Label	Element	Value
Risk Return Abstract	rr_RiskReturnAbstract
Document Type	dei_DocumentType	485BPOS
Document Period End Date	dei_DocumentPeriodEndDate	Dec. 31, 2017
Registrant Name	dei_EntityRegistrantName	PUTNAM TAX EXEMPT INCOME FUND
Central Index Key	dei_EntityCentralIndexKey	0000205802
Amendment Flag	dei_AmendmentFlag	false
Trading Symbol	dei_TradingSymbol	PTAEX
Document Creation Date	dei_DocumentCreationDate	Jan. 30, 2018
Document Effective Date	dei_DocumentEffectiveDate	Jan. 30, 2018
Prospectus Date	rr_ProspectusDate	Jan. 30, 2018
Putnam Tax Exempt Income Fund
Risk Return Abstract	rr_RiskReturnAbstract
Risk/Return [Heading]	rr_RiskReturnHeading	Fund Summary
Objective [Heading]	rr_ObjectiveHeading	Goal
Objective, Primary [Text Block]	rr_ObjectivePrimaryTextBlock	Putnam Tax Exempt Income Fund seeks as high a level of current income exempt from federal income tax as we believe to be consistent with preservation of capital.
Expense [Heading]	rr_ExpenseHeading	Fees and expenses
Expense Narrative [Text Block]	rr_ExpenseNarrativeTextBlock	The following table describes the fees and expenses you may pay if you buy and hold shares of the fund. You may qualify for sales charge discounts if you and your family invest, or agree to invest in the future, at least $100,000 in class A shares or $50,000 in class M shares of Putnam funds. More information about these and other discounts is available from your financial advisor and in How do I buy fund shares? beginning on page 12 of the fund’s prospectus, in the Appendix to the fund’s prospectus, and in How to buy shares beginning on page II-1 of the fund’s statement of additional information (SAI).
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 you pay each year as a percentage of the value of your investment)
Portfolio Turnover [Heading]	rr_PortfolioTurnoverHeading	Portfolio turnover
Portfolio Turnover [Text Block]	rr_PortfolioTurnoverTextBlock	The fund pays transaction-related costs, such as commissions, when it buys and sells securities (or “turns over” its portfolio). A higher turnover rate may indicate higher transaction costs and may result in higher taxes when the fund’s shares are held in a taxable account. These costs, which are not reflected in annual fund operating expenses or the above example, affect fund performance. The fund’s turnover rate in the most recent fiscal year was 35%.
Portfolio Turnover, Rate	rr_PortfolioTurnoverRate	35.00%
Expense Breakpoint Discounts [Text]	rr_ExpenseBreakpointDiscounts	You may qualify for sales charge discounts if you and your family invest, or agree to invest in the future, at least $100,000 in class A shares or $50,000 in class M shares of Putnam funds. More information about these and other discounts is available from your financial advisor and in How do I buy fund shares? beginning on page 12 of the fund’s prospectus, in the Appendix to the fund’s prospectus, and in How to buy shares beginning on page II-1 of the fund’s statement of additional information (SAI).
Expense Breakpoint, Minimum Investment Required [Amount]	rr_ExpenseBreakpointMinimumInvestmentRequiredAmount	$ 50,000
Expense Example [Heading]	rr_ExpenseExampleHeading	Example
Expense Example Narrative [Text Block]	rr_ExpenseExampleNarrativeTextBlock	The following hypothetical example is intended to help you compare the cost of investing in the fund with the cost of investing in other funds. It assumes that you invest $10,000 in the fund for the time periods indicated and then, except as indicated, redeem all your shares at the end of those periods. It assumes a 5% return on your investment each year and that the fund’s operating expenses remain the same. Your actual costs may be higher or lower.
