Label	Element	Value
Risk/Return:	rr_RiskReturnAbstract
Registrant Name	dei_EntityRegistrantName	SIT U S GOVERNMENT SECURITIES FUND INC
Prospectus Date	rr_ProspectusDate	Aug. 01, 2016
SIT MUTUAL FUNDS II INC | Sit Tax-Free Income Fund
Risk/Return:	rr_RiskReturnAbstract
Risk/Return [Heading]	rr_RiskReturnHeading	SUMMARY INFORMATION SIT TAX-FREE INCOME FUND
Objective [Heading]	rr_ObjectiveHeading	INVESTMENT OBJECTIVE
Objective, Primary [Text Block]	rr_ObjectivePrimaryTextBlock	The Fund seeks high current income that is exempt from federal income tax consistent with preservation of capital.
Expense [Heading]	rr_ExpenseHeading	FEES AND EXPENSES OF THE FUND
Expense Narrative [Text Block]	rr_ExpenseNarrativeTextBlock	This table describes fees and expenses that you may pay if you buy and hold shares of the Fund.
Shareholder Fees Caption [Text]	rr_ShareholderFeesCaption	Shareholder Fees (fees paid directly from your investment)
Operating Expenses Caption [Text]	rr_OperatingExpensesCaption	Annual Fund Operating Expenses (expenses that you pay each year as a percentage of the value of your investment)
Portfolio Turnover [Heading]	rr_PortfolioTurnoverHeading	PORTFOLIO TURNOVER
Portfolio Turnover [Text Block]	rr_PortfolioTurnoverTextBlock	The Fund pays transactions costs, such as commissions, when it buys and sells securities (or “turns over” its portfolio). A higher portfolio turnover rate may indicate higher transaction costs and may result in higher taxes when Fund shares are held in a taxable account. These costs, which are not reflected in annual fund operating expenses in the example, affect the Fund’s performance. During the most recent fiscal year, the Fund’s portfolio turnover rate was 23.11% of the average value of the portfolio.
Portfolio Turnover, Rate	rr_PortfolioTurnoverRate	23.11%
Expenses Not Correlated to Ratio Due to Acquired Fund Fees [Text]	rr_ExpensesNotCorrelatedToRatioDueToAcquiredFundFees	The total annual fund operating expenses do not correlate to the ratio of expenses to average net assets shown in the Fund's Financial Highlights, which does not include Acquired Fund Fees and Expenses.
Expense Example [Heading]	rr_ExpenseExampleHeading	Example
Expense Example Narrative [Text Block]	rr_ExpenseExampleNarrativeTextBlock	This Example is intended to help you compare the cost of investing in the Fund with the cost of investing in other mutual funds. It assumes that you invest $10,000 in the Fund for the time periods indicated, that your investment has a 5% return each year, that the Fund’s operating expenses remain the same, and that you redeem all of your shares at the end of those periods. Although your actual costs and returns may be higher or lower, based on these assumptions your costs would be:
Strategy [Heading]	rr_StrategyHeading	PRINCIPAL INVESTMENT STRATEGIES
Strategy Narrative [Text Block]	rr_StrategyNarrativeTextBlock	The Fund seeks to achieve its objective by investing primarily in municipal securities that generate interest income that is exempt from both regular federal income tax and federal alternative minimum tax. During normal market conditions, the Fund invests 100% (and, as a fundamental policy, no less than 80%) of its net assets in such tax-exempt municipal securities. Municipal securities are debt obligations issued by or for U.S. states, territories, and possessions and the District of Columbia, and their political subdivisions, agencies, and instrumentalities. The Fund invests both in revenue bonds, which are backed by and payable only from the revenues derived from a specific facility or specific revenue source, and in general obligation bonds, which are secured by the full faith, credit and taxation power of the issuing municipality. The Fund generally invests a significant portion of its assets in obligations of municipal housing authorities, which include single family and multi-family mortgage revenue bonds, revenue bonds of health care-related facilities, and revenue bonds of educational institutions, which include higher education institutions, public, private and charter schools, and student loan-backed bonds. The Fund primarily invests in securities rated investment-grade at the time of purchase or, if unrated, determined to be of comparable quality by the Fund's investment adviser (the "Adviser"). Investment-grade securities are rated within the four highest grades by the major rating agencies. However, the Fund may invest up to 25% of its assets in municipal securities rated below investment grade (commonly referred to as junk bonds) or determined to be of comparable quality by the Adviser, but the Fund may not invest in securities rated lower than B3 by Moody's Investors Service, or B- by Standard and Poor's or Fitch Ratings, or, if unrated, determined by the Adviser to be of comparable quality. The Fund may invest in closed-end funds which invest in the same types of securities in which the Fund may invest directly. In selecting securities for the Fund, the Adviser seeks securities providing high tax-exempt income. The Adviser attempts to maintain an average effective duration for the portfolio of approximately 3 to 8 years. Duration is a measure of total price sensitivity relative to changes in interest rates. Portfolios with longer durations are typically more sensitive to changes in interest rates. The Adviser's economic outlook and interest rate forecast, as well as their evaluation of a security's structure, credit quality, yield, maturity, and liquidity, are all factors considered when making investment decisions. The Fund's dollar-weighted average maturity will, under normal market conditions, range between 10 and 20 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.
