| Form N-CSR Cover | 6 Months Ended |
| Sep. 30, 2024 | |
| Shareholder Report [Line Items] | |
| Document Type | N-CSRS |
| Amendment Flag | false |
| Registrant Name | SIT U S GOVERNMENT SECURITIES FUND INC |
| Entity Central Index Key | 0000809981 |
| Entity Investment Company Type | N-1A |
| Document Period End Date | Sep. 30, 2024 |
| Shareholder Report | 6 Months Ended |
| Sep. 30, 2024 USD ($) Holding | |
| Shareholder Report [Line Items] | |
| Document Type | N-CSRS |
| Amendment Flag | false |
| Registrant Name | SIT U S GOVERNMENT SECURITIES FUND INC |
| Entity Central Index Key | 0000809981 |
| Entity Investment Company Type | N-1A |
| Document Period End Date | Sep. 30, 2024 |
| C000032004 | |
| Shareholder Report [Line Items] | |
| Fund Name | Sit U.S. Government Securities Fund |
| Class Name | Class S |
| No Trading Symbol [Flag] | true |
| Annual or Semi-Annual Statement [Text Block] | This semi-annual shareholder report contains important information about Sit U.S. Government Securities Fund for the period of April 1, 2024 to September 30, 2024. |
| Shareholder Report Annual or Semi-Annual | Semi-Annual Shareholder Report |
| Additional Information [Text Block] | You can find additional information about the Fund at www.sitfunds.com/documents |
| Additional Information Phone Number | 1-800-332-5580 |
| Expenses [Text Block] | What were the Fund’s costs for the last six months? (based on a hypothetical $10,000 investment) Class Name Costs of a $10,000 investment Costs paid as a percentage of a $10,000 investment Sit U.S. Government Securities Fund - Class S $41 0.80% |
| Expenses Paid, Amount | $ 41 |
| Expense Ratio, Percent | 0.80% |
| Average Annual Return [Table Text Block] | 6 Months 1 Year 5 Years 10 Years Sit U.S. Government Securities Fund - Class S 4.60% 8.99% 1.12% 1.48% Bloomberg U.S. Aggregate Bond Index* 5.26% 11.57% 0.33% 1.84% Bloomberg Intermediate Government Bond Index 4.56% 8.33% 0.84% 1.50% |
| Material Change Date | Sep. 30, 2024 |
| AssetsNet | $ 227,989,425 |
| Holdings Count | Holding | 188 |
| Advisory Fees Paid, Amount | $ 234,766 |
| InvestmentCompanyPortfolioTurnover | 11.64% |
| Additional Fund Statistics [Text Block] | Fund Statistics (as of 09/30/24) Total Net Assets $227,989,425 # of Portfolio Holdings 188 Portfolio Turnover Rate 11.64% Investment Advisory Fees Paid $234,766 Average Maturity 23.9 Years Effective Duration Footnote Reference 3.6 Years Footnote Description Footnote 1 Duration is a measure of estimated price sensitivity relative to changes in interest rates. Portfolios with longer durations are typically more sensitive to changes in interest rates. For example, if interest rates rise by 1%, the fair value of a security with an effective duration of 5 years would decrease by 5%, with all other factors being constant. The correlation between duration and price sensitivity is greater for securities rated investment-grade than it is for securities rated below investment-grade. Duration estimates are based on assumptions by Sit Investment Associates, Inc. (the "Adviser") and are subject to a number of limitations. Effective duration is calculated based on historical price changes of securities held by the Fund, and therefore is a more accurate estimate of price sensitivity provided interest rates remain within their historical range. |
