Label

Element

Value

Risk/Return:

rr_RiskReturnAbstract

Registrant Name

dei_EntityRegistrantName

THORNBURG INVESTMENT TRUST

Prospectus Date

rr_ProspectusDate

Feb. 01, 2015

Thornburg International Growth Fund | R3 R4 R5 R6

Risk/Return:

rr_RiskReturnAbstract

Risk/Return [Heading]

rr_RiskReturnHeading

FUND SUMMARY

International Growth Fund

Objective [Heading]

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Investment Goal

Objective, Primary [Text Block]

rr_ObjectivePrimaryTextBlock

The Fund seeks long-term growth of capital by investing in equity securities selected for their growth potential.

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.

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 106.18% of the average value of its portfolio.

Portfolio Turnover, Rate

rr_PortfolioTurnoverRate

106.18%

Expense Example [Heading]

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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:

Strategy [Heading]

rr_StrategyHeading

Principal Investment Strategies

Strategy Narrative [Text Block]

rr_StrategyNarrativeTextBlock

The Fund expects to invest primarily in equity securities from issuers around the world (primarily common stocks) selected for their growth potential and, under normal market conditions, invests at least 75% of its assets in foreign securities or depository receipts of foreign securities. However, the Fund may own a variety of securities, including domestic equity securities and partnership interests. The Fund may invest in developing countries and in smaller companies with market capitalizations of less than $500 million.

The Fund’s investment advisor, Thornburg Investment Management, Inc. (“Thornburg”) intends to invest in companies that it believes will have growing revenues and earnings. The Fund can invest in companies of any size, from larger, well-established companies to smaller, emerging growth companies.

Thornburg primarily uses individual issuer and industry analysis to make investment decisions. Among the specific factors considered by Thornburg in identifying securities for inclusion in the Fund are:

■	earnings growth potential	■	price/revenue ratio
■	business model	■	PE/growth rate ratio
■	industry growth potential	■	price/cash flow ratio
■	industry leadership	■	enterprise value/EBITDA (earnings before interest, taxes, depreciation and amortization) ratio
■	asset appreciation potential	■	management strength
■	potential size of business	■	debt/capital ratio
■	value based on earnings growth discount model
■	price/earnings ratio

The Fund typically makes equity investments in the following three types of companies:

Growth Industry Leaders: Companies in this category often have leadership positions in growing markets. In some cases these companies may have dominant market share. These companies tend to be larger and more established.

Consistent Growers: Companies in this category generally exhibit steady earnings or revenue growth, or both. These companies often have subscription or other recurring revenue profiles. Given their business models, these companies may outperform in weak markets.

Emerging Growth Companies: Companies often addressing a new market or carving out a niche in an existing market. Companies in this category may experience rapid growth, and tend to be smaller, earlier stage companies. Thornburg expects that companies in this category should generate high returns over time, but with higher volatility.

In conjunction with individual issuer analysis, Thornburg may identify economic sectors it expects to experience growth. At times this approach may produce a focus on certain industries, such as technology, financial services, healthcare or biotechnology. The exposure to particular economic sectors or industries likely will vary over time. Investment decisions are also based on domestic and international economic developments, outlooks for securities markets, interest rates and inflation, and the supply and demand for debt and equity securities.

Debt obligations, usually with associated equity features, occasionally will be considered for investment when Thornburg believes them to be more attractive than equity alternatives. The Fund may purchase debt obligations of any maturity and of any credit quality, including “high yield” or “junk” bonds. There is no minimum credit quality or rating of debt obligation the Fund may purchase.

Risk [Heading]

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Principal Investment Risks

Risk Narrative [Text Block]

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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 governmental agency. Accordingly, the loss of money is a risk of investing in the Fund. The value of the Fund’s shares varies 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.

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.

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. The value of a security may change in response to developments affecting entire economies, markets or industries, including changes in interest rates, political and legal developments, and general market volatility.

Risks Affecting Specific Issuers – The value of an equity security or debt obligation may decline in response to developments affecting the specific issuer of the security or obligation, even if the overall industry or economy is unaffected. These developments may include a variety of factors, including but not limited to management issues or other corporate disruption, a decline in revenues or profitability, an increase in costs, or an adverse effect on the issuer’s competitive position.

