Label	Element	Value
Risk/Return:	rr_RiskReturnAbstract
Registrant Name	dei_EntityRegistrantName	THORNBURG INVESTMENT TRUST
Prospectus Date	rr_ProspectusDate	Feb. 01, 2015
Thornburg Strategic Municipal Income Fund
Risk/Return:	rr_RiskReturnAbstract
Risk/Return [Heading]	rr_RiskReturnHeading	FUND SUMMARY Strategic Municipal Income Fund
Objective [Heading]	rr_ObjectiveHeading	Investment Goal
Objective, Primary [Text Block]	rr_ObjectivePrimaryTextBlock	The Fund seeks a high level of current income exempt from federal individual income tax.
Expense [Heading]	rr_ExpenseHeading	Fees and Expenses of the Fund
Expense Narrative [Text Block]	rr_ExpenseNarrativeTextBlock	This table describes the fees and expenses that you may pay if you buy and hold shares of the Fund. You may qualify for discounts from the sales charges applicable to Class A shares if you or other qualifying account holders invest, or agree to invest in the future, at least $50,000 in the Thornburg Funds. More information about these and other discounts is available from your financial professional and in “Opening Your Account – Buying Fund Shares” on page 82 of this Prospectus.
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)
Fee Waiver or Reimbursement over Assets, Date of Termination	rr_FeeWaiverOrReimbursementOverAssetsDateOfTermination	February 1, 2016
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 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 21.89% of the average value of its portfolio.
Portfolio Turnover, Rate	rr_PortfolioTurnoverRate	21.89%
Expense Breakpoint Discounts [Text]	rr_ExpenseBreakpointDiscounts	You may qualify for discounts from the sales charges applicable to Class A shares if you or other qualifying account holders invest, or agree to invest in the future, at least $50,000 in the Thornburg Funds. More information about these and other discounts is available from your financial professional and in “Opening Your Account – Buying Fund Shares” on page 82 of this Prospectus.
Expense Breakpoint, Minimum Investment Required [Amount]	rr_ExpenseBreakpointMinimumInvestmentRequiredAmount	50,000
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. The Example assumes that you invest $10,000 in the Fund for the time periods indicated and then redeem all of your shares at the end of those periods. The Example also assumes that your investment has a 5% return each year, dividends and distributions are reinvested, and that the Fund’s operating expenses remain the same. Although your actual costs may be higher or lower, based on these assumptions (and giving effect to fee waivers and expense reimbursements in the first year), your costs would be:
Expense Example, No Redemption, By Year, Caption [Text]	rr_ExpenseExampleNoRedemptionByYearCaption	You would pay the following expenses if you did not redeem your Class C shares:
Strategy [Heading]	rr_StrategyHeading	Principal Investment Strategies
Strategy Narrative [Text Block]	rr_StrategyNarrativeTextBlock	Thornburg Investment Management, Inc. (“Thornburg”) actively manages the Fund’s investments in pursuing the Fund’s investment goal. The Fund invests principally in a portfolio of municipal obligations issued by states and state agencies, local governments and their agencies, and by certain United States territories and possessions. Investment decisions are based upon outlooks for interest rates and securities markets, the supply of municipal debt obligations, the difference in yields between higher and lower-rated obligations, and analysis of specific obligations. The Fund invests in obligations and participations in obligations of any credit quality. The Fund may invest up to 50 percent of its portfolio in lower-quality debt obligations rated at the time of purchase as below investment grade (sometimes called “junk” bonds or “high yield” bonds) or, if unrated, issued by obligors with comparable below investment grade obligations outstanding or deemed by Thornburg to be comparable to obligors with outstanding below investment grade obligations. The Fund may also invest in obligations that are in default at the time of purchase. “Participations” are undivided interests in pools of securities where the underlying credit support passes through to the participants. The Fund may invest in municipal obligations of any maturity, but seeks to maintain a portfolio of investments having a dollar-weighted average effective duration of normally one to ten years. Duration is a measure of estimated sensitivity to interest rate changes. A portfolio with a longer average effective duration will typically be more sensitive to interest rate changes than a portfolio with a shorter average effective duration. During temporary periods the Fund’s average effective duration and average portfolio maturity may be reduced for defensive purposes. There is no limitation on the duration or maturity of any specific security the Fund may purchase. The Fund may dispose of any security before it matures. The Fund attempts to reduce changes in its share value through credit analysis, selection and diversification. The Fund normally invests 100% of its assets in municipal obligations, the income from which is exempt from the regular federal income tax. The Fund may invest up to 20% of its assets in taxable securities which would produce income not exempt from federal income tax because of market conditions, pending investment of idle funds or to afford liquidity. The Fund’s temporary taxable investments may exceed 20% of its assets when made for defensive purposes during periods of abnormal market conditions. If the Fund found it necessary to own taxable investments, some of its income would be subject to federal income tax. Gains realized on investments held by the Fund and not offset by realized losses will be subject to federal income tax. The Fund also may invest in derivative instruments such as options, futures contracts, options on futures contracts, and swap agreements (including, but not limited to, credit default swap agreements), to the extent Thornburg believes such investments may assist the Fund in pursuing its investment goal.