Risk [Heading]	rr_RiskHeading	PRINCIPAL INVESTMENT RISKS
Risk Narrative [Text Block]	rr_RiskNarrativeTextBlock	As with all mutual funds investing in bonds, the price and yield of the Fund may change daily due to interest rate changes and other factors. You could lose money by investing in the Fund. The principal risks of investing in the Fund are as follows: Call Risk: Many bonds may be redeemed ("called") at the option of the issuer before their stated maturity date. In general, an issuer will call its bonds if they can be refinanced by issuing new bonds which bear a lower interest rate. The Fund may then be forced to invest the unanticipated proceeds at lower interest rates, resulting in a decline in the Fund's income. Credit Risk: The issuers or guarantors of securities owned by the Fund may default on the payment of principal or interest, or experience a decline in credit quality, causing the value of the Fund to decrease. Cybersecurity Risk: Cybersecurity breaches may allow an unauthorized party to gain access to Fund assets, shareholder data, or proprietary information, or cause the Fund and/or its service providers to suffer data corruption or lose operational functionality. High-Yield Risk: The Fund may invest up to 25% of its assets in municipal securities rated below investment-grade or if nonrated, determined to be of comparable quality by the Adviser. Debt securities rated below investment-grade are commonly known as junk bonds. Junk bonds are considered predominately speculative and involve greater risk of default or price changes due to changes in the issuer's creditworthiness. Income Risk: The income you earn from the Fund may decline due to declining interest rates. Interest Rate Risk: An increase in interest rates may lower the Fund's value and the overall return on your investment. The magnitude of this decrease is often greater for longer-term fixed income securities than shorter-term securities. The Fund's exposure to risks associated with rising interest rates may be increased due to the current period of historically low interest rates in the U.S. and the effect of potential government fiscal policy initiatives and resulting market reaction to those initiatives. Liquidity Risk: The Fund may not be able to pay redemption proceeds within the time periods described in this Prospectus because of an inability to sell certain debt securities with more limited trading opportunities at a favorable price or time. The reduction in dealer market-making capacity and other recent events have caused the markets for some debt securities to experience increased price volatility and reduced liquidity. Consequently, the Fund may have to accept a lower price to sell a security, sell other securities to raise cash, or give up an investment opportunity, any of which could have a negative effect on the Fund's performance. Infrequent trading may also lead to greater price volatility. Management Risk: A strategy used by the investment management team may not produce the intended results. Market Risk: The market value of securities may fall or fail to rise. Market risk may affect a single issuer, sector of the economy, industry, or the market as a whole. The market value of securities may fluctuate, sometimes rapidly and unpredictably. Political, Economic and Tax Risk: Because the Fund invests primarily in municipal securities issued by states and their political subdivisions, the Fund may be particularly affected by the political and economic conditions and developments in those states. Since the Fund primarily invests in municipal securities, the value of the Fund may be more adversely affected than other funds by future changes in federal or state income tax laws. Revenue Bond Risk: The revenue bonds in which the Fund invests may entail greater credit risk than the Fund's investments in general obligation bonds. In particular, weaknesses in federal housing subsidy programs and their administration may result in a decrease of subsidies available for the payment of principal and interest on certain multi-family housing authority bonds. Sector Concentration Risk: Because the Fund may invest a significant portion of their assets in health care facility bonds, housing authority bonds, and education bonds, the Fund may be more affected by events influencing these sectors than a fund that is more diversified across numerous sectors. Valuation Risk: The Fund may hold securities for which prices from pricing services may be unavailable or are deemed unreliable, in which case the Fund's procedures for valuing investments provide that the Adviser shall use the fair value of such securities for valuing investments. There is a risk that the fair value determined by the Adviser or the price determined by the pricing service may be different than the actual sale prices of such securities.
