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Label Element Value
Prospectus [Line Items] rr_ProspectusLineItems  
Document Type dei_DocumentType 485BPOS
Document Period End Date dei_DocumentPeriodEndDate Dec. 31, 2021
Entity Registrant Name dei_EntityRegistrantName Putnam Mortgage Securities Fund
Entity Central Index Key dei_EntityCentralIndexKey 0000732337
Entity Inv Company Type dei_EntityInvCompanyType N-1A
Amendment Flag dei_AmendmentFlag false
Putnam Mortgage Securities Fund  
Prospectus [Line Items] rr_ProspectusLineItems  
Risk/Return [Heading] rr_RiskReturnHeading Fund summary
Objective [Heading] rr_ObjectiveHeading Goal
Objective, Primary [Text Block] rr_ObjectivePrimaryTextBlock

Putnam Mortgage Securities Fund seeks as high a level of current income as Putnam Investment Management, LLC believes is consistent with preservation of capital.

Expense [Heading] rr_ExpenseHeading Fees and expenses
Expense Narrative [Text Block] rr_ExpenseNarrativeTextBlock

The following tables describe the fees and expenses you may pay if you buy, hold and sell shares of the fund. You may pay other fees, such as brokerage commissions and other fees to financial intermediaries, which are not reflected in the tables and examples below. 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 of Putnam funds. More information about these and other discounts is available from your financial professional and in How do I buy fund shares? beginning on page 18 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 863%.

Portfolio Turnover, Rate rr_PortfolioTurnoverRate 863.00%
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 of Putnam funds.
Expense Breakpoint, Minimum Investment Required [Amount] rr_ExpenseBreakpointMinimumInvestmentRequiredAmount $ 100,000
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
Strategy Narrative [Text Block] rr_StrategyNarrativeTextBlock

Investments

We invest mainly in mortgages, mortgage-related fixed income securities and related derivatives that are either investment-grade or below-investment-grade in quality (sometimes referred to as “junk bonds”). Under normal circumstances, we invest at least 80% of the fund’s net assets (plus any borrowings for investment purposes) in mortgages, mortgage-related fixed income securities and related derivatives (i.e., derivatives used to acquire exposure to, or whose underlying securities are, mortgages or mortgage-related securities). The fund generally uses the net unrealized gain or loss, or market value, of mortgage-related derivatives for purposes of this policy, but may use the notional value of a derivative if that is determined to be a more appropriate measure of the fund’s investment exposure. This policy may be changed only after 60 days’ notice to shareholders.

We expect to invest in mortgage-backed investments that are obligations of U.S. government agencies and instrumentalities and accordingly are backed by the full faith and credit of the United States (e.g., Ginnie Mae mortgage-backed bonds) as well as in mortgage-backed investments that are backed by only the credit of a



 



federal agency or government-sponsored entity (e.g., Fannie Mae and Freddie Mac mortgage-backed bonds), and that have short- to long-term maturities.

We also expect to invest in lower-rated, higher-yielding mortgage-backed securities, including non-agency residential mortgage-backed securities (which may be backed by non-qualified or “sub-prime” mortgages), commercial mortgage-backed securities, and collateralized mortgage obligations (including interest only, principal only, and other prepayment derivatives). Non-agency (i.e., privately issued) securities typically are lower-rated and higher yielding than securities issued or backed by agencies such as Ginnie Mae, Fannie Mae or Freddie Mac. The fund currently has significant investment exposure to commercial mortgage-backed securities. While our emphasis will be on mortgage-backed securities, we may also invest to a lesser extent in other types of asset-backed securities.

We may consider, among other factors, credit, interest rate, prepayment and liquidity risks, as well as general market conditions, when deciding whether to buy or sell investments. We typically use to a significant extent derivatives, including credit default swaps, interest rate swaps, total return swaps, to-be-announced (TBA) commitments, futures, options and swaptions, including on mortgage-backed securities and indices, for both hedging and non-hedging purposes, including to obtain or adjust exposure to mortgage-backed 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 investments in the fund’s portfolio may fall or fail to rise over extended periods of time for a variety of reasons, including general economic, political or financial market conditions, investor sentiment and market perceptions, government actions, geopolitical events or changes, outbreaks of infectious illnesses or other widespread public health issues, and factors related to a specific issuer, geography, industry or sector, such as the housing or real estate markets. These and other factors may lead to increased volatility and reduced liquidity in the fund’s portfolio holdings, may negatively impact the fund’s performance, and may exacerbate other risks to which the fund is subject.