Foreign Investment Risk – Investments in securities of foreign issuers may involve risks including adverse fluctuations in currency exchange rates, political instability, confiscations, taxes or restrictions on currency exchange, difficulty in selling foreign investments, and reduced legal protection.

Developing Country Risk – The risks which may affect investments in foreign issuers (see “Foreign Investment Risk,” above) may be more pronounced for investments in developing countries because the economies of those countries are usually less diversified, communications, transportation and economic infrastructures are less developed, and developing countries ordinarily have less established legal, political, business and social frameworks. At times the prices of equity securities or debt obligations of a developing country issuer may be extremely volatile. An issuer domiciled in a developed country may be similarly affected by these developing country risks to the extent that the issuer conducts a significant percentage of its business in developing countries.

Small and Mid-Cap Company Risk – Investments in small-capitalization companies and mid-capitalization companies may involve additional risks, which may be relatively higher with smaller companies. These additional risks may result from limited product lines, more limited access to markets and financial resources, greater vulnerability to competition and changes in markets, lack of management depth, increased volatility in share price, and possible difficulties in valuing or selling these investments.

Credit Risk – If debt obligations held by the Fund are downgraded by ratings agencies or go into default, or if management action, legislation or other government action reduces the ability of issuers to pay principal and interest when due, the value of those debt obligations may decline and the Fund’s share value and any dividends paid by the Fund may be reduced. Because the ability of an issuer of a lower-rated or unrated debt obligation (including particularly “junk” or “high yield” bonds) to pay principal and interest when due is typically less certain than for an issuer of a higher-rated debt obligation, lower-rated and unrated debt obligations are generally more vulnerable than higher-rated debt obligations to default, to ratings downgrades, and to liquidity risk.

Interest Rate Risk – When interest rates increase, the value of the Fund’s investments in debt obligations may decline and the Fund’s share value may be reduced. This effect is typically more pronounced for intermediate and longer-term debt obligations. Decreases in market interest rates may result in prepayments of debt obligations the Fund acquires, requiring the Fund to reinvest at lower interest rates.

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 investments promptly, or may only be able to sell investments at less than desired prices. This risk may be more pronounced for the Fund’s investments in developing countries.

Additional information about Fund investments, investment strategies, and risks of investing in the Fund appears below beginning on page 38.

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 governmental agency.

Bar Chart and Performance Table [Heading]

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Past Performance of the Fund

Performance Narrative [Text Block]

rr_PerformanceNarrativeTextBlock

The following information provides some indication of the risks of investing in International Growth 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 R3 shares vary in each full year shown. The average annual total return figures compare Class R3, Class R4, Class R5 and Class R6 share performance to the Morgan Stanley Capital International (MSCI) All Country (AC) World ex-U.S. Growth Index, a market capitalization weighted index which includes growth companies in developed and emerging markets throughout the world, excluding the United States. 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 International Growth 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 R3, Class R4, Class R5 and Class R6 share performance to the Morgan Stanley Capital International (MSCI) All Country (AC) World ex-U.S. Growth Index, a market capitalization weighted index which includes growth companies in developed and emerging markets throughout the world, excluding the United States.

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]

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Annual Total Returns – Class R3 Shares

Bar Chart Closing [Text Block]

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Highest quarterly results for time period shown: 28.18%
(quarter ended 6-30-09).

Lowest quarterly results for time period shown: -12.87%
(quarter ended 9-30-11).

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 highest historical 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 Narrative

rr_PerformanceTableNarrativeTextBlock

After-tax returns are calculated using the highest historical 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.

