Label

Element

Value

Risk/Return:

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Registrant Name

dei_EntityRegistrantName

THORNBURG INVESTMENT TRUST

Prospectus Date

rr_ProspectusDate

Feb. 01, 2015

Thornburg Core Growth Fund | A C I

Risk/Return:

rr_RiskReturnAbstract

Risk/Return [Heading]

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FUND SUMMARY

Growth Fund

Objective [Heading]

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Investment Goal

Objective, Primary [Text Block]

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

Expense [Heading]

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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 100.62% of the average value of its portfolio.

Portfolio Turnover, Rate

rr_PortfolioTurnoverRate

100.62%

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]

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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]

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The Fund expects to invest primarily in domestic equity securities (primarily common stocks) selected for their growth potential. However, the Fund may own a variety of securities, including foreign equity securities and partnership interests. The Fund may invest in developing countries.

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, health-care 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]

rr_RiskHeading

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.

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.

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. These risks may be more pronounced for investments in developing countries.

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.

Additional information about Fund investments, investment strategies, and risks of investing in the Fund appears below beginning on page 74.

Risk Lose Money [Text]

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Accordingly, the loss of money is a risk of investing in the Fund.

Risk Not Insured Depository Institution [Text]

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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.

Bar Chart and Performance Table [Heading]

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Past Performance of the Fund

Performance Narrative [Text Block]

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The following information provides some indication of the risks of investing in 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 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 Russell 3000 Growth 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]

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The following information provides some indication of the risks of investing in Growth Fund by showing how the Fund’s investment results vary from year to year.

Performance Additional Market Index [Text]

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The average annual total return figures compare Class A, Class C and Class I share performance to the Russell 3000 Growth 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]

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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 A Shares

Bar Chart Does Not Reflect Sales Loads [Text]

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The sales charge for Class A shares is not reflected in the returns shown in the bar chart, and the returns would be less if the charge was taken into account.

Bar Chart Closing [Text Block]

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Highest quarterly results for time period shown: 24.12%
(quarter ended 6-30-09).

Lowest quarterly results for time period shown: -26.95%
(quarter ended 12-31-08).

The sales charge for Class A shares is not reflected in the returns shown in the bar chart, and the returns would be less if the charge was taken into account.

Performance Table Heading

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Average Annual Total Returns (periods ended 12-31-14)

Performance Table Uses Highest Federal Rate

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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 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

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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. 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 Core Growth Fund | A C I | Class A

Risk/Return:

rr_RiskReturnAbstract

Maximum Sales Charge (Load) Imposed on Purchases (as a percentage of offering price)

rr_MaximumSalesChargeImposedOnPurchasesOverOfferingPrice

4.50%

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

0.86%

Distribution and Service (12b-1) Fees

rr_DistributionAndService12b1FeesOverAssets

0.25%

Other Expenses

rr_OtherExpensesOverAssets

0.29%

Total Annual Fund Operating Expenses

rr_ExpensesOverAssets

1.40%

Fee Waiver/Expense Reimbursement

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Total Annual Fund Operating Expenses After Fee Waiver/Expense Reimbursement

rr_NetExpensesOverAssets

1.40%

Expenses Deferred Charges [Text Block]

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A 1.00% 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

586

3 Years

rr_ExpenseExampleYear03

873

5 Years

rr_ExpenseExampleYear05

1,181

10 Years

rr_ExpenseExampleYear10

2,054

2005

rr_AnnualReturn2005

22.09%

2006

rr_AnnualReturn2006

18.11%

2007

rr_AnnualReturn2007

11.42%

2008

rr_AnnualReturn2008

(50.95%)

2009

rr_AnnualReturn2009

45.39%

2010

rr_AnnualReturn2010

9.73%

2011

rr_AnnualReturn2011

1.35%

2012

rr_AnnualReturn2012

21.42%

2013

rr_AnnualReturn2013

42.59%

2014

rr_AnnualReturn2014

0.07%

Highest Quarterly Return, Label

rr_HighestQuarterlyReturnLabel

Highest quarterly results

Highest Quarterly Return, Date

rr_BarChartHighestQuarterlyReturnDate

Jun. 30, 2009

Highest Quarterly Return

rr_BarChartHighestQuarterlyReturn

24.12%

Lowest Quarterly Return, Label

rr_LowestQuarterlyReturnLabel

Lowest quarterly results

Lowest Quarterly Return, Date

rr_BarChartLowestQuarterlyReturnDate

Dec. 31, 2008

Lowest Quarterly Return

rr_BarChartLowestQuarterlyReturn

(26.95%)

1 Year

rr_AverageAnnualReturnYear01

(4.44%)