The risks associated with bond investments include interest rate risk, which is the risk that the value of the fund’s investments is likely to fall if interest rates rise. Bond investments are also subject to credit risk, which is the risk that issuers of the fund’s investments may default on payment of interest or principal. Bond investments may be more susceptible to downgrades or defaults during economic downturns or other periods of economic stress. Default risk is generally higher for non-qualified mortgages. 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, and may be considered speculative.

Mortgage- and asset-backed investments, unlike traditional debt investments, are also subject to prepayment risk, which means that they may increase in value less than other bonds when interest rates decline and decline in value more than



 



other bonds when interest rates rise. We may have to invest the proceeds from prepaid investments, including mortgage- and asset-backed investments, in other investments with less attractive terms and yields. The fund’s investments in mortgage-backed securities and asset-backed securities, and in certain other securities and derivatives, may be or become illiquid.

The fund’s concentration in an industry group composed of privately issued residential and commercial mortgage-backed securities and mortgage-backed securities issued or guaranteed by the U.S. government or its agencies or instrumentalities may make the fund’s net asset value more susceptible to economic, market, political and other developments affecting the residential and commercial real estate markets, and the servicing of mortgage loans secured by real estate properties. During periods of difficult economic conditions, delinquencies and losses on commercial mortgage-backed investments in particular generally increase, including as a result of the effects of those conditions on commercial real estate markets, the ability of commercial tenants to make loan payments, and the ability of a property to attract and retain commercial tenants.

Our use of derivatives may increase the risks of investing in the fund by increasing investment exposure (which may be considered leverage) or, in the case of many over-the-counter instruments, because of the potential inability to terminate or sell derivatives positions and the potential failure of the other party to the instrument to meet its obligations. The risk of a party failing to meet its obligations may increase if the fund has significant exposure to that counterparty. The value of derivatives may move in unexpected ways due to unanticipated market movements, the use of leverage, imperfect correlation between the derivative instrument and the reference asset, or other factors, especially in unusual market conditions, and volatility in the value of derivatives could adversely impact the fund’s returns, obligations and exposures. Derivatives are also subject to other risks, including liquidity risk (e.g., liquidity demands arising from the requirement to make payments to a derivative counterparty), operational risk (e.g., settlement issues or system failures) and legal risk (e.g., insufficient legal documentation or contract enforceability issues).

There is no guarantee that the investment techniques, analyses, or judgments that we apply in making investment decisions for the fund will produce the intended outcome or that the investments we select for the fund will perform as well as other securities that were not selected for the fund. We, or the fund’s other service providers, may experience disruptions or operating errors that could negatively impact the fund.

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. Before April 19, 2018, the fund was managed with a materially different



 



investment strategy and may have achieved materially different performance results under its current investment strategy from that shown for periods before this date. 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.
Highest Quarterly Return, Label rr_HighestQuarterlyReturnLabel Best calendar quarter
Highest Quarterly Return, Date rr_BarChartHighestQuarterlyReturnDate Dec. 31, 2020
Highest Quarterly Return rr_BarChartHighestQuarterlyReturn 4.54%
Lowest Quarterly Return, Label rr_LowestQuarterlyReturnLabel Worst calendar quarter
Lowest Quarterly Return, Date rr_BarChartLowestQuarterlyReturnDate Mar. 31, 2020
Lowest Quarterly Return rr_BarChartLowestQuarterlyReturn (8.89%)
Performance Table Heading rr_PerformanceTableHeading Average annual total returns after sales charges (for periods ended 12/31/22)
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
The Putnam Mortgage Securities Linked Benchmark represents the performance of the Bloomberg GNMA Index through April 18, 2018, and the performance of the Bloomberg U.S. MBS Index thereafter.
All Bloomberg indices provided by Bloomberg Index Services Limited.
BLOOMBERG® is a trademark and service mark of Bloomberg Finance L.P. and its affiliates (collectively “Bloomberg”). Bloomberg, or Bloomberg’s licensors, own all proprietary rights in the Bloomberg Indices. Neither Bloomberg nor Bloomberg’s licensors approve or endorse this material or guarantee the accuracy or completeness of any information herein, or make any warranty, express or implied, as to the results to be obtained therefrom, and to the maximum extent allowed by law, neither shall have any liability or responsibility for injury or damages arising in connection therewith.