Risk [Heading]	rr_RiskHeading	Principal Investment Risks
Risk Narrative [Text Block]	rr_RiskNarrativeTextBlock	An investment in the Fund is not a deposit in any bank and is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency. Accordingly, the loss of money is a risk of investing in the Fund. The value of the Fund’s shares and its dividends may fluctuate from day to day and over time, and when you sell your shares they may be worth less than what you paid for them. The following is a summary of the principal risks of investing in the Fund. Please note that because the Fund’s objective is to provide high current income, the Fund invests with an emphasis on income, rather than stability of net asset value. Management Risk – The Fund is an actively managed portfolio, and the value of the Fund may be reduced if Thornburg pursues unsuccessful investments or fails to correctly identify risks affecting the broad economy or specific issuers in which the Fund invests. Interest Rate Risk – When interest rates increase, the value of the Fund’s investments may decline and the Fund’s share value may be reduced. This effect is typically more pronounced for intermediate and longer-term obligations. When interest rates decrease, the Fund’s dividends may decline. Decreases in market interest rates may also result in prepayments of obligations the Fund acquires, requiring the Fund to reinvest at lower interest rates. Credit Risk – If obligations held by the Fund are downgraded by ratings agencies or go into default, or if legislation or other government action reduces the ability of issuers to pay principal and interest when due, the value of those obligations may decline and the Fund’s share value and the dividends paid by the Fund may be reduced. Because the ability of an issuer of a lower-rated or unrated obligation to pay principal and interest when due is typically less certain than for an issuer of a higher-rated obligation, lower-rated and unrated obligations are generally more vulnerable than higher-rated obligations to default, to ratings downgrades, and to liquidity risk. Municipal leases held by the Fund may be subject to non-appropriation provisions which permit governmental issuers to discontinue payments to the Fund under the leases. High Yield Risk – Debt obligations that are rated below investment grade and unrated obligations of similar credit quality (commonly referred to as “junk” or “high yield” bonds) may have a substantial risk of loss. These obligations are generally considered to be speculative with respect to the issuer’s ability to pay interest and principal when due. These obligations may be subject to greater price volatility than investment grade obligations, and their prices may decline significantly in periods of general economic difficulty or in response to adverse publicity, changes in investor perceptions or other factors. These obligations may also be subject to greater liquidity risk. Market and Economic Risk – The value of the Fund’s investments may decline and its share value may be reduced due to changes in general economic and market conditions. This effect is typically more pronounced for the Fund’s investments in lower-rated and unrated municipal obligations (including particularly “junk” or “high yield” bonds), the value of which may fluctuate more significantly in response to poor economic growth or other changes in market conditions, political, economic and legal developments, and developments affecting specific issuers. Liquidity Risk – Due to a lack of demand in the marketplace or other factors, the Fund may not be able to sell some or all of the obligations promptly, or may only be able to sell obligations at less than desired prices. The market for lower-rated and unrated obligations (including particularly “junk” or “high yield” bonds) may be less liquid than the market for other obligations, making it difficult for the Fund to value its investment in a lower-rated or unrated obligation or to sell the investment in a timely manner or at an acceptable price. Derivatives Risk – The Fund’s investments in derivatives involve the risks associated with the securities or other assets underlying the derivatives, and also may involve risks different or greater than the risks affecting the underlying assets, including the inability or unwillingness of the other party to a derivative to perform its obligations to the Fund, the Fund’s inability or delays in selling or closing positions in derivatives, and difficulties in valuing derivatives. Additional information about Fund investments, investment strategies and risks of investing in the Fund appears below beginning on page 74.
Risk Lose Money [Text]	rr_RiskLoseMoney	Accordingly, the loss of money is a risk of investing in the Fund.
Risk Not Insured Depository Institution [Text]	rr_RiskNotInsuredDepositoryInstitution	An investment in the Fund is not a deposit in any bank and is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency.
