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Prospectus: rr_ProspectusTable  
Document Type dei_DocumentType 485BPOS
Document Period End Date dei_DocumentPeriodEndDate Dec. 31, 2021
Registrant Name dei_EntityRegistrantName Morgan Stanley U.S. Government Securities Trust
Entity Inv Company Type dei_EntityInvCompanyType N-1A
Central Index Key dei_EntityCentralIndexKey 0000730044
Amendment Flag dei_AmendmentFlag false
Document Creation Date dei_DocumentCreationDate Apr. 28, 2022
Document Effective Date dei_DocumentEffectiveDate Apr. 29, 2022
Prospectus Date rr_ProspectusDate Apr. 29, 2022
(Morgan Stanley U.S. Government Securities Trust - Classes A, L, I and C) | (Morgan Stanley U.S. Government Securities Trust)  
Prospectus: rr_ProspectusTable  
Objective [Heading] rr_ObjectiveHeading Investment Objective
Objective, Primary [Text Block] rr_ObjectivePrimaryTextBlock
Morgan Stanley U.S. Government Securities Trust (the “Fund”) seeks a high level of current income consistent with safety of principal.
Expense [Heading] rr_ExpenseHeading Fees and Expenses
Expense Narrative [Text Block] rr_ExpenseNarrativeTextBlock
The table below describes the fees and expenses that you may pay if you buy, hold and sell shares of the Fund. You may pay fees other than the fees and expenses of the Fund, such as brokerage commissions and other fees charged by financial intermediaries, which are not reflected in the tables and examples below.
For purchases of Class A shares, you may qualify for a sales charge discount if the cumulative net asset value per share (“NAV”) of Class A shares of the Fund being purchased in a single transaction, together with the NAV of any Class A, Class L and Class C shares of the Fund already held in Related Accounts (as defined in the section of this Prospectus entitled “Shareholder Information—Sales Charges Applicable to Purchases of Class A Shares”) as of the date of the transaction as well as Class A, Class L and Class C shares of any other Morgan Stanley Multi-Class Fund excluding Morgan Stanley Institutional Fund Trust Short Duration Income, Ultra-Short Income and Ultra-Short Municipal Income Portfolios (as defined in the section of the Prospectus entitled “Shareholder Information—How to Exchange Shares—Permissible Fund Exchanges”) and including shares of Morgan Stanley Money Market Funds (as defined in the section of the Prospectus entitled “Shareholder Information—How to Exchange Shares—Permissible Fund Exchanges”) that you acquired in an exchange of Class A shares of the Fund or Class A shares of another Morgan Stanley Multi-Class Fund excluding Morgan Stanley Institutional Fund Trust Short Duration Income, Ultra-Short Income and Ultra-Short Municipal Income Portfolios already held in Related Accounts as of the date of the transaction, amounts to $100,000 or more. More information about this combined purchase discount and other discounts is available from your financial intermediary and on page 25 of the Prospectus in the section entitled “Shareholder Information— Share Class Arrangements” and in the section of the Fund’s Statement of Additional Information (“SAI”) entitled “Purchase, Redemption and Pricing of Shares.” In addition, Appendix A attached to the Prospectus contains more information regarding financial intermediary-specific sales charge waivers and discounts.
Class I shares may be available on brokerage platforms of firms that have agreements with the Fund’s principal underwriter permitting such firms to (i) offer Class I shares solely when acting as an agent for the investor and (ii) impose on an investor transacting in Class I shares through such platforms a commission and/or other forms of compensation to the broker. Shares of the Fund are available in other share classes that have different fees and expenses.
Expense Breakpoint Discounts [Text] rr_ExpenseBreakpointDiscounts For purchases of Class A shares, you may qualify for a sales charge discount if the cumulative net asset value per share (“NAV”) of Class A shares of the Fund being purchased in a single transaction, together with the NAV of any Class A, Class L and Class C shares of the Fund already held in Related Accounts (as defined in the section of this Prospectus entitled “Shareholder Information—Sales Charges Applicable to Purchases of Class A Shares”) as of the date of the transaction as well as Class A, Class L and Class C shares of any other Morgan Stanley Multi-Class Fund excluding Morgan Stanley Institutional Fund Trust Short Duration Income, Ultra-Short Income and Ultra-Short Municipal Income Portfolios (as defined in the section of the Prospectus entitled “Shareholder Information—How to Exchange Shares—Permissible Fund Exchanges”) and including shares of Morgan Stanley Money Market Funds (as defined in the section of the Prospectus entitled “Shareholder Information—How to Exchange Shares—Permissible Fund Exchanges”) that you acquired in an exchange of Class A shares of the Fund or Class A shares of another Morgan Stanley Multi-Class Fund excluding Morgan Stanley Institutional Fund Trust Short Duration Income, Ultra-Short Income and Ultra-Short Municipal Income Portfolios already held in Related Accounts as of the date of the transaction, amounts to $100,000 or more.