5 Years

rr_AverageAnnualReturnYear05

12.97%

10 Years

rr_AverageAnnualReturnYear10

7.75%

Thornburg Core Growth Fund | A C I | 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

1.00%

^[2]

Management Fees

rr_ManagementFeesOverAssets

0.86%

Distribution and Service (12b-1) Fees

rr_DistributionAndService12b1FeesOverAssets

1.00%

Other Expenses

rr_OtherExpensesOverAssets

0.28%

Total Annual Fund Operating Expenses

rr_ExpensesOverAssets

2.14%

Fee Waiver/Expense Reimbursement

rr_FeeWaiverOrReimbursementOverAssets

Total Annual Fund Operating Expenses After Fee Waiver/Expense Reimbursement

rr_NetExpensesOverAssets

2.14%

Expenses Deferred Charges [Text Block]

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

1 Year

rr_ExpenseExampleYear01

317

3 Years

rr_ExpenseExampleYear03

670

5 Years

rr_ExpenseExampleYear05

1,149

10 Years

rr_ExpenseExampleYear10

2,472

1 Year

rr_ExpenseExampleNoRedemptionYear01

217

3 Years

rr_ExpenseExampleNoRedemptionYear03

670

5 Years

rr_ExpenseExampleNoRedemptionYear05

1,149

10 Years

rr_ExpenseExampleNoRedemptionYear10

2,472

1 Year

rr_AverageAnnualReturnYear01

(1.64%)

5 Years

rr_AverageAnnualReturnYear05

13.17%

10 Years

rr_AverageAnnualReturnYear10

7.43%

Thornburg Core Growth Fund | A C I | 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

0.86%

Distribution and Service (12b-1) Fees

rr_DistributionAndService12b1FeesOverAssets

none

Other Expenses

rr_OtherExpensesOverAssets

0.17%

Total Annual Fund Operating Expenses

rr_ExpensesOverAssets

1.03%

Fee Waiver/Expense Reimbursement

rr_FeeWaiverOrReimbursementOverAssets

(0.04%)

^[3]

Total Annual Fund Operating Expenses After Fee Waiver/Expense Reimbursement

rr_NetExpensesOverAssets

0.99%

1 Year

rr_ExpenseExampleYear01

101

3 Years

rr_ExpenseExampleYear03

324

5 Years

rr_ExpenseExampleYear05

565

10 Years

rr_ExpenseExampleYear10

1,256

1 Year

rr_AverageAnnualReturnYear01

0.52%

5 Years

rr_AverageAnnualReturnYear05

14.54%

10 Years

rr_AverageAnnualReturnYear10

8.76%

Thornburg Core Growth Fund | A C I | Return After Taxes on Distributions | Class A

Risk/Return:

rr_RiskReturnAbstract

1 Year

rr_AverageAnnualReturnYear01

(4.43%)

5 Years

rr_AverageAnnualReturnYear05

12.97%

10 Years

rr_AverageAnnualReturnYear10

7.71%

Thornburg Core Growth Fund | A C I | Return After Taxes on Distributions and Sale of Fund Shares | Class A

Risk/Return:

rr_RiskReturnAbstract

1 Year

rr_AverageAnnualReturnYear01

(2.51%)

5 Years

rr_AverageAnnualReturnYear05

10.40%

10 Years

rr_AverageAnnualReturnYear10

6.31%

Thornburg Core Growth Fund | A C I | Russell 3000 Growth Index (reflects no deduction for fees, expenses, or taxes) | Class A

Risk/Return:

rr_RiskReturnAbstract

1 Year

rr_AverageAnnualReturnYear01

12.44%

5 Years

rr_AverageAnnualReturnYear05

15.89%

10 Years

rr_AverageAnnualReturnYear10

8.50%

Thornburg Core Growth Fund | A C I | Russell 3000 Growth Index (reflects no deduction for fees, expenses, or taxes) | Class C

Risk/Return:

rr_RiskReturnAbstract

1 Year

rr_AverageAnnualReturnYear01

12.44%

5 Years

rr_AverageAnnualReturnYear05

15.89%

10 Years

rr_AverageAnnualReturnYear10

8.50%

Thornburg Core Growth Fund | A C I | Russell 3000 Growth Index (reflects no deduction for fees, expenses, or taxes) | Class I

Risk/Return:

rr_RiskReturnAbstract

1 Year

rr_AverageAnnualReturnYear01

12.44%

5 Years

rr_AverageAnnualReturnYear05

15.89%

10 Years

rr_AverageAnnualReturnYear10

8.50%

[1] A 1.00% 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] Imposed only on redemptions of Class C shares within 12 months of purchase.

[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 I expenses do not exceed 0.99%. 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.