Strategy [Heading]	rr_StrategyHeading	Investments, risks, and performance Investments
Strategy Narrative [Text Block]	rr_StrategyNarrativeTextBlock	We invest mainly in bonds that pay interest that is exempt from federal income tax (but that may be subject to federal alternative minimum tax (AMT)), are investment-grade in quality, and have intermediate- to long-term maturities (i.e., three years or longer). Under normal circumstances, we invest at least 80% of the fund’s net assets in tax-exempt investments, which for purposes of this policy exclude investments paying interest subject to the federal AMT for individuals. This investment policy cannot be changed without the approval of the fund’s shareholders. We may consider, among other factors, credit, interest rate and prepayment risks, as well as general market conditions, when deciding whether to buy or sell investments.
Risk [Heading]	rr_RiskHeading	Risks
Risk Narrative [Text Block]	rr_RiskNarrativeTextBlock	It is important to understand that you can lose money by investing in the fund. The value of bonds in the fund’s portfolio may fall or fail to rise over extended periods of time for a variety of reasons, including general financial market conditions, changing market perceptions (including perceptions about the risk of default and expectations about monetary policy or interest rates), changes in government intervention in the financial markets, and factors related to a specific issuer or industry. These and other factors may lead to increased volatility and reduced liquidity in the fund’s portfolio holdings. The risks associated with bond investments include interest rate risk, which means the value of the fund’s investments is likely to fall if interest rates rise. Bond investments also are subject to credit risk, which is the risk that the issuers of the fund’s investments may default on payment of interest or principal. Since the fund invests in tax-exempt bonds, which, to be treated as tax-exempt under the Internal Revenue Code, may be issued only by limited types of issuers for limited types of projects, the fund’s investments may be focused in certain market segments. Consequently, the fund may be more vulnerable to fluctuations in the values of the securities it holds than a fund that invests more broadly. Interest rate risk is generally greater for longer-term bonds, and credit risk is generally greater for below-investment-grade bonds, which can be more sensitive to changes in markets, credit conditions and interest rates. Interest the fund receives might be taxable. The fund may not achieve its goal, and it is not intended to be a complete investment program. An investment in the fund is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency.
Risk Lose Money [Text]	rr_RiskLoseMoney	It is important to understand that you can lose money by investing in the fund.
Risk Not Insured Depository Institution [Text]	rr_RiskNotInsuredDepositoryInstitution	An investment in the fund is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency.
Bar Chart and Performance Table [Heading]	rr_BarChartAndPerformanceTableHeading	Performance
Performance Narrative [Text Block]	rr_PerformanceNarrativeTextBlock	The performance information below gives some indication of the risks associated with an investment in the fund by showing the fund’s performance year to year and over time. The bar chart does not reflect the impact of sales charges. If it did, performance would be lower. Please remember that past performance is not necessarily an indication of future results. Monthly performance figures for the fund are available at putnam.com.
Performance Information Illustrates Variability of Returns [Text]	rr_PerformanceInformationIllustratesVariabilityOfReturns	The performance information below gives some indication of the risks associated with an investment in the fund by showing the fund’s performance year to year and over time.
Performance Availability Website Address [Text]	rr_PerformanceAvailabilityWebSiteAddress	putnam.com
Performance Past Does Not Indicate Future [Text]	rr_PerformancePastDoesNotIndicateFuture	Please remember that past performance is not necessarily an indication of future results.
Bar Chart [Heading]	rr_BarChartHeading	Annual total returns for class A shares before sales charges
Bar Chart Does Not Reflect Sales Loads [Text]	rr_BarChartDoesNotReflectSalesLoads	The bar chart does not reflect the impact of sales charges. If it did, performance would be lower.
Bar Chart Closing [Text Block]	rr_BarChartClosingTextBlock	Best calendar quarter Q3 2009   9.43% Worst calendar quarter Q4 2008  −4.81%
Highest Quarterly Return, Label	rr_HighestQuarterlyReturnLabel	Best calendar quarter
Highest Quarterly Return, Date	rr_BarChartHighestQuarterlyReturnDate	Sep. 30, 2009
Highest Quarterly Return	rr_BarChartHighestQuarterlyReturn	9.43%
Lowest Quarterly Return, Label	rr_LowestQuarterlyReturnLabel	Worst calendar quarter
Lowest Quarterly Return, Date	rr_BarChartLowestQuarterlyReturnDate	Dec. 31, 2008
Lowest Quarterly Return	rr_BarChartLowestQuarterlyReturn	(4.81%)
Performance Table Heading	rr_PerformanceTableHeading	Average annual total returns after sales charges (for periods ending 12/31/17)
Performance Table Uses Highest Federal Rate	rr_PerformanceTableUsesHighestFederalRate	After-tax returns reflect the historical highest individual federal marginal income tax rates and do not reflect 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.