Expense Breakpoint, Minimum Investment Required [Amount] rr_ExpenseBreakpointMinimumInvestmentRequiredAmount $ 100,000
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)
Expense Example [Heading] rr_ExpenseExampleHeading Example
Expense Example Narrative [Text Block] rr_ExpenseExampleNarrativeTextBlock
The example below is intended to help you compare the cost of investing in the Fund with the cost of investing in other mutual funds.
The example assumes that you invest $10,000 in the Fund, your investment has a 5% return each year and that the Fund’s operating expenses remain the same (except that the example incorporates the fee waiver and/or expense reimbursement arrangement for only the first year). After eight years, Class C shares of the Fund generally will convert automatically to Class A shares of the Fund. The example for Class C shares reflects the conversion to Class A shares after eight years. Please refer to the section of the Prospectus entitled “Shareholder Information—Conversion Features” for more information. Although your actual costs may be higher or lower, based on these assumptions your costs would be:
Expense Example by, Year, Caption [Text] rr_ExpenseExampleByYearCaption If You SOLD Your Shares
Expense Example, No Redemption, By Year, Caption [Text] rr_ExpenseExampleNoRedemptionByYearCaption If You HELD Your Shares
Portfolio Turnover [Heading] rr_PortfolioTurnoverHeading Portfolio Turnover
Portfolio Turnover [Text Block] rr_PortfolioTurnoverTextBlock
The Fund pays transaction 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 Total Annual Fund Operating Expenses or in the Example, affect the Fund’s performance. During the most recent fiscal year, the Fund’s portfolio turnover rate was 376% of the average value of its portfolio.
Portfolio Turnover, Rate rr_PortfolioTurnoverRate 376.00%
Strategy [Heading] rr_StrategyHeading Principal Investment Strategies
Strategy Narrative [Text Block] rr_StrategyNarrativeTextBlock
The Fund normally invests at least 80% of its net assets in a portfolio of U.S. government securities. This policy may be changed without shareholder approval; however, you would be notified upon 60 days’ notice in writing of any changes. In making investment decisions, the Adviser considers economic developments, interest rate trends and other factors. The U.S. government securities that the Fund may purchase include: U.S. Treasury bills, notes and bonds, all of which are direct obligations of the U.S. Government; securities (including mortgage-backed securities) issued by agencies and instrumentalities of the U.S. Government which are backed by the full faith and credit of the United States; securities (including mortgage-backed securities) issued by agencies and instrumentalities which are not backed by the full faith and credit of the United States, but whose issuing agency or instrumentality has the right to borrow, to meet its obligations, from the U.S. Treasury; securities issued by agencies and instrumentalities which are backed solely by the credit of the issuing agency or instrumentality; municipal securities; and securities guaranteed by the U.S. Government or its agencies and instrumentalities or securities supported by the U.S. Government in some other way, such as the discretionary authority of the U.S. Government to purchase certain obligations of the agency or instrumentality. The Fund’s investments may include zero coupon securities, which are purchased at a discount and generally accrue interest, but make no payment until maturity.
The mortgage-backed securities in which the Fund may invest include mortgage pass-through securities, commercial mortgage-backed securities (“CMBS”), collateralized mortgage obligations (“CMOs”), stripped mortgage-backed securities (“SMBS”) and inverse floating rate obligations (“inverse floaters”). In addition, the Fund may invest in to-be-announced pass-through mortgage securities, which settle on a delayed delivery basis (“TBAs”). The Fund may also invest in asset-backed securities. The Fund may invest in restricted and illiquid securities.
The Fund may, but it is not required to, use derivative instruments for risk management purposes. The Fund’s use of derivatives may involve the purchase and sale of derivative instruments such as futures, options, swaps and other related instruments and techniques. These derivative instruments will be counted toward the Fund’s 80% policy discussed above to the extent they have economic characteristics similar to the securities included within that policy.
The Fund is not a money market fund, does not seek to maintain a stable NAV and does not qualify for the tax relief afforded to money market funds by the U.S. Treasury.
Risk [Heading] rr_RiskHeading Principal Risks
Risk Narrative [Text Block] rr_RiskNarrativeTextBlock
There is no assurance that the Fund will achieve its investment objective, and you can lose money investing in this Fund. The principal risks of investing in the Fund include:
•
U.S. Government Securities. Different types of U.S. government securities are subject to different levels of credit risk, including the risk of default, depending on the nature of the particular government support for that security. For example, a U.S. government-sponsored entity, such as Federal National Mortgage Association or Federal Home Loan Mortgage Corporation, although chartered or sponsored by an Act of Congress, may issue securities that are neither insured nor guaranteed by the U.S. Treasury and, therefore, are not backed by the full faith and credit of the United States. With respect to U.S. government securities that are not backed by the full faith and credit of the United States, there is the risk that the U.S. Government will not provide financial support to such U.S. government agencies, instrumentalities or sponsored enterprises if it is not obligated to do so by law.