Bar Chart and Performance Table [Heading]	rr_BarChartAndPerformanceTableHeading	Past Performance of the Fund
Performance Narrative [Text Block]	rr_PerformanceNarrativeTextBlock	The following information provides some indication of the risks of investing in Strategic Municipal Income Fund by showing how the Fund’s investment results vary from year to year. The bar chart shows how the annual total returns for Class A shares have been different in each full year shown. The average annual total return figures compare Class A, Class C and Class I share performance to the Bank of America Merrill Lynch Municipal Master Index, a broad measure of market performance. Past performance (before and after taxes) is not necessarily an indication of how the Fund will perform in the future. The performance information shown below is as of the calendar year ended December 31, 2014. Updated performance information may be obtained on the Thornburg website at www.thornburg.com or by calling 1-800-847-0200.
Performance Information Illustrates Variability of Returns [Text]	rr_PerformanceInformationIllustratesVariabilityOfReturns	The following information provides some indication of the risks of investing in Strategic Municipal Income Fund by showing how the Fund’s investment results vary from year to year.
Performance Additional Market Index [Text]	rr_PerformanceAdditionalMarketIndex	The average annual total return figures compare Class A, Class C and Class I share performance to the Bank of America Merrill Lynch Municipal Master Index, a broad measure of market performance.
Performance Availability Phone [Text]	rr_PerformanceAvailabilityPhone	1-800-847-0200
Performance Availability Website Address [Text]	rr_PerformanceAvailabilityWebSiteAddress	www.thornburg.com
Performance Past Does Not Indicate Future [Text]	rr_PerformancePastDoesNotIndicateFuture	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 – Class A Shares
Bar Chart Does Not Reflect Sales Loads [Text]	rr_BarChartDoesNotReflectSalesLoads	The sales charge for Class A shares is not reflected in the returns shown in the bar chart above, and the returns would be less if the charge was taken into account.
Bar Chart Closing [Text Block]	rr_BarChartClosingTextBlock	Highest quarterly results for time period shown: 11.53% (quarter ended 9-30-09). Lowest quarterly results for time period shown: -5.38% (quarter ended 12-31-10). The sales charge for Class A shares is not reflected in the returns shown in the bar chart above, and the returns would be less if the charge was taken into account.
Performance Table Heading	rr_PerformanceTableHeading	Average Annual Total Returns (periods ended 12-31-14)
Performance Table Uses Highest Federal Rate	rr_PerformanceTableUsesHighestFederalRate	After-tax returns are calculated using the historical highest individual federal marginal income tax rates, and do not reflect state or local income taxes.
Performance Table Not Relevant to Tax Deferred	rr_PerformanceTableNotRelevantToTaxDeferred	Actual after-tax returns depend on an investor’s own tax situation and may differ from the returns shown, and 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.
Performance Table One Class of after Tax Shown [Text]	rr_PerformanceTableOneClassOfAfterTaxShown	The after-tax returns shown relate only to Class A shares, and after-tax returns will vary for Class C and Class I shares because the returns of the classes are different.
Performance Table Narrative	rr_PerformanceTableNarrativeTextBlock	After-tax returns are calculated using the historical highest individual federal marginal income tax rates, and do not reflect state or local income taxes. Actual after-tax returns depend on an investor’s own tax situation and may differ from the returns shown, and 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 after-tax returns shown relate only to Class A shares, and after-tax returns will vary for Class C and Class I shares because the returns of the classes are different.
Thornburg Strategic Municipal Income Fund | Class A
Risk/Return:	rr_RiskReturnAbstract
Maximum Sales Charge (Load) Imposed on Purchases (as a percentage of offering price)	rr_MaximumSalesChargeImposedOnPurchasesOverOfferingPrice	2.00%
Maximum Deferred Sales Charge (Load) (as a percentage of redemption proceeds or original purchase price, whichever is lower)	rr_MaximumDeferredSalesChargeOverOther	none	[1]
Management Fees	rr_ManagementFeesOverAssets	0.75%
Distribution and Service (12b-1) Fees	rr_DistributionAndService12b1FeesOverAssets	0.25%
Other Expenses	rr_OtherExpensesOverAssets	0.31%	[2]
Total Annual Fund Operating Expenses	rr_ExpensesOverAssets	1.31%
Fee Waiver/Expense Reimbursement	rr_FeeWaiverOrReimbursementOverAssets	(0.06%)	[3]
Total Annual Fund Operating Expenses After Fee Waiver/Expense Reimbursement	rr_NetExpensesOverAssets	1.25%
Expenses Deferred Charges [Text Block]	rr_ExpensesDeferredChargesTextBlock	A 0.50% contingent deferred sales charge (CDSC) is imposed on redemptions of any part or all of a purchase of $1 million or more within 12 months of purchase.