Performance Table One Class of after Tax Shown [Text]	rr_PerformanceTableOneClassOfAfterTaxShown	After-tax returns are shown for class A shares only and will vary for other classes.
Performance Table Closing [Text Block]	rr_PerformanceTableClosingTextBlock	After-tax returns reflect the historical highest individual federal marginal income tax rates and do not reflect state and local taxes. Actual after-tax returns depend on an investor’s tax situation and may differ from those shown. After-tax returns are shown for class A shares only and will vary for other classes. These after-tax returns do not apply if you hold your fund shares through a 401(k) plan, an IRA, or another tax-advantaged arrangement. Class B share performance reflects conversion to class A shares after eight years.
Putnam Tax Exempt Income Fund | Class A
Risk Return Abstract	rr_RiskReturnAbstract
Maximum sales charge (load) imposed on purchases (as a percentage of offering price)	rr_MaximumSalesChargeImposedOnPurchasesOverOfferingPrice	4.00%
Maximum deferred sales charge (load) (as a percentage of original purchase price or redemption proceeds, whichever is lower)	rr_MaximumDeferredSalesChargeOverOther	1.00%	[1]
Management fees	rr_ManagementFeesOverAssets	0.44%
Distribution and service (12b-1) fees	rr_DistributionAndService12b1FeesOverAssets	0.22%	[2]
Other expenses	rr_OtherExpensesOverAssets	0.11%	[3]
Total annual fund operating expenses	rr_ExpensesOverAssets	0.77%
Expense Example, with Redemption, 1 Year	rr_ExpenseExampleYear01	$ 475
Expense Example, with Redemption, 3 Years	rr_ExpenseExampleYear03	636
Expense Example, with Redemption, 5 Years	rr_ExpenseExampleYear05	811
Expense Example, with Redemption, 10 Years	rr_ExpenseExampleYear10	$ 1,316
Annual Return 2008	rr_AnnualReturn2008	(9.51%)
Annual Return 2009	rr_AnnualReturn2009	17.94%
Annual Return 2010	rr_AnnualReturn2010	2.49%
Annual Return 2011	rr_AnnualReturn2011	10.78%
Annual Return 2012	rr_AnnualReturn2012	8.80%
Annual Return 2013	rr_AnnualReturn2013	(4.10%)
Annual Return 2014	rr_AnnualReturn2014	9.93%
Annual Return 2015	rr_AnnualReturn2015	2.65%
Annual Return 2016	rr_AnnualReturn2016	0.38%
Annual Return 2017	rr_AnnualReturn2017	5.88%
1 Year	rr_AverageAnnualReturnYear01	1.64%
5 Years	rr_AverageAnnualReturnYear05	2.00%
10 Years	rr_AverageAnnualReturnYear10	3.83%
Putnam Tax Exempt Income Fund | Class B
Risk Return Abstract	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 original purchase price or redemption proceeds, whichever is lower)	rr_MaximumDeferredSalesChargeOverOther	5.00%	[4]
Management fees	rr_ManagementFeesOverAssets	0.44%
Distribution and service (12b-1) fees	rr_DistributionAndService12b1FeesOverAssets	0.85%
Other expenses	rr_OtherExpensesOverAssets	0.11%	[3]
Total annual fund operating expenses	rr_ExpensesOverAssets	1.40%
Expense Example, with Redemption, 1 Year	rr_ExpenseExampleYear01	$ 643
Expense Example, with Redemption, 3 Years	rr_ExpenseExampleYear03	743
Expense Example, with Redemption, 5 Years	rr_ExpenseExampleYear05	966
Expense Example, with Redemption, 10 Years	rr_ExpenseExampleYear10	1,508
Expense Example, No Redemption, 1 Year	rr_ExpenseExampleNoRedemptionYear01	143
Expense Example, No Redemption, 3 Years	rr_ExpenseExampleNoRedemptionYear03	443