•
Zero Coupon Securities. The interest earned on zero coupon securities is, implicitly, automatically compounded and paid out at maturity. While such compounding at a constant rate eliminates the risk of receiving lower yields upon reinvestment of interest if prevailing interest rates decline, the owner of a zero coupon security will be unable to participate in higher yields upon reinvestment of interest received on interest-paying securities if prevailing interest rates rise. A zero coupon security pays no interest to its holder during its life. Therefore, to the extent the Fund invests in zero coupon securities, it will not receive current cash available for distribution to shareholders. In addition, zero coupon securities are subject to substantially greater price fluctuations during periods of changing prevailing interest rates than are comparable securities which pay interest on a current basis.
•
Credit and Interest Rate Risk. Credit risk refers to the possibility that the issuer or guarantor of a security will be unable or unwilling or perceived to be unable or unwilling to make interest payments and/or repay the principal on its debt. In such instances, the value of the Fund could decline and the Fund could lose money. Interest rate risk refers to the decline in the value of a fixed-income security resulting from changes in the general level of interest rates. When the general level of interest rates goes up, the prices of most fixed-income securities go down. When the general level of interest rates goes down, the prices of most fixed-income securities go up. The Fund may invest in variable and floating rate loans and other variable and floating rate securities. Although these instruments are generally less sensitive to interest rate changes than fixed rate instruments, the value of variable and floating rate loans and other securities may decline if their interest rates do not rise as quickly, or as much, as general interest rates. The Fund may face a heightened level of interest rate risk in times of monetary policy change and/or uncertainty, such as when the Federal Reserve Board adjusts a quantitative easing program and/or changes rates. A changing interest rate environment increases certain risks, including the potential for periods of volatility, increased redemptions, shortened durations (i.e., prepayment risk) and extended durations (i.e., extension risk). During periods when interest rates are low or there are negative interest rates, the Fund’s yield (and total return) also may be low or otherwise adversely affected or the Fund may be unable to maintain positive returns. Credit ratings may not be an accurate assessment of liquidity or credit risk. Although credit quality may not accurately reflect the true credit risk of an instrument, a change in the credit rating of an instrument or an issuer can have a rapid, adverse effect on the instrument’s liquidity and make it more difficult for the Fund to sell at an advantageous price or time.
•
Asset-Backed Securities. Asset-backed securities involve the risk that various federal and state consumer laws and other legal and economic factors may result in the collateral backing the securities being insufficient to support payment on the securities. Some asset-backed securities also entail prepayment risk and extension risk, which may vary depending on the type of asset.
•
Mortgage-Backed Securities. Mortgage-backed securities entail prepayment risk, which generally increases during a period of falling interest rates. Rising interest rates tend to discourage refinancings, with the result that the average life and volatility of mortgage-backed securities will increase and market price will decrease. Rates of prepayment, faster or slower than expected by the Adviser, could reduce the Fund’s yield, increase the volatility of the Fund and/or cause a decline in NAV. Mortgage-backed securities are also subject to extension risk, which is the risk that rising interest rates could cause mortgages or other obligations underlying the securities to be prepaid more slowly than expected, thereby lengthening the duration of such securities, increasing their sensitivity to interest rate changes and causing their prices to decline. Certain mortgage-backed securities may be more volatile and less liquid than other traditional types of debt securities. In addition, mortgage-backed securities are subject to credit risk. The Fund may invest in non-agency mortgage-backed securities offered by non-governmental issuers, such as commercial banks, savings and loan institutions, private mortgage insurance companies, mortgage bankers and other secondary market issuers. Non-agency mortgage-backed securities are not subject to the same underwriting requirements for the underlying mortgages that are applicable to those mortgage-backed securities that have a government or government-sponsored entity guarantee. As a result, the mortgage loans underlying non-agency mortgage-backed securities may, and frequently do, have less favorable collateral, credit risk or other underwriting characteristics than government or government-sponsored mortgage-backed securities and have wider variances in a number of terms including interest rate, term, size, purpose and borrower characteristics. An unexpectedly high rate of defaults on the mortgages held by a mortgage pool may adversely affect the value of a mortgage-backed security and could result
in losses to the Fund. The risk of such defaults is generally higher in the case of mortgage pools that include subprime mortgages. Furthermore, mortgage-backed securities may be subject to risks associated with the assets underlying those securities, such as a decline in value. Investments in mortgage-backed securities may give rise to a form of leverage (indebtedness) and may cause the Fund’s portfolio turnover rate to appear higher. Leverage may cause the Fund to be more volatile than if the Fund had not been leveraged. The risks associated with mortgage-backed securities typically become elevated during periods of distressed economic, market, health and labor conditions. In particular, increased levels of unemployment, delays and delinquencies in payments of mortgage and rent obligations, and uncertainty regarding the effects and extent of government intervention with respect to mortgage payments and other economic matters may adversely affect the Fund’s investments in mortgage-backed securities.