Risk Lose Money [Text]	rr_RiskLoseMoney	You could lose money by investing in the Fund.
Bar Chart and Performance Table [Heading]	rr_BarChartAndPerformanceTableHeading	HISTORICAL PERFORMANCE
Performance Narrative [Text Block]	rr_PerformanceNarrativeTextBlock	The following tables provide information on the Fund’s volatility and performance. The Fund’s past performance before and after taxes is not necessarily an indication of how the Fund will perform in the future. The bar chart below is intended to provide you with an indication of the risks of investing in the Fund by showing changes in the Fund’s performance from year to year. The table below compares the Fund’s performance over different time periods to that of the Fund’s benchmark index, which is a broad measure of market performance. The table includes returns both before and after taxes. After-tax returns are calculated using historical highest individual federal marginal income tax rates and do not reflect the impact of state and local taxes. Actual after-tax returns depend on an investor’s tax situation and may differ from those shown. After-tax returns shown are not relevant to investors who hold their Fund shares through tax-deferred arrangements, such as 401(k) plans or individual retirement accounts. The performance information reflects Fund expenses, and assumes that all distributions have been reinvested. Performance reflects fee waivers in effect. If these fee waivers were not in place, performance would be reduced. The benchmark is an unmanaged index, has no expenses, and it is not possible to invest directly in an index.
Performance Information Illustrates Variability of Returns [Text]	rr_PerformanceInformationIllustratesVariabilityOfReturns	The bar chart below is intended to provide you with an indication of the risks of investing in the Fund by showing changes in the Fund’s performance from year to year. The table below compares the Fund’s performance over different time periods to that of the Fund’s benchmark index, which is a broad measure of market performance.
Performance Past Does Not Indicate Future [Text]	rr_PerformancePastDoesNotIndicateFuture	The Fund’s past performance before and after taxes is not necessarily an indication of how the Fund will perform in the future.
Bar Chart [Heading]	rr_BarChartHeading	Annual Total Returns for calendar years ended December 31
Bar Chart Closing [Text Block]	rr_BarChartClosingTextBlock	The Fund’s year-to-date return as of 6/30/16 (not annualized) was 5.04%. Best Quarter: 9.71% (3Q09). Worst Quarter: -11.66% (4Q08).
Performance Table Heading	rr_PerformanceTableHeading	Average Annual Total Returns for periods ended December 31, 2015
Performance Table Uses Highest Federal Rate	rr_PerformanceTableUsesHighestFederalRate	After-tax returns are calculated using historical highest individual federal marginal income tax rates and do not reflect the impact of state and local taxes.
Performance Table Not Relevant to Tax Deferred	rr_PerformanceTableNotRelevantToTaxDeferred	Actual after-tax returns depend on an investor’s tax situation and may differ from those shown. After-tax returns shown are not relevant to investors who hold their Fund shares through tax-deferred arrangements, such as 401(k) plans or individual retirement accounts.