| Holdings [Text Block] | Portfolio Composition (% of Net Assets) Collateralized Mortgage Obligations 70.1 % Government National Mortgage Association 9.6 Federal National Mortgage Association 8.0 Federal Home Loan Mortgage Corporation 7.9 U.S. Treasury / Federal Agency Securities 2.5 Asset-Backed Securities 0.7 Cash & Other Net Assets 1.2 Total 100.0 % |
| Material Fund Change [Text Block] | Material Fund Changes There were no material changes to the Fund during the six-month period ended September 30, 2024. |
| Summary of Change Legend [Text Block] | There were no material changes to the Fund during the six-month period ended September 30, 2024. |
| C000217198 | |
| Shareholder Report [Line Items] | |
| Fund Name | Sit U.S. Government Securities Fund |
| Class Name | Class Y |
| No Trading Symbol [Flag] | true |
| Annual or Semi-Annual Statement [Text Block] | This semi-annual shareholder report contains important information about Sit U.S. Government Securities Fund for the period of April 1, 2024 to September 30, 2024. |
| Shareholder Report Annual or Semi-Annual | Semi-Annual Shareholder Report |
| Additional Information [Text Block] | You can find additional information about the Fund at www.sitfunds.com/documents |
| Additional Information Phone Number | 1-800-332-5580 |
| Expenses [Text Block] | What were the Fund’s costs for the last six months? (based on a hypothetical $10,000 investment) Class Name Costs of a $10,000 investment Costs paid as a percentage of a $10,000 investment Sit U.S. Government Securities Fund - Class Y $28 0.55% |
| Expenses Paid, Amount | $ 28 |
| Expense Ratio, Percent | 0.55% |
| Average Annual Return [Table Text Block] | 6 Months 1 Year Since Inception 1/1/2020 Sit U.S. Government Securities Fund - Class Y 4.63% 9.28% 1.41% Bloomberg U.S. Aggregate Bond Index* 5.26% 11.57% 0.31% Bloomberg Intermediate Government Bond Index 4.56% 8.33% 0.88% |
| Material Change Date | Sep. 30, 2024 |
| AssetsNet | $ 227,989,425 |
| Holdings Count | Holding | 188 |
| Advisory Fees Paid, Amount | $ 234,766 |
| InvestmentCompanyPortfolioTurnover | 11.64% |