•
Commercial Mortgage-Backed Securities. CMBS are subject to credit risk and prepayment risk. Although prepayment risk is present, it is of a lesser degree in the CMBS market than in the residential mortgage market; commercial real estate property loans often contain provisions which substantially reduce the likelihood that such securities will be prepaid (e.g., significant prepayment penalties on loans and, in some cases, prohibition on principal payments for several years following origination).
•
Collateralized Mortgage Obligations. CMOs are comprised of various tranches, the expected cash flows of which have varying degrees of predictability as compared with the underlying mortgage loans or mortgage pass-through entities. The less predictable the cash flow, the higher the yield and the greater the risk. In addition, if the collateral securing CMOs or any third-party guarantees is insufficient to make payments, the Fund could sustain a loss. Like other mortgage backed-securities, some CMOs are subject to credit risk. The Fund invests in both Agency and Non-Agency bonds. Many Agency bonds do not have credit risk as they are government guaranteed.
•
Stripped Mortgage-Backed Securities. Investments in each class of SMBS are extremely sensitive to changes in interest rates. The interest-only or “IO” class tends to decrease in value substantially if interest rates decline and prepayment rates become more rapid. The principal-only or “PO” class tends to decrease in value substantially if interest rates increase and the rate of prepayment decreases. If the Fund invests in SMBS and interest rates move in a manner not anticipated by Fund management, it is possible that the Fund could lose all or substantially all of its investment. Additionally, some SMBS entail credit risk. The Fund invests in both Agency and Non-Agency bonds. Many Agency bonds do not have credit risk as they are government guaranteed.
•
Inverse Floaters. Inverse floating rate obligations are obligations that pay interest at rates that vary inversely with changes in market rates of interest. Because the interest rate paid to holders of such obligations is generally determined by subtracting a variable or floating rate from a predetermined amount, the interest rate paid to holders of such obligations will decrease as such variable or floating rate increases and increase as such variable or floating rate decreases.
•
Municipal Obligations. To the extent the Fund invests in municipal obligations issued by state and local governments and their agencies, the Fund may be susceptible to political, economic, regulatory or other factors affecting issuers of these municipal obligations. To the extent that the Fund invests in municipal obligations of issuers in the same economic sector, it could be more sensitive to economic, business or political developments that affect such sector. During certain economic and other conditions, the financial resources of many issuers of municipal securities may be significantly stressed, which could impair any such issuer’s ability to meet its financial obligations when due and adversely impact the value of its securities held by the Fund.
•
Liquidity. The Fund may make investments that are illiquid or restricted or that may become less liquid in response to overall economic conditions or adverse investor perceptions, and which may entail greater risk than investments in other types of securities. These investments may be more difficult to value or sell, particularly in times of market turmoil, and there may be little trading in the secondary market available for particular securities. Liquidity risk may be magnified in a market where credit spread and interest rate volatility is rising and where investor redemptions from fixed-income mutual funds may be higher than normal. If the Fund is forced to sell an illiquid or restricted security to fund redemptions or for other cash needs, it may be forced to sell the security at a loss or for less than its fair value.
•
Derivatives. A derivative instrument often has risks similar to its underlying asset and may have additional risks, including imperfect correlation between the value of the derivative and the underlying asset, risks of default by the counterparty to certain transactions, magnification of losses incurred due to changes in the market value of the securities, instruments, indices or interest rates to which the derivative instrument relates, risks that the transactions may not be liquid, risks arising from margin requirements and risks arising from mispricing or valuation complexity. Certain derivative transactions may give rise to a form of leverage. Leverage magnifies the potential for gain and the risk of loss.
•
Market and Geopolitical Risk. The value of your investment in the Fund is based on the values of the Fund’s investments, which may change due to economic and other events that affect markets generally, as well as those that affect particular regions, countries, industries, companies or governments. These events may be sudden and unexpected, and could adversely affect the liquidity of the Fund’s investments, which may in turn impact valuation, the Fund’s ability to sell securities and/or its ability to meet redemptions. The risks associated with these developments may be magnified if certain social, political, economic and other conditions and events (such as war, natural disasters, epidemics and pandemics, terrorism, conflicts and social unrest) adversely interrupt the global economy and financial markets. It is difficult to predict when events affecting the U.S. or global financial markets may occur, the effects that such events may have and the duration of those effects (which may last for extended periods). These events may negatively impact broad segments of businesses and populations and have a significant and rapid negative
impact on the performance of the Fund’s investments, adversely affect and increase the volatility of the Fund’s share price and exacerbate pre-existing risks to the Fund.
•
Portfolio Turnover. Consistent with its investment policies, the Fund will purchase and sell securities without regard to the effect on portfolio turnover. Higher portfolio turnover will cause the Fund to incur additional transaction costs.
Shares of the Fund are not bank deposits and are not guaranteed or insured by the Federal Deposit Insurance Corporation or any other government agency.
Risk Lose Money [Text] rr_RiskLoseMoney There is no assurance that the Fund will achieve its investment objective, and you can lose money investing in this Fund.