SIT MUTUAL FUNDS II INC | Sit Tax-Free Income Fund | Sit Tax-Free Income Fund
Risk/Return:	rr_RiskReturnAbstract
Shareholder Fees (fees paid directly from your investment)	rr_ShareholderFeeOther	none
Management Fees	rr_ManagementFeesOverAssets	0.80%
Acquired Fund Fees and Expenses	rr_AcquiredFundFeesAndExpensesOverAssets	0.09%	[1]
Total Annual Fund Operating Expenses	rr_ExpensesOverAssets	0.89%
1 Year	rr_ExpenseExampleYear01	$ 91
3 Years	rr_ExpenseExampleYear03	285
5 Years	rr_ExpenseExampleYear05	495
10 Years	rr_ExpenseExampleYear10	$ 1,100
2006	rr_AnnualReturn2006	3.54%
2007	rr_AnnualReturn2007	0.26%
2008	rr_AnnualReturn2008	(15.77%)
2009	rr_AnnualReturn2009	22.42%
2010	rr_AnnualReturn2010	2.49%
2011	rr_AnnualReturn2011	10.73%
2012	rr_AnnualReturn2012	9.93%
2013	rr_AnnualReturn2013	(4.46%)
2014	rr_AnnualReturn2014	14.60%
2015	rr_AnnualReturn2015	3.91%
Year to Date Return, Label	rr_YearToDateReturnLabel	year-to-date return
Bar Chart, Year to Date Return, Date	rr_BarChartYearToDateReturnDate	Jun. 30, 2016
Bar Chart, Year to Date Return	rr_BarChartYearToDateReturn	5.04%
Highest Quarterly Return, Label	rr_HighestQuarterlyReturnLabel	Best Quarter:
Highest Quarterly Return, Date	rr_BarChartHighestQuarterlyReturnDate	Sep. 30, 2009
Highest Quarterly Return	rr_BarChartHighestQuarterlyReturn	9.71%
Lowest Quarterly Return, Label	rr_LowestQuarterlyReturnLabel	Worst Quarter:
Lowest Quarterly Return, Date	rr_BarChartLowestQuarterlyReturnDate	Dec. 31, 2008
Lowest Quarterly Return	rr_BarChartLowestQuarterlyReturn	(11.66%)
1 Year	rr_AverageAnnualReturnYear01	3.91%
5 Years	rr_AverageAnnualReturnYear05	6.73%
10 Years	rr_AverageAnnualReturnYear10	4.27%
SIT MUTUAL FUNDS II INC | Sit Tax-Free Income Fund | Return after taxes on distributions | Sit Tax-Free Income Fund
Risk/Return:	rr_RiskReturnAbstract
1 Year	rr_AverageAnnualReturnYear01	3.90%
5 Years	rr_AverageAnnualReturnYear05	6.72%
10 Years	rr_AverageAnnualReturnYear10	4.26%
SIT MUTUAL FUNDS II INC | Sit Tax-Free Income Fund | Return after taxes on distributions and sale of Fund shares | Sit Tax-Free Income Fund
Risk/Return:	rr_RiskReturnAbstract
1 Year	rr_AverageAnnualReturnYear01	3.79%
5 Years	rr_AverageAnnualReturnYear05	6.19%
10 Years	rr_AverageAnnualReturnYear10	4.22%
SIT MUTUAL FUNDS II INC | Sit Tax-Free Income Fund | Barclays 5-Year Municipal Bond Index (reflects no deduction for fees, expenses or taxes)
Risk/Return:	rr_RiskReturnAbstract
1 Year	rr_AverageAnnualReturnYear01	2.43%
5 Years	rr_AverageAnnualReturnYear05	3.25%
10 Years	rr_AverageAnnualReturnYear10	4.12%

[1] The total annual fund operating expenses do not correlate to the ratio of expenses to average net assets shown in the Fund's Financial Highlights, which does not include Acquired Fund Fees and Expenses.