1 Year	rr_ExpenseExampleYear01	325
3 Years	rr_ExpenseExampleYear03	601
5 Years	rr_ExpenseExampleYear05	898
10 Years	rr_ExpenseExampleYear10	1,742
2010	rr_AnnualReturn2010	3.06%
2011	rr_AnnualReturn2011	12.40%
2012	rr_AnnualReturn2012	9.76%
2013	rr_AnnualReturn2013	(2.80%)
2014	rr_AnnualReturn2014	9.86%
Highest Quarterly Return, Label	rr_HighestQuarterlyReturnLabel	Highest quarterly results
Highest Quarterly Return, Date	rr_BarChartHighestQuarterlyReturnDate	Sep. 30, 2009
Highest Quarterly Return	rr_BarChartHighestQuarterlyReturn	11.53%
Lowest Quarterly Return, Label	rr_LowestQuarterlyReturnLabel	Lowest quarterly results
Lowest Quarterly Return, Date	rr_BarChartLowestQuarterlyReturnDate	Dec. 31, 2010
Lowest Quarterly Return	rr_BarChartLowestQuarterlyReturn	(5.38%)
1 Year	rr_AverageAnnualReturnYear01	7.67%
5 Years	rr_AverageAnnualReturnYear05	5.87%
Since Inception	rr_AverageAnnualReturnSinceInception	8.14%
Inception Date	rr_AverageAnnualReturnInceptionDate	Apr. 01, 2009
Thornburg Strategic Municipal Income Fund | Class C
Risk/Return:	rr_RiskReturnAbstract
Maximum Sales Charge (Load) Imposed on Purchases (as a percentage of offering price)	rr_MaximumSalesChargeImposedOnPurchasesOverOfferingPrice	none
Maximum Deferred Sales Charge (Load) (as a percentage of redemption proceeds or original purchase price, whichever is lower)	rr_MaximumDeferredSalesChargeOverOther	0.60%	[4]
Management Fees	rr_ManagementFeesOverAssets	0.75%
Distribution and Service (12b-1) Fees	rr_DistributionAndService12b1FeesOverAssets	0.60%
Other Expenses	rr_OtherExpensesOverAssets	0.37%	[2]
Total Annual Fund Operating Expenses	rr_ExpensesOverAssets	1.72%
Fee Waiver/Expense Reimbursement	rr_FeeWaiverOrReimbursementOverAssets	(0.17%)	[3]
Total Annual Fund Operating Expenses After Fee Waiver/Expense Reimbursement	rr_NetExpensesOverAssets	1.55%
Expenses Deferred Charges [Text Block]	rr_ExpensesDeferredChargesTextBlock	Imposed only on redemptions of Class C shares within 12 months of purchase.
1 Year	rr_ExpenseExampleYear01	218
3 Years	rr_ExpenseExampleYear03	525
5 Years	rr_ExpenseExampleYear05	917
10 Years	rr_ExpenseExampleYear10	2,016
1 Year	rr_ExpenseExampleNoRedemptionYear01	158
3 Years	rr_ExpenseExampleNoRedemptionYear03	525
5 Years	rr_ExpenseExampleNoRedemptionYear05	917
10 Years	rr_ExpenseExampleNoRedemptionYear10	2,016
1 Year	rr_AverageAnnualReturnYear01	9.00%
5 Years	rr_AverageAnnualReturnYear05	6.00%
Since Inception	rr_AverageAnnualReturnSinceInception	8.22%
Inception Date	rr_AverageAnnualReturnInceptionDate	Apr. 01, 2009
Thornburg Strategic Municipal Income Fund | Class I
Risk/Return:	rr_RiskReturnAbstract
Maximum Sales Charge (Load) Imposed on Purchases (as a percentage of offering price)	rr_MaximumSalesChargeImposedOnPurchasesOverOfferingPrice	none
Maximum Deferred Sales Charge (Load) (as a percentage of redemption proceeds or original purchase price, whichever is lower)	rr_MaximumDeferredSalesChargeOverOther	none
Management Fees	rr_ManagementFeesOverAssets	0.75%
Distribution and Service (12b-1) Fees	rr_DistributionAndService12b1FeesOverAssets	none
Other Expenses	rr_OtherExpensesOverAssets	0.18%	[2]
Total Annual Fund Operating Expenses	rr_ExpensesOverAssets	0.93%
Fee Waiver/Expense Reimbursement	rr_FeeWaiverOrReimbursementOverAssets
Total Annual Fund Operating Expenses After Fee Waiver/Expense Reimbursement	rr_NetExpensesOverAssets	0.93%
1 Year	rr_ExpenseExampleYear01	95
3 Years	rr_ExpenseExampleYear03	296