Risk Not Insured Depository Institution [Text] rr_RiskNotInsuredDepositoryInstitution Shares of the Fund are not bank deposits and are not guaranteed or insured by the Federal Deposit Insurance Corporation or any other government agency.
Bar Chart and Performance Table [Heading] rr_BarChartAndPerformanceTableHeading Past Performance
Performance Narrative [Text Block] rr_PerformanceNarrativeTextBlock
The bar chart and table below provide some indication of the risks of investing in the Fund by showing changes in the Fund’s Class A shares’ performance from year-to-year and by showing how the Fund’s average annual returns for the past one, five and 10 year periods and since inception compare with those of a broad measure of market performance, as well as an index that represents a group of similar mutual funds, over time. The performance of the other classes, which is shown in the table below, will differ because the classes have different ongoing fees. The performance information in the bar chart does not reflect the deduction of sales charges; if these amounts were reflected, returns would be less than shown. The Fund’s returns in the table include the maximum applicable sales charge for Class A and Class C and assume you sold your shares at the end of each period (unless otherwise noted). The Fund’s past performance, before and after taxes, is not necessarily an indication of how the Fund will perform in the future. Updated performance information is available online at www.morganstanley.com/im or by calling toll-free 1-800-548-7786.
Performance Information Illustrates Variability of Returns [Text] rr_PerformanceInformationIllustratesVariabilityOfReturns The bar chart and table below provide some indication of the risks of investing in the Fund by showing changes in the Fund’s Class A shares’ performance from year-to-year and by showing how the Fund’s average annual returns for the past one, five and 10 year periods and since inception compare with those of a broad measure of market performance, as well as an index that represents a group of similar mutual funds, over time.
Performance Availability Phone [Text] rr_PerformanceAvailabilityPhone 1-800-548-7786
Performance Availability Website Address [Text] rr_PerformanceAvailabilityWebSiteAddress www.morganstanley.com/im
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—Calendar Years
Bar Chart Does Not Reflect Sales Loads [Text] rr_BarChartDoesNotReflectSalesLoads The performance information in the bar chart does not reflect the deduction of sales charges; if these amounts were reflected, returns would be less than shown.
Performance Table Does Reflect Sales Loads rr_PerformanceTableDoesReflectSalesLoads The Fund’s returns in the table include the maximum applicable sales charge for Class A and Class C and assume you sold your shares at the end of each period (unless otherwise noted).
Performance Table Uses Highest Federal Rate rr_PerformanceTableUsesHighestFederalRate The after-tax returns shown in the table above are calculated using the historical highest individual federal marginal income tax rates during the period shown 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, and after-tax returns are not relevant to investors who hold their Fund shares through tax-deferred arrangements, such as 401(k) plans or individual retirement accounts (“IRAs”).
Performance Table One Class of after Tax Shown [Text] rr_PerformanceTableOneClassOfAfterTaxShown After-tax returns for the Fund’s other classes will vary from the Class A shares’ returns.
Performance Table Explanation after Tax Higher rr_PerformanceTableExplanationAfterTaxHigher After-tax returns may be higher than before-tax returns due to foreign tax credits and/or an assumed benefit from capital losses that would have been realized had Fund shares been sold at the end of the relevant periods, as applicable.
Bar Chart Closing [Text Block] rr_BarChartClosingTextBlock
High Quarter
03/31/20
2.55
Low Quarter
12/31/16
-2.79
Performance Table Heading rr_PerformanceTableHeading Average Annual Total Returns1(for the periods ended December 31, 2021) [1]
Performance Table Closing [Text Block] rr_PerformanceTableClosingTextBlock
The after-tax returns shown in the table above are calculated using the historical highest individual federal marginal income tax rates during the period shown and do not reflect the impact of state and local taxes. After-tax returns for the Fund’s other classes will vary from the Class A shares’ returns. Actual after-tax returns depend on an investor’s tax situation and may differ from those shown, and after-tax returns are not relevant to investors who hold their Fund shares through tax-deferred arrangements, such as 401(k) plans or individual retirement accounts (“IRAs”). After-tax returns may be higher than before-tax returns due to foreign tax credits and/or an assumed benefit from capital losses that would have been realized had Fund shares been sold at the end of the relevant periods, as applicable.