Thornburg International Growth Fund | R3 R4 R5 R6 | Class R3

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

0.88%

Distribution and Service (12b-1) Fees

rr_DistributionAndService12b1FeesOverAssets

0.50%

Other Expenses

rr_OtherExpensesOverAssets

0.48%

Total Annual Fund Operating Expenses

rr_ExpensesOverAssets

1.86%

Fee Waiver/Expense Reimbursement

rr_FeeWaiverOrReimbursementOverAssets

(0.36%)

^[1]

Total Annual Fund Operating Expenses After Fee Waiver/Expense Reimbursement

rr_NetExpensesOverAssets

1.50%

1 Year

rr_ExpenseExampleYear01

153

3 Years

rr_ExpenseExampleYear03

550

5 Years

rr_ExpenseExampleYear05

972

10 Years

rr_ExpenseExampleYear10

2,150

2009

rr_AnnualReturn2009

44.98%

2010

rr_AnnualReturn2010

27.46%

2011

rr_AnnualReturn2011

(3.05%)

2012

rr_AnnualReturn2012

21.88%

2013

rr_AnnualReturn2013

36.21%

2014

rr_AnnualReturn2014

(12.56%)

Highest Quarterly Return, Label

rr_HighestQuarterlyReturnLabel

Highest quarterly results

Highest Quarterly Return, Date

rr_BarChartHighestQuarterlyReturnDate

Jun. 30, 2009

Highest Quarterly Return

rr_BarChartHighestQuarterlyReturn

28.18%

Lowest Quarterly Return, Label

rr_LowestQuarterlyReturnLabel

Lowest quarterly results

Lowest Quarterly Return, Date

rr_BarChartLowestQuarterlyReturnDate

Sep. 30, 2011

Lowest Quarterly Return

rr_BarChartLowestQuarterlyReturn

(12.87%)

1 Year

rr_AverageAnnualReturnYear01

(12.56%)

5 Years

rr_AverageAnnualReturnYear05

12.40%

Since Inception

rr_AverageAnnualReturnSinceInception

5.38%

Inception Date

rr_AverageAnnualReturnInceptionDate

Feb. 01, 2008

Thornburg International Growth Fund | R3 R4 R5 R6 | Class R4

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

0.88%

Distribution and Service (12b-1) Fees

rr_DistributionAndService12b1FeesOverAssets

0.25%

Other Expenses

rr_OtherExpensesOverAssets

0.50%

Total Annual Fund Operating Expenses

rr_ExpensesOverAssets

1.63%

Fee Waiver/Expense Reimbursement

rr_FeeWaiverOrReimbursementOverAssets

(0.23%)

^[1]

Total Annual Fund Operating Expenses After Fee Waiver/Expense Reimbursement

rr_NetExpensesOverAssets

1.40%

1 Year

rr_ExpenseExampleYear01

143

3 Years

rr_ExpenseExampleYear03

492

5 Years

rr_ExpenseExampleYear05

865

10 Years

rr_ExpenseExampleYear10

1,914

1 Year

rr_AverageAnnualReturnYear01

(12.53%)

5 Years

rr_AverageAnnualReturnYear05

12.53%

Since Inception

rr_AverageAnnualReturnSinceInception

5.48%

Inception Date

rr_AverageAnnualReturnInceptionDate

Feb. 01, 2008

Thornburg International Growth Fund | R3 R4 R5 R6 | Class R5

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

0.88%

Distribution and Service (12b-1) Fees

rr_DistributionAndService12b1FeesOverAssets

none

Other Expenses

rr_OtherExpensesOverAssets

0.30%

Total Annual Fund Operating Expenses

rr_ExpensesOverAssets

1.18%

Fee Waiver/Expense Reimbursement

rr_FeeWaiverOrReimbursementOverAssets

(0.19%)

^[1]

Total Annual Fund Operating Expenses After Fee Waiver/Expense Reimbursement

rr_NetExpensesOverAssets

0.99%

1 Year

rr_ExpenseExampleYear01

101

3 Years

rr_ExpenseExampleYear03

356

5 Years

rr_ExpenseExampleYear05

631

10 Years

rr_ExpenseExampleYear10

1,415

1 Year

rr_AverageAnnualReturnYear01

(12.14%)

5 Years

rr_AverageAnnualReturnYear05

12.97%

Since Inception

rr_AverageAnnualReturnSinceInception

5.90%

Inception Date

rr_AverageAnnualReturnInceptionDate

Feb. 01, 2008

Thornburg International Growth Fund | R3 R4 R5 R6 | Class R6

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

0.88%

Distribution and Service (12b-1) Fees

rr_DistributionAndService12b1FeesOverAssets

none

Other Expenses

rr_OtherExpensesOverAssets

0.46%

Total Annual Fund Operating Expenses

rr_ExpensesOverAssets

1.34%

Fee Waiver/Expense Reimbursement

rr_FeeWaiverOrReimbursementOverAssets

(0.45%)