Expense Example, No Redemption, 5 Years	rr_ExpenseExampleNoRedemptionYear05	766
Expense Example, No Redemption, 10 Years	rr_ExpenseExampleNoRedemptionYear10	$ 1,508
1 Year	rr_AverageAnnualReturnYear01	0.21%
5 Years	rr_AverageAnnualReturnYear05	1.83%
10 Years	rr_AverageAnnualReturnYear10	3.73%
Putnam Tax Exempt Income Fund | Class C
Risk Return Abstract	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 original purchase price or redemption proceeds, whichever is lower)	rr_MaximumDeferredSalesChargeOverOther	1.00%	[5]
Management fees	rr_ManagementFeesOverAssets	0.44%
Distribution and service (12b-1) fees	rr_DistributionAndService12b1FeesOverAssets	1.00%
Other expenses	rr_OtherExpensesOverAssets	0.11%	[3]
Total annual fund operating expenses	rr_ExpensesOverAssets	1.55%
Expense Example, with Redemption, 1 Year	rr_ExpenseExampleYear01	$ 258
Expense Example, with Redemption, 3 Years	rr_ExpenseExampleYear03	490
Expense Example, with Redemption, 5 Years	rr_ExpenseExampleYear05	845
Expense Example, with Redemption, 10 Years	rr_ExpenseExampleYear10	1,845
Expense Example, No Redemption, 1 Year	rr_ExpenseExampleNoRedemptionYear01	158
Expense Example, No Redemption, 3 Years	rr_ExpenseExampleNoRedemptionYear03	490
Expense Example, No Redemption, 5 Years	rr_ExpenseExampleNoRedemptionYear05	845
Expense Example, No Redemption, 10 Years	rr_ExpenseExampleNoRedemptionYear10	$ 1,845
1 Year	rr_AverageAnnualReturnYear01	4.04%
5 Years	rr_AverageAnnualReturnYear05	2.03%
10 Years	rr_AverageAnnualReturnYear10	3.43%
Putnam Tax Exempt Income Fund | Class M
Risk Return Abstract	rr_RiskReturnAbstract
Maximum sales charge (load) imposed on purchases (as a percentage of offering price)	rr_MaximumSalesChargeImposedOnPurchasesOverOfferingPrice	3.25%
Maximum deferred sales charge (load) (as a percentage of original purchase price or redemption proceeds, whichever is lower)	rr_MaximumDeferredSalesChargeOverOther	none
Management fees	rr_ManagementFeesOverAssets	0.44%
Distribution and service (12b-1) fees	rr_DistributionAndService12b1FeesOverAssets	0.50%
Other expenses	rr_OtherExpensesOverAssets	0.11%	[3]
Total annual fund operating expenses	rr_ExpensesOverAssets	1.05%
Expense Example, with Redemption, 1 Year	rr_ExpenseExampleYear01	$ 429
Expense Example, with Redemption, 3 Years	rr_ExpenseExampleYear03	648
Expense Example, with Redemption, 5 Years	rr_ExpenseExampleYear05	886
Expense Example, with Redemption, 10 Years	rr_ExpenseExampleYear10	$ 1,566
1 Year	rr_AverageAnnualReturnYear01	2.25%
5 Years	rr_AverageAnnualReturnYear05	1.86%
10 Years	rr_AverageAnnualReturnYear10	3.60%
Putnam Tax Exempt Income Fund | Class T
Risk Return Abstract	rr_RiskReturnAbstract
Maximum sales charge (load) imposed on purchases (as a percentage of offering price)	rr_MaximumSalesChargeImposedOnPurchasesOverOfferingPrice	2.50%
Maximum deferred sales charge (load) (as a percentage of original purchase price or redemption proceeds, whichever is lower)	rr_MaximumDeferredSalesChargeOverOther	none
Management fees	rr_ManagementFeesOverAssets	0.44%
Distribution and service (12b-1) fees	rr_DistributionAndService12b1FeesOverAssets	0.25%
Other expenses	rr_OtherExpensesOverAssets	0.11%	[3],[6]