 



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 and C share performance reflects conversion to class A shares after eight years.
Putnam Mortgage Securities Fund | Bloomberg U.S. MBS Index (no deduction for fees, expenses or taxes)  
Prospectus [Line Items] rr_ProspectusLineItems  
1 Year rr_AverageAnnualReturnYear01 (11.81%)
5 Years rr_AverageAnnualReturnYear05 (0.53%)
10 Years rr_AverageAnnualReturnYear10 0.74%
Putnam Mortgage Securities Fund | Putnam Mortgage Securities Linked Benchmark (no deduction for fees, expenses or taxes)  
Prospectus [Line Items] rr_ProspectusLineItems  
1 Year rr_AverageAnnualReturnYear01 (11.81%)
5 Years rr_AverageAnnualReturnYear05 (0.52%)
10 Years rr_AverageAnnualReturnYear10 0.58%
Putnam Mortgage Securities Fund | Class A  
Prospectus [Line Items] rr_ProspectusLineItems  
Maximum Sales Charge Imposed on Purchases (as a percentage of Offering Price) rr_MaximumSalesChargeImposedOnPurchasesOverOfferingPrice 4.00%
Maximum Deferred Sales Charge (as a percentage) rr_MaximumDeferredSalesChargeOverOther 1.00% [1]
Management Fees (as a percentage of Assets) rr_ManagementFeesOverAssets 0.39%
Distribution and Service (12b-1) Fees rr_DistributionAndService12b1FeesOverAssets 0.25%
Other Expenses (as a percentage of Assets): rr_OtherExpensesOverAssets 0.28%
Expenses (as a percentage of Assets) rr_ExpensesOverAssets 0.92%
Expense Example, with Redemption, 1 Year rr_ExpenseExampleYear01 $ 490
Expense Example, with Redemption, 3 Years rr_ExpenseExampleYear03 682
Expense Example, with Redemption, 5 Years rr_ExpenseExampleYear05 889
Expense Example, with Redemption, 10 Years rr_ExpenseExampleYear10 $ 1,486
Annual Return 2013 rr_AnnualReturn2013 (0.48%)
Annual Return 2014 rr_AnnualReturn2014 5.49%
Annual Return 2015 rr_AnnualReturn2015 (0.26%)
Annual Return 2016 rr_AnnualReturn2016 0.63%
Annual Return 2017 rr_AnnualReturn2017 0.82%
Annual Return 2018 rr_AnnualReturn2018 (0.79%)
Annual Return 2019 rr_AnnualReturn2019 12.40%
Annual Return 2020 rr_AnnualReturn2020 (0.71%)
Annual Return 2021 rr_AnnualReturn2021 (4.22%)
Annual Return 2022 rr_AnnualReturn2022 (10.82%)
1 Year rr_AverageAnnualReturnYear01 (14.39%)
5 Years rr_AverageAnnualReturnYear05 (1.92%)
10 Years rr_AverageAnnualReturnYear10 (0.36%)
Putnam Mortgage Securities Fund | Class A | After Taxes on Distributions  
Prospectus [Line Items] rr_ProspectusLineItems  
1 Year rr_AverageAnnualReturnYear01 (18.35%)
5 Years rr_AverageAnnualReturnYear05 (3.94%)
10 Years rr_AverageAnnualReturnYear10 (1.90%)
Putnam Mortgage Securities Fund | Class A | After Taxes on Distributions and Sales  
Prospectus [Line Items] rr_ProspectusLineItems  
1 Year rr_AverageAnnualReturnYear01 (8.45%)
5 Years rr_AverageAnnualReturnYear05 (2.19%)
10 Years rr_AverageAnnualReturnYear10 (0.87%)
Putnam Mortgage Securities Fund | Class B  
Prospectus [Line Items] rr_ProspectusLineItems  
Maximum Sales Charge Imposed on Purchases (as a percentage of Offering Price) rr_MaximumSalesChargeImposedOnPurchasesOverOfferingPrice none