| Additional Fund Statistics [Text Block] | Fund Statistics (as of 09/30/24) Total Net Assets $227,989,425 # of Portfolio Holdings 188 Portfolio Turnover Rate 11.64% Investment Advisory Fees Paid $234,766 Average Maturity 23.9 Years Effective Duration Footnote Reference 3.6 Years Footnote Description Footnote 1 Duration is a measure of estimated price sensitivity relative to changes in interest rates. Portfolios with longer durations are typically more sensitive to changes in interest rates. For example, if interest rates rise by 1%, the fair value of a security with an effective duration of 5 years would decrease by 5%, with all other factors being constant. The correlation between duration and price sensitivity is greater for securities rated investment-grade than it is for securities rated below investment-grade. Duration estimates are based on assumptions by Sit Investment Associates, Inc. (the "Adviser") and are subject to a number of limitations. Effective duration is calculated based on historical price changes of securities held by the Fund, and therefore is a more accurate estimate of price sensitivity provided interest rates remain within their historical range. |
| Holdings [Text Block] | Portfolio Composition (% of Net Assets) Collateralized Mortgage Obligations 70.1 % Government National Mortgage Association 9.6 Federal National Mortgage Association 8.0 Federal Home Loan Mortgage Corporation 7.9 U.S. Treasury / Federal Agency Securities 2.5 Asset-Backed Securities 0.7 Cash & Other Net Assets 1.2 Total 100.0 % |
| Material Fund Change [Text Block] | Material Fund Changes There were no material changes to the Fund during the six-month period ended September 30, 2024. |
| Summary of Change Legend [Text Block] | There were no material changes to the Fund during the six-month period ended September 30, 2024. |
| Shareholder Report, Line Graph (Details) - USD ($) | 1 Months Ended | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Sep. 30, 2024 | Aug. 31, 2024 | Jul. 31, 2024 | Jun. 30, 2024 | May 31, 2024 | Apr. 30, 2024 | Mar. 31, 2024 | Feb. 29, 2024 | Jan. 31, 2024 | Dec. 31, 2023 | Nov. 30, 2023 | Oct. 31, 2023 | Sep. 30, 2023 | Aug. 31, 2023 | Jul. 31, 2023 | Jun. 30, 2023 | May 31, 2023 | Apr. 30, 2023 | Mar. 31, 2023 | Feb. 28, 2023 | Jan. 31, 2023 | Dec. 31, 2022 | Nov. 30, 2022 | Oct. 31, 2022 | Sep. 30, 2022 | Aug. 31, 2022 | Jul. 31, 2022 | Jun. 30, 2022 | May 31, 2022 | Apr. 30, 2022 | Mar. 31, 2022 | Feb. 28, 2022 | Jan. 31, 2022 | Dec. 31, 2021 | Nov. 30, 2021 | Oct. 31, 2021 | Sep. 30, 2021 | Aug. 31, 2021 | Jul. 31, 2021 | Jun. 30, 2021 | May 31, 2021 | Apr. 30, 2021 | Mar. 31, 2021 | Feb. 28, 2021 | Jan. 31, 2021 | Dec. 31, 2020 | Nov. 30, 2020 | Oct. 31, 2020 | Sep. 30, 2020 | Aug. 31, 2020 | Jul. 31, 2020 | Jun. 30, 2020 | May 31, 2020 | Apr. 30, 2020 | Mar. 31, 2020 | Feb. 29, 2020 | Jan. 31, 2020 | Dec. 31, 2019 | Nov. 30, 2019 | Oct. 31, 2019 | Sep. 30, 2019 | Aug. 31, 2019 | Jul. 31, 2019 | Jun. 30, 2019 | May 31, 2019 | Apr. 30, 2019 | Mar. 31, 2019 | Feb. 28, 2019 | Jan. 31, 2019 | Dec. 31, 2018 | Nov. 30, 2018 | Oct. 31, 2018 | Sep. 30, 2018 | Aug. 31, 2018 | Jul. 31, 2018 | Jun. 30, 2018 | May 31, 2018 | Apr. 30, 2018 | Mar. 31, 2018 | Feb. 28, 2018 | Jan. 31, 2018 | Dec. 31, 2017 | Nov. 30, 2017 | Oct. 31, 2017 | Sep. 30, 2017 | Aug. 31, 2017 | Jul. 31, 2017 | Jun. 30, 2017 | May 31, 2017 | Apr. 30, 2017 | Mar. 31, 2017 | Feb. 28, 2017 | Jan. 31, 2017 | Dec. 31, 2016 | Nov. 30, 2016 | Oct. 31, 2016 | Sep. 30, 2016 | Aug. 31, 2016 | Jul. 31, 2016 | Jun. 30, 2016 | May 31, 2016 | Apr. 30, 2016 | Mar. 31, 2016 | Feb. 29, 2016 | Jan. 31, 2016 | Dec. 31, 2015 | Nov. 30, 2015 | Oct. 31, 2015 | Sep. 30, 2015 | Aug. 31, 2015 | Jul. 31, 2015 | Jun. 30, 2015 | May 31, 2015 | Apr. 30, 2015 | Mar. 31, 2015 | Feb. 28, 2015 | Jan. 31, 2015 | Dec. 31, 2014 | Nov. 30, 2014 | Oct. 31, 2014 | Sep. 30, 2014 | |
| C000032004 | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Account Value [Line Items] | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Line Graph and Table Measure Name | Sit U.S. Government Securities Fund - Class S | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Account Value | $ 11,577 | $ 11,463 | $ 11,319 | $ 11,149 | $ 11,029 | $ 10,941 | $ 11,068 | $ 11,003 | $ 11,087 | $ 11,084 | $ 10,783 | $ 10,504 | $ 10,622 | $ 10,787 | $ 10,790 | $ 10,795 | $ 10,873 | $ 10,926 | $ 10,889 | $ 10,765 | $ 10,872 | $ 10,739 | $ 10,751 | $ 10,652 | $ 10,721 | $ 10,938 | $ 11,132 | $ 11,050 | $ 11,065 | $ 11,033 | $ 11,099 | $ 11,190 | $ 11,195 | $ 11,239 | $ 11,309 | $ 11,315 | $ 11,323 | $ 11,387 | $ 11,412 | $ 11,372 | $ 11,369 | $ 11,362 | $ 11,344 | $ 11,352 | $ 11,352 | $ 11,348 | $ 11,348 | $ 11,337 | $ 11,368 | $ 11,365 | $ 11,373 | $ 11,343 | $ 11,326 | $ 11,323 | $ 11,294 | $ 11,215 | $ 11,059 | $ 10,951 | $ 10,941 | $ 10,950 | $ 10,951 | $ 10,983 | $ 10,845 | $ 10,870 | $ 10,822 | $ 10,741 | $ 10,730 | $ 10,641 | $ 10,629 | $ 10,597 | $ 10,509 | $ 10,454 | $ 10,461 | $ 10,464 | $ 10,408 | $ 10,417 | $ 10,402 | $ 10,377 | $ 10,387 | $ 10,363 | $ 10,364 | $ 10,413 | $ 10,390 | $ 10,384 | $ 10,376 | $ 10,392 | $ 10,360 | $ 10,355 | $ 10,362 | $ 10,337 | $ 10,309 | $ 10,333 | $ 10,280 | $ 10,276 | $ 10,274 | $ 10,375 | $ 10,400 | $ 10,389 | $ 10,391 | $ 10,381 | $ 10,319 | $ 10,311 | $ 10,306 | $ 10,300 | $ 10,264 | $ 10,205 | $ 10,187 | $ 10,174 | $ 10,183 | $ 10,160 | $ 10,155 | $ 10,132 | $ 10,141 | $ 10,135 | $ 10,141 | $ 10,094 | $ 10,095 | $ 10,062 | $ 10,031 | $ 10,008 | $ 10,000 |
| C000217198 | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Account Value [Line Items] | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Line Graph and Table Measure Name | Sit U.S. Government Securities Fund - Class Y | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Account Value | $ 10,690 | 10,592 | 10,446 | 10,288 | 10,185 | 10,101 | 10,216 | 10,144 | 10,229 | 10,214 | 9,935 | 9,685 | 9,782 | 9,932 | 9,943 | 9,945 | 10,004 | 10,061 | 10,025 | 9,909 | 9,996 | 9,871 | 9,890 | 9,787 | 9,858 | 10,055 | 10,231 | 10,145 | 10,165 | 10,133 | 10,196 | 10,277 | 10,270 | 10,318 | 10,372 | 10,384 | 10,390 | 10,446 | 10,457 | 10,428 | 10,422 | 10,414 | 10,395 | 10,400 | 10,398 | 10,392 | 10,389 | 10,377 | 10,403 | 10,397 | 10,401 | 10,372 | 