyers or sellers or when dealers are unwilling or unable to make a market for certain securities, and if the Fund is forced to sell these investments to meet redemption requests or for other cash needs, the Fund may suffer a loss. Banking regulations may also cause certain dealers to reduce their inventories of certain securities, which may further decrease the Fund’s ability to buy or sell such securities. All of these risks may increase during periods of market turmoil and could have a negative effect on the Fund’s performance.
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| PREPAYMENT OR CALL RISK [Member] | |||||
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| Risk [Text Block] | PREPAYMENT (OR CALL) RISK is the risk that an issuer could exercise its right to pay principal on callable debt securities held by the Fund earlier than expected. Issuers may be more likely to prepay when interest rates fall, when credit spreads change, or when an issuer’s credit quality improves. If this happens, the Fund will not benefit from the rise in the market price of the securities that normally accompanies a decline in interest rates, and will be forced to reinvest prepayment proceeds in lower yielding securities, which may reduce the Fund’s returns. The Fund may also lose any premium it paid to purchase the securities.
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| SECTOR RISK [Member] | |||||
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| Risk [Text Block] | SECTOR RISK is the risk that companies in similar businesses may be similarly affected by particular economic or market events, which may, in certain circumstances, cause the value of securities of all companies in a particular sector of the market to decrease.
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| VALUATION RISK [Member] | |||||
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| Risk [Text Block] | VALUATION RISK is the risk that the sale price the Fund could receive for a portfolio security may differ from the Fund’s valuation of the security, particularly for securities that trade in low volume or volatile markets or that are valued using a fair value methodology. It is possible that the Fund could incur a loss because a portfolio security is sold at a discount to the value ascribed to it for purposes of calculating the Fund’s net asset value. Fair value methodologies may involve more subjectivity than the use of market quotations when fair valuing an investment. In addition, the value of the securities in the Fund’s portfolio may change on days when shareholders will not be able to purchase or sell the Fund’s shares.
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| DERIVATIVES RISKS [Member] | |||||
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| Risk [Text Block] | DERIVATIVES RISK is the risk that derivatives may pose risks in addition to and greater than those associated with investing directly in securities, currencies and other instruments, may be illiquid or less liquid, more volatile, more difficult to value and leveraged so that small changes in the value of the underlying instrument may produce disproportionate losses to the Fund. Derivatives are also subject to counterparty risk, which is the risk that the other party to the transaction will not perform its contractual obligations. The use of derivatives is a highly specialized activity that involves investment techniques and risks different from those associated with investments in more traditional securities and instruments.
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| TBA TRANSACTIONS RISK [Member] | |||||
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| Risk [Text Block] | TBA TRANSACTIONS RISK is the risk of loss if the securities received are less favorable than what was anticipated by the Fund when entering into the TBA transaction. TBA transactions also involve the risk that the counter-party will fail to deliver the securities, exposing the Fund to further losses. Default or bankruptcy of a counterparty to a TBA transaction would expose the Fund to possible losses because of adverse market action, expenses or delays in connection with the purchase or sale of the pools of mortgage pass-through securities specified in the TBA transaction. Whether or not the Fund takes delivery of the securities at the termination date of a TBA transaction, the Fund will nonetheless be exposed to changes in the value of the underlying investments during the term of the agreement.
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| SECURITIES LENDING RISK [Member] | |||||
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| Risk [Text Block] | SECURITIES LENDING RISK is the risk that the Fund may lose money because the borrower of the loaned securities fails to return the securities in a timely manner or at all. The Fund could also lose money in the event of a decline in the value of collateral provided for loaned securities or a decline in the value of any investments made with cash collateral.
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| MARKET RISKS [Member] | |||||
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| Risk [Text Block] | MARKET RISK is the risk that the value of the Fund’s investments may increase or decrease in response to expected, real or perceived economic, political or financial events in the U.S. or global markets. The frequency and magnitude of such changes in value cannot be predicted. Certain securities and other investments held by the Fund may experience increased volatility, illiquidity, or other potentially adverse effects in response to changing market conditions, inflation, elevated levels of government debt, changes in interest rates, lack of liquidity in the bond or equity markets or volatility in the equity markets. Market disruptions caused by local or regional events such as financial institution failures, changes in trade regulation or economic sanctions, internal unrest and discord, war, acts of terrorism, the spread of infectious illness (including epidemics and pandemics) or other public health issues, recessions, the threat or occurrence of a government shutdown, or other events or adverse investor sentiment could have a significant impact on the Fund and its investments. During periods of market disruption or other abnormal market conditions, the Fund’s exposure to risks described elsewhere in this summary will likely increase.
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| Risk Lose Money [Member] | |||||
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| Risk [Text Block] | As with any mutual fund, it is possible to lose money on an investment in the Fund. | ||||
| Risk Not Insured Depository Institution [Member] | |||||
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| Risk [Text Block] | An investment in the Fund is not a deposit of any bank and is not insured or guaranteed by the Federal Deposit Insurance Corporation, any other government agency, or The Northern Trust Company, its affiliates, subsidiaries or any other bank. | ||||
| Risk Nondiversified Status [Member] | |||||
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| Risk [Text Block] | NON‑DIVERSIFICATION RISK Under the 1940 Act, a fund designated as “diversified” must limit its holdings such that the securities of issuers which individually represent more than 5% of its total assets must in the aggregate represent less than 25% of its total assets. The Fund is “diversified” for purposes of the 1940 Act. However, in seeking to track its Index, the Fund may become “non‑diversified,” as defined in the 1940 Act, solely as a result of a change in relative market capitalization or index weighting of one or more constituents of the Index. A non‑diversified fund can invest a greater portion of its assets in the obligations or securities of a small portion of its assets in the obligations or securities of a small number of issuers or any single issuer than a diversified fund can. In such circumstances, a change in the value of one or a few issuers’ securities will therefore affect the value of the Fund more than if it was a diversified fund.
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