Maximum Deferred Sales Charge (as a percentage) rr_MaximumDeferredSalesChargeOverOther 5.00% [2]
Management Fees (as a percentage of Assets) rr_ManagementFeesOverAssets 0.39%
Distribution and Service (12b-1) Fees rr_DistributionAndService12b1FeesOverAssets 1.00%
Other Expenses (as a percentage of Assets): rr_OtherExpensesOverAssets 0.28%
Expenses (as a percentage of Assets) rr_ExpensesOverAssets 1.67%
Expense Example, with Redemption, 1 Year rr_ExpenseExampleYear01 $ 670
Expense Example, with Redemption, 3 Years rr_ExpenseExampleYear03 826
Expense Example, with Redemption, 5 Years rr_ExpenseExampleYear05 1,107
Expense Example, with Redemption, 10 Years rr_ExpenseExampleYear10 1,777
Expense Example, No Redemption, 1 Year rr_ExpenseExampleNoRedemptionYear01 170
Expense Example, No Redemption, 3 Years rr_ExpenseExampleNoRedemptionYear03 526
Expense Example, No Redemption, 5 Years rr_ExpenseExampleNoRedemptionYear05 907
Expense Example, No Redemption, 10 Years rr_ExpenseExampleNoRedemptionYear10 $ 1,777
1 Year rr_AverageAnnualReturnYear01 (15.45%)
5 Years rr_AverageAnnualReturnYear05 (2.16%)
10 Years rr_AverageAnnualReturnYear10 (0.54%)
Putnam Mortgage Securities Fund | Class C  
Prospectus [Line Items] rr_ProspectusLineItems  
Maximum Sales Charge Imposed on Purchases (as a percentage of Offering Price) rr_MaximumSalesChargeImposedOnPurchasesOverOfferingPrice none
Maximum Deferred Sales Charge (as a percentage) rr_MaximumDeferredSalesChargeOverOther 1.00% [3]
Management Fees (as a percentage of Assets) rr_ManagementFeesOverAssets 0.39%
Distribution and Service (12b-1) Fees rr_DistributionAndService12b1FeesOverAssets 1.00%
Other Expenses (as a percentage of Assets): rr_OtherExpensesOverAssets 0.28%
Expenses (as a percentage of Assets) rr_ExpensesOverAssets 1.67%
Expense Example, with Redemption, 1 Year rr_ExpenseExampleYear01 $ 270
Expense Example, with Redemption, 3 Years rr_ExpenseExampleYear03 526
Expense Example, with Redemption, 5 Years rr_ExpenseExampleYear05 907
Expense Example, with Redemption, 10 Years rr_ExpenseExampleYear10 1,777
Expense Example, No Redemption, 1 Year rr_ExpenseExampleNoRedemptionYear01 170
Expense Example, No Redemption, 3 Years rr_ExpenseExampleNoRedemptionYear03 526
Expense Example, No Redemption, 5 Years rr_ExpenseExampleNoRedemptionYear05 907
Expense Example, No Redemption, 10 Years rr_ExpenseExampleNoRedemptionYear10 $ 1,777
1 Year rr_AverageAnnualReturnYear01 (12.22%)
5 Years rr_AverageAnnualReturnYear05 (1.85%)
10 Years rr_AverageAnnualReturnYear10 (0.55%)
Putnam Mortgage Securities Fund | Class R  
Prospectus [Line Items] rr_ProspectusLineItems  
Maximum Sales Charge Imposed on Purchases (as a percentage of Offering Price) rr_MaximumSalesChargeImposedOnPurchasesOverOfferingPrice none
Maximum Deferred Sales Charge (as a percentage) rr_MaximumDeferredSalesChargeOverOther none
Management Fees (as a percentage of Assets) rr_ManagementFeesOverAssets 0.39%
Distribution and Service (12b-1) Fees rr_DistributionAndService12b1FeesOverAssets 0.50%
Other Expenses (as a percentage of Assets): rr_OtherExpensesOverAssets 0.28%