^[1]

Total Annual Fund Operating Expenses After Fee Waiver/Expense Reimbursement

rr_NetExpensesOverAssets

0.89%

1 Year

rr_ExpenseExampleYear01

91

3 Years

rr_ExpenseExampleYear03

380

5 Years

rr_ExpenseExampleYear05

691

10 Years

rr_ExpenseExampleYear10

1,573

1 Year

rr_AverageAnnualReturnYear01

(12.05%)

Since Inception

rr_AverageAnnualReturnSinceInception

7.36%

Inception Date

rr_AverageAnnualReturnInceptionDate

Feb. 01, 2013

Thornburg International Growth Fund | R3 R4 R5 R6 | Return After Taxes on Distributions | Class R3

Risk/Return:

rr_RiskReturnAbstract

1 Year

rr_AverageAnnualReturnYear01

(13.62%)

5 Years

rr_AverageAnnualReturnYear05

11.91%

Since Inception

rr_AverageAnnualReturnSinceInception

4.89%

Inception Date

rr_AverageAnnualReturnInceptionDate

Feb. 01, 2008

Thornburg International Growth Fund | R3 R4 R5 R6 | Return After Taxes on Distributions and Sale of Fund Shares | Class R3

Risk/Return:

rr_RiskReturnAbstract

1 Year

rr_AverageAnnualReturnYear01

(6.21%)

5 Years

rr_AverageAnnualReturnYear05

9.88%

Since Inception

rr_AverageAnnualReturnSinceInception

4.11%

Inception Date

rr_AverageAnnualReturnInceptionDate

Feb. 01, 2008

Thornburg International Growth Fund | R3 R4 R5 R6 | MSCI AC World ex-U.S. Growth Index (reflects no deduction for fees, expenses, or taxes) | Class R3

Risk/Return:

rr_RiskReturnAbstract

1 Year

rr_AverageAnnualReturnYear01

(2.65%)

5 Years

rr_AverageAnnualReturnYear05

5.19%

Since Inception

rr_AverageAnnualReturnSinceInception

1.13%

Inception Date

rr_AverageAnnualReturnInceptionDate

Feb. 01, 2008

Thornburg International Growth Fund | R3 R4 R5 R6 | MSCI AC World ex-U.S. Growth Index (reflects no deduction for fees, expenses, or taxes) | Class R4

Risk/Return:

rr_RiskReturnAbstract

1 Year

rr_AverageAnnualReturnYear01

(2.65%)

5 Years

rr_AverageAnnualReturnYear05

5.19%

Since Inception

rr_AverageAnnualReturnSinceInception

1.13%

Inception Date

rr_AverageAnnualReturnInceptionDate

Feb. 01, 2008

Thornburg International Growth Fund | R3 R4 R5 R6 | MSCI AC World ex-U.S. Growth Index (reflects no deduction for fees, expenses, or taxes) | Class R5

Risk/Return:

rr_RiskReturnAbstract

1 Year

rr_AverageAnnualReturnYear01

(2.65%)

5 Years

rr_AverageAnnualReturnYear05

5.19%

Since Inception

rr_AverageAnnualReturnSinceInception

1.13%

Inception Date

rr_AverageAnnualReturnInceptionDate

Feb. 01, 2008

Thornburg International Growth Fund | R3 R4 R5 R6 | MSCI AC World ex-U.S. Growth Index (reflects no deduction for fees, expenses, or taxes) | Class R6

Risk/Return:

rr_RiskReturnAbstract

1 Year

rr_AverageAnnualReturnYear01

(2.65%)

Since Inception

rr_AverageAnnualReturnSinceInception

4.38%

Inception Date

rr_AverageAnnualReturnInceptionDate

Feb. 01, 2013

[1] 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 R3, Class R4, Class R5 and Class R6 expenses do not exceed 1.50%, 1.40%, 0.99%, and 0.89%, respectively. 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.