S000012098 [Member] Investment Strategy - S000012098 [Member]	Apr. 30, 2025
Prospectus [Line Items]
Strategy [Heading]	Principal Investment Strategies
Strategy Narrative [Text Block]	Under normal circumstances, the Fund invests at least 80% of its net assets (including the amount of any borrowings for investment purposes) in securities that comprise the FTSE USBIG Treasury Index (the Index). The Index is an unmanaged index composed of U.S. Treasury notes and bonds with remaining maturities of at least one year and outstanding principal of at least $5 billion and which are included in the FTSE U.S. Broad Investment-Grade Bond Index. Different securities have different weightings in the Index. Securities in the Index are weighted by market value; that is, the price per bond or note multiplied by the number of bonds or notes outstanding. In seeking to match the performance of the Index, before fees and expenses, the investment manager attempts to allocate the Fund’s assets among securities in the Index. The Fund will not hold all of the securities in the Index.

S000012098 [Member] Investment Strategy - S000012098 [Member]

Apr. 30, 2025

Principal Investment Strategies

Under normal circumstances, the Fund invests at least 80% of its net assets (including the amount of any borrowings for investment purposes) in securities that comprise the FTSE USBIG Treasury Index (the Index). The Index is an unmanaged index composed of U.S. Treasury notes and bonds with remaining maturities of at least one year and outstanding principal of at least $5 billion and which are included in the FTSE U.S. Broad Investment-Grade Bond Index. Different securities have different weightings in the Index. Securities in the Index are weighted by market value; that is, the price per bond or note multiplied by the number of bonds or notes outstanding.

In seeking to match the performance of the Index, before fees and expenses, the investment manager attempts to allocate the Fund’s assets among securities in the Index. The Fund will not hold all of the securities in the Index.