(Morgan Stanley U.S. Government Securities Trust - Classes A, L, I and C) | (Morgan Stanley U.S. Government Securities Trust) | Bloomberg U.S. Government/Mortgage Index (reflects no deduction for fees, expenses or taxes)  
Prospectus: rr_ProspectusTable  
Past 1 Year rr_AverageAnnualReturnYear01 (1.77%) [2]
Past 5 Years rr_AverageAnnualReturnYear05 2.85% [2]
Past 10 Years rr_AverageAnnualReturnYear10 2.21% [2]
Since Inception rr_AverageAnnualReturnSinceInception 4.59% [2],[3]
(Morgan Stanley U.S. Government Securities Trust - Classes A, L, I and C) | (Morgan Stanley U.S. Government Securities Trust) | Lipper General U.S. Government Funds Index (reflects no deduction for taxes)  
Prospectus: rr_ProspectusTable  
Past 1 Year rr_AverageAnnualReturnYear01 (2.18%) [4]
Past 5 Years rr_AverageAnnualReturnYear05 2.89% [4]
Past 10 Years rr_AverageAnnualReturnYear10 2.19% [4]
Since Inception rr_AverageAnnualReturnSinceInception 4.07% [3],[4]
(Morgan Stanley U.S. Government Securities Trust - Classes A, L, I and C) | (Morgan Stanley U.S. Government Securities Trust) | Class A  
Prospectus: rr_ProspectusTable  
Maximum Sales Charge Imposed on Purchases (as a percentage of Offering Price) rr_MaximumSalesChargeImposedOnPurchasesOverOfferingPrice 3.25%
Maximum Deferred Sales Charge (as a percentage) rr_MaximumDeferredSalesChargeOverOther none [5]
Management Fees (as a percentage of Assets) rr_ManagementFeesOverAssets 0.42%
Distribution and Service (12b-1) Fees rr_DistributionAndService12b1FeesOverAssets 0.25%
Other Expenses (as a percentage of Assets): rr_OtherExpensesOverAssets 0.31%
Total Annual Fund Operating Expenses rr_ExpensesOverAssets 0.98% [6]
Fee Waiver or Reimbursement rr_FeeWaiverOrReimbursementOverAssets 0.13% [6]
Total Annual Fund Operating Expenses After Expense Reimbursement rr_NetExpensesOverAssets 0.85% [6]
Expenses Deferred Charges [Text Block] rr_ExpensesDeferredChargesTextBlock
Investments in Class A shares that are not subject to any sales charges at the time of purchase are subject to a contingent deferred sales charge (“CDSC”) of 0.75% that will be imposed if you sell your shares within 12 months after the last day of the month of purchase, except for certain specific circumstances. See “Shareholder Information—Share Class Arrangements” for further information about the CDSC waiver categories.
Expense Example, with Redemption, 1 Year rr_ExpenseExampleYear01 $ 409
Expense Example, with Redemption, 3 Years rr_ExpenseExampleYear03 614
Expense Example, with Redemption, 5 Years rr_ExpenseExampleYear05 837
Expense Example, with Redemption, 10 Years rr_ExpenseExampleYear10 1,476
Expense Example, No Redemption, 1 Year rr_ExpenseExampleNoRedemptionYear01 409
Expense Example, No Redemption, 3 Years rr_ExpenseExampleNoRedemptionYear03 614
Expense Example, No Redemption, 5 Years rr_ExpenseExampleNoRedemptionYear05 837
Expense Example, No Redemption, 10 Years rr_ExpenseExampleNoRedemptionYear10 $ 1,476
Annual Return 2012 rr_AnnualReturn2012 3.86%
Annual Return 2013 rr_AnnualReturn2013 (2.10%)
Annual Return 2014 rr_AnnualReturn2014 5.60%
Annual Return 2015 rr_AnnualReturn2015 0.10%
Annual Return 2016 rr_AnnualReturn2016 2.40%
Annual Return 2017 rr_AnnualReturn2017 2.66%
Annual Return 2018 rr_AnnualReturn2018 (0.01%)
Annual Return 2019 rr_AnnualReturn2019 6.43%
Annual Return 2020 rr_AnnualReturn2020 5.55%
Annual Return 2021 rr_AnnualReturn2021 (2.06%)
Highest Quarterly Return, Label rr_HighestQuarterlyReturnLabel High Quarter
Highest Quarterly Return rr_BarChartHighestQuarterlyReturn 2.55%
Highest Quarterly Return, Date rr_BarChartHighestQuarterlyReturnDate Mar. 31, 2020
Lowest Quarterly Return, Label rr_LowestQuarterlyReturnLabel Low Quarter
Lowest Quarterly Return rr_BarChartLowestQuarterlyReturn (2.79%)
Lowest Quarterly Return, Date rr_BarChartLowestQuarterlyReturnDate Dec. 31, 2016
Past 1 Year rr_AverageAnnualReturnYear01 (5.24%) [7]
Past 5 Years rr_AverageAnnualReturnYear05 1.79% [7]
Past 10 Years rr_AverageAnnualReturnYear10 1.86% [7]
Since Inception rr_AverageAnnualReturnSinceInception 3.50% [7]
(Morgan Stanley U.S. Government Securities Trust - Classes A, L, I and C) | (Morgan Stanley U.S. Government Securities Trust) | Class A | Return After Taxes on Distributions  
Prospectus: rr_ProspectusTable  
Past 1 Year rr_AverageAnnualReturnYear01 (6.11%) [7],[8]
