The Fund seeks to achieve its objective by investing exclusively in U.S. government securities, which are securities issued, guaranteed or insured by the U.S. government, its agencies or instrumentalities.

The Fund invests a substantial portion of its assets in pass-through securities. Pass-through securities are formed when mortgages or other debt instruments are pooled together and undivided interests in the pool are sold to investors, such as the Fund. Pass-through securities in which the Fund invests include mortgage-backed securities such as those issued by Government National Mortgage Association (GNMA), Federal National Mortgage Association (FNMA) and Federal Home Loan Mortgage Corporation (FHLMC). GNMA is an agency of the U.S. government and its securities are backed by the full faith and credit of the U.S. government. FNMA and FHLMC are U.S. government sponsored enterprises and their securities are backed by their credit. Other types of U.S. government securities in which the Fund may invest include U.S. Treasury securities, U.S. government agency collateralized mortgage obligations and other U.S. government agency securities.

In selecting securities for the Fund, Sit Investment Associates, Inc. (the “Adviser”) seeks securities providing high current income relative to yields currently available in the market. In making purchase and sales decisions for the Fund, the Adviser considers its economic outlook and interest rate forecast, as well as its evaluation of a security’s prepayment risk, yield, maturity, and liquidity.

The Adviser attempts to maintain an average effective duration for the portfolio of approximately 0 to 5 years. Duration is a measure of total price sensitivity relative to changes in interest rates. For example, if interest rates rise by 1%, the market value of a security with an effective duration of 3 years would decrease by 3%, with all other factors being constant. Portfolios with longer durations are typically more sensitive to changes in interest rates. The Adviser may hedge the Fund’s duration by investing in interest rate futures and options, but not in excess of 5% of the Fund’s net assets.

The Fund’s dollar-weighted average maturity will, under normal market conditions, range between 15 and 25 years. However, since the Fund’s securities are subject to various types of call provisions which make their expected average lives shorter than their stated maturity dates, the Adviser believes that the Fund’s average effective duration is a more accurate measure of the Fund’s price sensitivity to changes in interest rates than the Fund’s dollar-weighted average maturity.

Investment Strategy - SIT U S GOVERNMENT SECURITIES FUND INC	Mar. 31, 2025
Prospectus [Line Items]
Strategy [Heading]	PRINCIPAL INVESTMENT STRATEGIES
Strategy Narrative [Text Block]	The Fund seeks to achieve its objective by investing exclusively in U.S. government securities, which are securities issued, guaranteed or insured by the U.S. government, its agencies or instrumentalities. The Fund invests a substantial portion of its assets in pass-through securities. Pass-through securities are formed when mortgages or other debt instruments are pooled together and undivided interests in the pool are sold to investors, such as the Fund. Pass-through securities in which the Fund invests include mortgage-backed securities such as those issued by Government National Mortgage Association (GNMA), Federal National Mortgage Association (FNMA) and Federal Home Loan Mortgage Corporation (FHLMC). GNMA is an agency of the U.S. government and its securities are backed by the full faith and credit of the U.S. government. FNMA and FHLMC are U.S. government sponsored enterprises and their securities are backed by their credit. Other types of U.S. government securities in which the Fund may invest include U.S. Treasury securities, U.S. government agency collateralized mortgage obligations and other U.S. government agency securities. In selecting securities for the Fund, Sit Investment Associates, Inc. (the “Adviser”) seeks securities providing high current income relative to yields currently available in the market. In making purchase and sales decisions for the Fund, the Adviser considers its economic outlook and interest rate forecast, as well as its evaluation of a security’s prepayment risk, yield, maturity, and liquidity. The Adviser attempts to maintain an average effective duration for the portfolio of approximately 0 to 5 years. Duration is a measure of total price sensitivity relative to changes in interest rates. For example, if interest rates rise by 1%, the market value of a security with an effective duration of 3 years would decrease by 3%, with all other factors being constant. Portfolios with longer durations are typically more sensitive to changes in interest rates. The Adviser may hedge the Fund’s duration by investing in interest rate futures and options, but not in excess of 5% of the Fund’s net assets. The Fund’s dollar-weighted average maturity will, under normal market conditions, range between 15 and 25 years. However, since the Fund’s securities are subject to various types of call provisions which make their expected average lives shorter than their stated maturity dates, the Adviser believes that the Fund’s average effective duration is a more accurate measure of the Fund’s price sensitivity to changes in interest rates than the Fund’s dollar-weighted average maturity.