5 Years	rr_ExpenseExampleYear05	515
10 Years	rr_ExpenseExampleYear10	1,143
1 Year	rr_AverageAnnualReturnYear01	10.21%
5 Years	rr_AverageAnnualReturnYear05	6.61%
Since Inception	rr_AverageAnnualReturnSinceInception	8.84%
Inception Date	rr_AverageAnnualReturnInceptionDate	Apr. 01, 2009
Thornburg Strategic Municipal Income Fund | Return After Taxes on Distributions | Class A
Risk/Return:	rr_RiskReturnAbstract
1 Year	rr_AverageAnnualReturnYear01	7.64%
5 Years	rr_AverageAnnualReturnYear05	5.82%
Since Inception	rr_AverageAnnualReturnSinceInception	8.04%
Inception Date	rr_AverageAnnualReturnInceptionDate	Apr. 01, 2009
Thornburg Strategic Municipal Income Fund | Return After Taxes on Distributions and Sale of Fund Shares | Class A
Risk/Return:	rr_RiskReturnAbstract
1 Year	rr_AverageAnnualReturnYear01	5.53%
5 Years	rr_AverageAnnualReturnYear05	5.35%
Since Inception	rr_AverageAnnualReturnSinceInception	7.23%
Inception Date	rr_AverageAnnualReturnInceptionDate	Apr. 01, 2009
Thornburg Strategic Municipal Income Fund | BofA Merrill Lynch Master Index (reflects no deduction for fees, expenses, or taxes) | Class A
Risk/Return:	rr_RiskReturnAbstract
1 Year	rr_AverageAnnualReturnYear01	9.78%
5 Years	rr_AverageAnnualReturnYear05	5.39%
Since Inception	rr_AverageAnnualReturnSinceInception	6.32%
Inception Date	rr_AverageAnnualReturnInceptionDate	Apr. 01, 2009
Thornburg Strategic Municipal Income Fund | BofA Merrill Lynch Master Index (reflects no deduction for fees, expenses, or taxes) | Class C
Risk/Return:	rr_RiskReturnAbstract
1 Year	rr_AverageAnnualReturnYear01	9.78%
5 Years	rr_AverageAnnualReturnYear05	5.39%
Since Inception	rr_AverageAnnualReturnSinceInception	6.32%
Inception Date	rr_AverageAnnualReturnInceptionDate	Apr. 01, 2009
Thornburg Strategic Municipal Income Fund | BofA Merrill Lynch Master Index (reflects no deduction for fees, expenses, or taxes) | Class I
Risk/Return:	rr_RiskReturnAbstract
1 Year	rr_AverageAnnualReturnYear01	9.78%
5 Years	rr_AverageAnnualReturnYear05	5.39%
Since Inception	rr_AverageAnnualReturnSinceInception	6.32%
Inception Date	rr_AverageAnnualReturnInceptionDate	Apr. 01, 2009

[1] A 0.50% contingent deferred sales charge (CDSC) is imposed on redemptions of any part or all of a purchase of $1 million or more within 12 months of purchase.

[2] Other expenses include "Acquired Fund Fees and Expenses," which are fees and expenses incurred indirectly by the Fund through the Fund's investment in other investment companies. Those expenses were less than 0.01% for the fiscal year ended September 30, 2014.

[3] Thornburg Investment Management, Inc. ("Thornburg") and/or Thornburg Securities Corporation ("TSC") have contractually agreed to waive fees and reimburse expenses incurred by the Fund so that actual Class A and Class C expenses do not exceed 1.25% and 1.55%, respectively, not including the effects of Acquired Fund Fees and Expenses. The agreement to waive fees and reimburse expenses may be terminated by the Fund at any time, but may not be terminated by Thornburg or TSC before February 1, 2016, unless Thornburg or TSC ceases to be the investment advisor or distributor of the Fund prior to that date. Thornburg and TSC retain the ability to be repaid by the Fund for fee waivers and expense reimbursements during a fiscal year if Fund expenses fall below the relevant percentage threshold before the end of that fiscal year.

[4] Imposed only on redemptions of Class C shares within 12 months of purchase.