Total annual fund operating expenses	rr_ExpensesOverAssets	0.80%
Expense Example, with Redemption, 1 Year	rr_ExpenseExampleYear01	$ 330
Expense Example, with Redemption, 3 Years	rr_ExpenseExampleYear03	499
Expense Example, with Redemption, 5 Years	rr_ExpenseExampleYear05	683
Expense Example, with Redemption, 10 Years	rr_ExpenseExampleYear10	$ 1,215
1 Year	rr_AverageAnnualReturnYear01	3.18%	[7]
5 Years	rr_AverageAnnualReturnYear05	2.27%	[7]
10 Years	rr_AverageAnnualReturnYear10	3.94%	[7]
Putnam Tax Exempt Income Fund | Class Y
Risk Return Abstract	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 original purchase price or redemption proceeds, whichever is lower)	rr_MaximumDeferredSalesChargeOverOther	none
Management fees	rr_ManagementFeesOverAssets	0.44%
Other expenses	rr_OtherExpensesOverAssets	0.11%	[3]
Total annual fund operating expenses	rr_ExpensesOverAssets	0.55%
Expense Example, with Redemption, 1 Year	rr_ExpenseExampleYear01	$ 56
Expense Example, with Redemption, 3 Years	rr_ExpenseExampleYear03	176
Expense Example, with Redemption, 5 Years	rr_ExpenseExampleYear05	307
Expense Example, with Redemption, 10 Years	rr_ExpenseExampleYear10	$ 689
1 Year	rr_AverageAnnualReturnYear01	6.09%
5 Years	rr_AverageAnnualReturnYear05	3.05%
10 Years	rr_AverageAnnualReturnYear10	4.49%
Putnam Tax Exempt Income Fund | after taxes on distributions | Class A
Risk Return Abstract	rr_RiskReturnAbstract
1 Year	rr_AverageAnnualReturnYear01	1.60%
5 Years	rr_AverageAnnualReturnYear05	1.97%
10 Years	rr_AverageAnnualReturnYear10	3.80%
Putnam Tax Exempt Income Fund | after taxes on distributions and sale of fund shares | Class A
Risk Return Abstract	rr_RiskReturnAbstract
1 Year	rr_AverageAnnualReturnYear01	2.34%
5 Years	rr_AverageAnnualReturnYear05	2.34%
10 Years	rr_AverageAnnualReturnYear10	3.86%
Putnam Tax Exempt Income Fund | Bloomberg Barclays Municipal Bond Index (no deduction for fees, expenses or taxes)
Risk Return Abstract	rr_RiskReturnAbstract
1 Year	rr_AverageAnnualReturnYear01	5.45%
5 Years	rr_AverageAnnualReturnYear05	3.02%
10 Years	rr_AverageAnnualReturnYear10	4.46%

[1] Applies only to certain redemptions of shares bought with no initial sales charge.

[2] Represents a blended rate.

[3] Restated to reflect current fees.

[4] This charge is phased out over six years.

[5] This charge is eliminated after one year.

[6] Other expenses are based on class A shares for the fund's last fiscal year, as restated to reflect current fees.

[7] Class T shares were not outstanding during the periods shown. Performance shown for class T shares is derived from the historical performance of class A shares, adjusted up to reflect the lower initial sales charge currently applicable to class T shares and adjusted down by 0.05% to reflect the higher 12b-1 fees applicable to class T shares. The 0.05% downward adjustment reflects the fact that the 12b-1 fees currently applicable to class T shares are 0.05% higher per annum than the 0.20% rate applicable to class A shares prior to April 1, 2005 and up to 0.05% higher per annum than the blended rate applicable to class A shares since April 1, 2005. However, because a 0.05% downward adjustment has been used for all periods, class T share performance since April 1, 2005 may be understated.