10,354 | 10,349 | 10,320 | 10,246 | 10,101 | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Bloomberg U.S. Aggregate Bond Index | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Account Value [Line Items] | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Account Value | 12,004 | 11,846 | 11,678 | 11,412 | 11,304 | 11,116 | 11,404 | 11,300 | 11,462 | 11,493 | 11,069 | 10,590 | 10,760 | 11,040 | 11,111 | 11,119 | 11,159 | 11,282 | 11,214 | 10,936 | 11,226 | 10,891 | 10,940 | 10,552 | 10,691 | 11,174 | 11,499 | 11,224 | 11,403 | 11,330 | 11,777 | 12,114 | 12,250 | 12,520 | 12,552 | 12,515 | 12,518 | 12,628 | 12,652 | 12,512 | 12,425 | 12,384 | 12,287 | 12,443 | 12,625 | 12,716 | 12,699 | 12,575 | 12,632 | 12,638 | 12,741 | 12,554 | 12,475 | 12,417 | 12,201 | 12,273 | 12,056 | 11,828 | 11,836 | 11,842 | 11,807 | 11,870 | 11,570 | 11,545 | 11,402 | 11,203 | 11,200 | 10,989 | 10,995 | 10,880 | 10,683 | 10,620 | 10,705 | 10,774 | 10,705 | 10,703 | 10,716 | 10,640 | 10,720 | 10,651 | 10,753 | 10,879 | 10,829 | 10,843 | 10,836 | 10,888 | 10,792 | 10,745 | 10,756 | 10,674 | 10,592 | 10,598 | 10,527 | 10,506 | 10,492 | 10,746 | 10,829 | 10,835 | 10,847 | 10,779 | 10,589 | 10,586 | 10,546 | 10,450 | 10,376 | 10,235 | 10,269 | 10,296 | 10,294 | 10,225 | 10,240 | 10,169 | 10,281 | 10,306 | 10,343 | 10,295 | 10,393 | 10,179 | 10,170 | 10,098 | 10,000 |
| Bloomberg U.S. Aggregate Bond Index | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Account Value [Line Items] | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Account Value | 10,149 | 10,015 | 9,873 | 9,648 | 9,557 | 9,398 | 9,641 | 9,553 | 9,690 | 9,717 | 9,359 | 8,953 | 9,097 | 9,334 | 9,394 | 9,400 | 9,434 | 9,538 | 9,481 | 9,246 | 9,491 | 9,208 | 9,250 | 8,921 | 9,038 | 9,447 | 9,721 | 9,489 | 9,641 | 9,579 | 9,957 | 10,241 | 10,357 | 10,585 | 10,612 | 10,581 | 10,584 | 10,676 | 10,696 | 10,578 | 10,504 | 10,470 | 10,388 | 10,519 | 10,674 | 10,751 | 10,736 | 10,632 | 10,679 | 10,685 | 10,772 | 10,614 | 10,547 | 10,498 | 10,315 | 10,376 | 10,192 | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Bloomberg Intermediate Government Bond Index | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Account Value [Line Items] | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Account Value | 11,609 | 11,496 | 11,373 | 11,168 | 11,075 | 10,953 | 11,103 | 11,049 | 11,167 | 11,142 | 10,915 | 10,681 | 10,717 | 10,819 | 10,813 | 10,801 | 10,901 | 10,982 | 10,924 | 10,665 | 10,853 | 10,683 | 10,708 | 10,528 | 10,576 | 10,833 | 11,053 | 10,909 | 10,990 | 10,916 | 11,092 | 11,378 | 11,429 | 11,577 | 11,608 | 11,578 | 11,645 | 11,716 | 11,735 | 11,645 | 11,654 | 11,617 | 11,574 | 11,652 | 11,749 | 11,777 | 11,773 | 11,759 | 11,802 | 11,793 | 11,816 | 11,779 | 11,769 | 11,747 | 11,715 | 11,481 | 11,295 | $ 11,138 | $ 11,141 | $ 11,169 | $ 11,136 | $ 11,186 | $ 10,988 | $ 11,007 | $ 10,917 | $ 10,760 | $ 10,755 | $ 10,625 | $ 10,633 | $ 