Expenses (as a percentage of Assets) rr_ExpensesOverAssets 1.17%
Expense Example, with Redemption, 1 Year rr_ExpenseExampleYear01 $ 119
Expense Example, with Redemption, 3 Years rr_ExpenseExampleYear03 372
Expense Example, with Redemption, 5 Years rr_ExpenseExampleYear05 644
Expense Example, with Redemption, 10 Years rr_ExpenseExampleYear10 $ 1,420
1 Year rr_AverageAnnualReturnYear01 (11.08%)
5 Years rr_AverageAnnualReturnYear05 (1.37%)
10 Years rr_AverageAnnualReturnYear10 (0.21%)
Putnam Mortgage Securities Fund | Class R6  
Prospectus [Line Items] rr_ProspectusLineItems  
Maximum Sales Charge Imposed on Purchases (as a percentage of Offering Price) rr_MaximumSalesChargeImposedOnPurchasesOverOfferingPrice none
Maximum Deferred Sales Charge (as a percentage) rr_MaximumDeferredSalesChargeOverOther none
Management Fees (as a percentage of Assets) rr_ManagementFeesOverAssets 0.39%
Distribution and Service (12b-1) Fees rr_DistributionAndService12b1FeesOverAssets none
Other Expenses (as a percentage of Assets): rr_OtherExpensesOverAssets 0.14%
Expenses (as a percentage of Assets) rr_ExpensesOverAssets 0.53%
Expense Example, with Redemption, 1 Year rr_ExpenseExampleYear01 $ 54
Expense Example, with Redemption, 3 Years rr_ExpenseExampleYear03 170
Expense Example, with Redemption, 5 Years rr_ExpenseExampleYear05 296
Expense Example, with Redemption, 10 Years rr_ExpenseExampleYear10 $ 665
1 Year rr_AverageAnnualReturnYear01 (10.45%) [4]
5 Years rr_AverageAnnualReturnYear05 (0.75%) [4]
10 Years rr_AverageAnnualReturnYear10 0.36% [4]
Putnam Mortgage Securities Fund | Class Y  
Prospectus [Line Items] rr_ProspectusLineItems  
Maximum Sales Charge Imposed on Purchases (as a percentage of Offering Price) rr_MaximumSalesChargeImposedOnPurchasesOverOfferingPrice none
Maximum Deferred Sales Charge (as a percentage) rr_MaximumDeferredSalesChargeOverOther none
Management Fees (as a percentage of Assets) rr_ManagementFeesOverAssets 0.39%
Distribution and Service (12b-1) Fees rr_DistributionAndService12b1FeesOverAssets none
Other Expenses (as a percentage of Assets): rr_OtherExpensesOverAssets 0.28%
Expenses (as a percentage of Assets) rr_ExpensesOverAssets 0.67%
Expense Example, with Redemption, 1 Year rr_ExpenseExampleYear01 $ 68
Expense Example, with Redemption, 3 Years rr_ExpenseExampleYear03 214
Expense Example, with Redemption, 5 Years rr_ExpenseExampleYear05 373
Expense Example, with Redemption, 10 Years rr_ExpenseExampleYear10 $ 835
1 Year rr_AverageAnnualReturnYear01 (10.58%)
5 Years rr_AverageAnnualReturnYear05 (0.87%)
10 Years rr_AverageAnnualReturnYear10 0.29%
[1] Applies only to certain redemptions of shares bought with no initial sales charge.
[2] This charge is phased out over six years.
[3] This charge is eliminated after one year.
[4] Performance for class R6 shares prior to their inception (4/20/18) is derived from the historical performance of class Y shares and has not been adjusted for the lower investor servicing fees applicable to class R6 shares; had it, returns would have been higher.