Past 5 Years rr_AverageAnnualReturnYear05 0.66% [7],[8]
Past 10 Years rr_AverageAnnualReturnYear10 0.78% [7],[8]
Since Inception rr_AverageAnnualReturnSinceInception 2.01% [7],[8]
(Morgan Stanley U.S. Government Securities Trust - Classes A, L, I and C) | (Morgan Stanley U.S. Government Securities Trust) | Class A | Return After Taxes on Distributions and Sale of Fund Shares  
Prospectus: rr_ProspectusTable  
Past 1 Year rr_AverageAnnualReturnYear01 (3.10%) [7]
Past 5 Years rr_AverageAnnualReturnYear05 0.88% [7]
Past 10 Years rr_AverageAnnualReturnYear10 0.94% [7]
Since Inception rr_AverageAnnualReturnSinceInception 2.08% [7]
(Morgan Stanley U.S. Government Securities Trust - Classes A, L, I and C) | (Morgan Stanley U.S. Government Securities Trust) | Class L  
Prospectus: rr_ProspectusTable  
Maximum Sales Charge Imposed on Purchases (as a percentage of Offering Price) rr_MaximumSalesChargeImposedOnPurchasesOverOfferingPrice none
Maximum Deferred Sales Charge (as a percentage) rr_MaximumDeferredSalesChargeOverOther none
Management Fees (as a percentage of Assets) rr_ManagementFeesOverAssets 0.42%
Distribution and Service (12b-1) Fees rr_DistributionAndService12b1FeesOverAssets 0.50%
Other Expenses (as a percentage of Assets): rr_OtherExpensesOverAssets 0.33%
Total Annual Fund Operating Expenses rr_ExpensesOverAssets 1.25% [6]
Fee Waiver or Reimbursement rr_FeeWaiverOrReimbursementOverAssets 0.13% [6]
Total Annual Fund Operating Expenses After Expense Reimbursement rr_NetExpensesOverAssets 1.12% [6]
Expense Example, with Redemption, 1 Year rr_ExpenseExampleYear01 $ 114
Expense Example, with Redemption, 3 Years rr_ExpenseExampleYear03 384
Expense Example, with Redemption, 5 Years rr_ExpenseExampleYear05 674
Expense Example, with Redemption, 10 Years rr_ExpenseExampleYear10 1,500
Expense Example, No Redemption, 1 Year rr_ExpenseExampleNoRedemptionYear01 114
Expense Example, No Redemption, 3 Years rr_ExpenseExampleNoRedemptionYear03 384
Expense Example, No Redemption, 5 Years rr_ExpenseExampleNoRedemptionYear05 674
Expense Example, No Redemption, 10 Years rr_ExpenseExampleNoRedemptionYear10 $ 1,500
Past 1 Year rr_AverageAnnualReturnYear01 (2.32%) [7]
Past 5 Years rr_AverageAnnualReturnYear05 2.17% [7]
Past 10 Years rr_AverageAnnualReturnYear10 1.89% [7]
Since Inception rr_AverageAnnualReturnSinceInception 3.22% [7]
(Morgan Stanley U.S. Government Securities Trust - Classes A, L, I and C) | (Morgan Stanley U.S. Government Securities Trust) | Class I  
Prospectus: rr_ProspectusTable  
Maximum Sales Charge Imposed on Purchases (as a percentage of Offering Price) rr_MaximumSalesChargeImposedOnPurchasesOverOfferingPrice none
Maximum Deferred Sales Charge (as a percentage) rr_MaximumDeferredSalesChargeOverOther none
Management Fees (as a percentage of Assets) rr_ManagementFeesOverAssets 0.42%
Distribution and Service (12b-1) Fees rr_DistributionAndService12b1FeesOverAssets none
Other Expenses (as a percentage of Assets): rr_OtherExpensesOverAssets 0.30%
Total Annual Fund Operating Expenses rr_ExpensesOverAssets 0.72% [6]
Fee Waiver or Reimbursement rr_FeeWaiverOrReimbursementOverAssets 0.20% [6]
Total Annual Fund Operating Expenses After Expense Reimbursement rr_NetExpensesOverAssets 0.52% [6]
Expense Example, with Redemption, 1 Year rr_ExpenseExampleYear01 $ 53
Expense Example, with Redemption, 3 Years rr_ExpenseExampleYear03 210
Expense Example, with Redemption, 5 Years rr_ExpenseExampleYear05 381
Expense Example, with Redemption, 10 Years rr_ExpenseExampleYear10 876
Expense Example, No Redemption, 1 Year rr_ExpenseExampleNoRedemptionYear01 53
Expense Example, No Redemption, 3 Years rr_ExpenseExampleNoRedemptionYear03 210
Expense Example, No Redemption, 5 Years rr_ExpenseExampleNoRedemptionYear05 381
Expense Example, No Redemption, 10 Years rr_ExpenseExampleNoRedemptionYear10 $ 876
Past 1 Year rr_AverageAnnualReturnYear01 (1.74%) [7]
Past 5 Years rr_AverageAnnualReturnYear05 2.78% [7]
Past 10 Years rr_AverageAnnualReturnYear10 2.51% [7]
Since Inception rr_AverageAnnualReturnSinceInception 3.92% [7]
(Morgan Stanley U.S. Government Securities Trust - Classes A, L, I and C) | (Morgan Stanley U.S. Government Securities Trust) | Class C  
Prospectus: rr_ProspectusTable  