10,587 | $ 10,433 | $ 10,362 | $ 10,358 | $ 10,410 | $ 10,349 | $ 10,369 | $ 10,370 | $ 10,304 | $ 10,363 | $ 10,309 | $ 10,339 | $ 10,439 | $ 10,436 | $ 10,468 | $ 10,481 | $ 10,542 | $ 10,478 | $ 10,445 | $ 10,472 | $ 10,431 | $ 10,377 | $ 10,372 | $ 10,342 | $ 10,321 | $ 10,324 | $ 10,501 | $ 10,551 | $ 10,533 | $ 10,581 | $ 10,577 | $ 10,429 | $ 10,444 | $ 10,447 | $ 10,427 | $ 10,376 | $ 10,214 | $ 10,233 | $ 10,267 | $ 10,300 | $ 10,224 | $ 10,218 | $ 10,177 | $ 10,217 | $ 10,212 | $ 10,221 | $ 10,167 | $ 10,255 | $ 10,095 | $ 10,121 | $ 10,070 | $ 10,000 |
| Bloomberg Intermediate Government Bond Index | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Account Value [Line Items] | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Account Value | $ 10,423 | $ 10,321 | $ 10,211 | $ 10,027 | $ 9,943 | $ 9,834 | $ 9,969 | $ 9,920 | $ 10,026 | $ 10,004 | $ 9,799 | $ 9,590 | $ 9,622 | $ 9,713 | $ 9,708 | $ 9,697 | $ 9,787 | $ 9,860 | $ 9,808 | $ 9,575 | $ 9,744 | $ 9,591 | $ 9,614 | $ 9,452 | $ 9,495 | $ 9,726 | $ 9,924 | $ 9,794 | $ 9,867 | $ 9,801 | $ 9,959 | $ 10,215 | $ 10,262 | $ 10,394 | $ 10,422 | $ 10,395 | $ 10,455 | $ 10,519 | $ 10,536 | $ 10,455 | $ 10,463 | $ 10,430 | $ 10,391 | $ 10,462 | $ 10,548 | $ 10,573 | $ 10,570 | $ 10,557 | $ 10,596 | $ 10,588 | $ 10,609 | $ 10,575 | $ 10,566 | $ 10,547 | $ 10,518 | $ 10,307 | $ 10,141 |
| Shareholder Report, Average Annual Return (Details) | 1 Months Ended | 6 Months Ended | 12 Months Ended | 57 Months Ended | 60 Months Ended | 120 Months Ended |
| Sep. 30, 2024 | Sep. 30, 2024 | Sep. 30, 2024 | Sep. 30, 2024 | Sep. 30, 2024 | Sep. 30, 2024 | |
| C000032004 | ||||||
| Average Annual Return [Line Items] | ||||||
| Line Graph and Table Measure Name | Sit U.S. Government Securities Fund - Class S | |||||
| C000217198 | ||||||
| Average Annual Return [Line Items] | ||||||
| Line Graph and Table Measure Name | Sit U.S. Government Securities Fund - Class Y | |||||
| Without Sales Load [Member] | C000032004 | ||||||
| Average Annual Return [Line Items] | ||||||
| Average Annual Return, Percent | 4.60% | 8.99% | 1.12% | 1.48% | ||
| Without Sales Load [Member] | C000217198 | ||||||
| Average Annual Return [Line Items] | ||||||
| Average Annual Return, Percent | 4.63% | 9.28% | 1.41% | |||
| Bloomberg U.S. Aggregate Bond Index | ||||||
| Average Annual Return [Line Items] | ||||||
| Average Annual Return, Percent | 5.26% | 11.57% | 0.33% | 1.84% | ||
| Bloomberg U.S. Aggregate Bond Index | ||||||
| Average Annual Return [Line Items] | ||||||
| Average Annual Return, Percent | 5.26% | 11.57% | 0.31% | |||
| Bloomberg Intermediate Government Bond Index | ||||||
| Average Annual Return [Line Items] | ||||||
| Average Annual Return, Percent | 4.56% | 8.33% | 0.84% | 1.50% | ||
| Bloomberg Intermediate Government Bond Index | ||||||
| Average Annual Return [Line Items] | ||||||
| Average Annual Return, Percent | 4.56% | 8.33% | 0.88% |