Maximum Sales Charge Imposed on Purchases (as a percentage of Offering Price) rr_MaximumSalesChargeImposedOnPurchasesOverOfferingPrice none
Maximum Deferred Sales Charge (as a percentage) rr_MaximumDeferredSalesChargeOverOther 1.00% [9]
Management Fees (as a percentage of Assets) rr_ManagementFeesOverAssets 0.42%
Distribution and Service (12b-1) Fees rr_DistributionAndService12b1FeesOverAssets 1.00%
Other Expenses (as a percentage of Assets): rr_OtherExpensesOverAssets 0.31%
Total Annual Fund Operating Expenses rr_ExpensesOverAssets 1.73% [6]
Fee Waiver or Reimbursement rr_FeeWaiverOrReimbursementOverAssets 0.11% [6]
Total Annual Fund Operating Expenses After Expense Reimbursement rr_NetExpensesOverAssets 1.62% [6]
Expenses Deferred Charges [Text Block] rr_ExpensesDeferredChargesTextBlock
The Class C CDSC is only applicable if you sell your shares within one year after purchase. See “Shareholder Information—Share Class Arrangements” for a complete discussion of the CDSC.
Expense Example, with Redemption, 1 Year rr_ExpenseExampleYear01 $ 265
Expense Example, with Redemption, 3 Years rr_ExpenseExampleYear03 534
Expense Example, with Redemption, 5 Years rr_ExpenseExampleYear05 928
Expense Example, with Redemption, 10 Years rr_ExpenseExampleYear10 1,833
Expense Example, No Redemption, 1 Year rr_ExpenseExampleNoRedemptionYear01 165
Expense Example, No Redemption, 3 Years rr_ExpenseExampleNoRedemptionYear03 534
Expense Example, No Redemption, 5 Years rr_ExpenseExampleNoRedemptionYear05 928
Expense Example, No Redemption, 10 Years rr_ExpenseExampleNoRedemptionYear10 $ 1,833
Past 1 Year rr_AverageAnnualReturnYear01 (3.76%) [7]
Past 5 Years rr_AverageAnnualReturnYear05 1.66% [7]
Past 10 Years rr_AverageAnnualReturnYear10 [7]
Since Inception rr_AverageAnnualReturnSinceInception 1.32% [7]
[1]
1 During 2016, the Fund received proceeds related to certain non-recurring litigation settlements. Had these settlements not occurred, the five year, 10 year and since inception (where applicable) returns before and after taxes for such periods would have been lower.
[2]
4 The Bloomberg U.S. Government/Mortgage Index includes Treasuries, government-related issues, and agency mortgage-backed pass-through securities issued by Ginnie Mae (GNMA), Fannie Mae (FNMA), and Freddie Mac (FHLMC). It is not possible to invest directly in an index.
[3]
5 Since inception reflects the inception date of Class A shares.
[4]
6 The Lipper General U.S. Government Funds Index is an equally weighted performance index of the largest qualifying funds (based on net assets) in the Lipper General U.S. Government Funds classification. There are currently 10 funds represented in this Index.
[5]
1
Investments in Class A shares that are not subject to any sales charges at the time of purchase are subject to a contingent deferred sales charge (“CDSC”) of 0.75% that will be imposed if you sell your shares within 12 months after the last day of the month of purchase, except for certain specific circumstances. See “Shareholder Information—Share Class Arrangements” for further information about the CDSC waiver categories.
[6]
3 The Fund’s “Adviser” and “Administrator,” Morgan Stanley Investment Management Inc., has agreed to reduce its advisory fee, its administration fee and/or reimburse the Fund so that Total Annual Fund Operating Expenses, excluding acquired fund fees and expenses (as applicable), certain investment related expenses, taxes, interest and other extraordinary expenses (including litigation), will not exceed 0.85% for Class A, 1.12% for Class L, 0.52% for Class I and 1.62% for Class C. The fee waivers and/or expense reimbursements will continue for at least one year from the date of this Prospectus or until such time as the Fund’s Board of Trustees acts to discontinue all or a portion of such waivers and/or reimbursements when it deems such action is appropriate.
[7]
2 Class A, L and I shares commenced operations on July 28, 1997. Class C shares commenced operations on April 30, 2015.
[8]
3 These returns do not reflect any tax consequences from a sale of your shares at the end of each period, but they do reflect any applicable sales charges on such a sale.
[9]
2
The Class C CDSC is only applicable if you sell your shares within one year after purchase. See “Shareholder Information—Share Class Arrangements” for a complete discussion of the CDSC.