Summary of Significant Accounting Policies (Policies)	12 Months Ended
Dec. 31, 2020
Accounting Policies [Abstract]
BASIS OF PRESENTATION	BASIS OF PRESENTATION. Our consolidated financial statements include the accounts of Diversified Healthcare Trust, we, us or our, and our subsidiaries, all of which, except for the joint venture discussed below, are 100% owned directly or indirectly by us. All intercompany transactions and balances with or among our consolidated subsidiaries have been eliminated. Accounting principles generally accepted in the United States, or GAAP, require us to make estimates and assumptions that may affect the amounts reported in these financial statements and related notes. The actual results could differ from these estimates. In March 2017, we entered a joint venture arrangement with an institutional investor for one of our life science properties located in Boston, Massachusetts. We have determined that this joint venture is a variable interest entity, or VIE, as defined under the Consolidation Topic of the Financial Accounting Standards Board, or FASB, Accounting Standards Codification, or the Codification. We concluded that we must consolidate this VIE because we are the entity with the power to direct the activities that most significantly impact the VIE's economic performance and we have the obligation to absorb losses of, and the right to receive benefits from, the VIE that could be significant to the VIE, and therefore are the primary beneficiary of the VIE. The assets of this VIE were $970,142 and $1,015,661 as of December 31, 2020 and 2019, respectively, and consist primarily of the net real estate owned by the joint venture. The liabilities of this VIE were $697,129 and $704,344 as of December 31, 2020 and 2019, respectively, and consist primarily of mortgage debts secured by the property. The investor's interest in this consolidated entity is reflected as a noncontrolling interest in our consolidated financial statements.
REAL ESTATE PROPERTIES	REAL ESTATE PROPERTIES. We record properties at our cost and calculate depreciation on real estate investments on a straight line basis over estimated useful lives generally up to 40 years. We allocate the purchase prices of our properties to land, building and improvements based on determinations of the fair values of these assets assuming the properties are vacant. We determine the fair value of each property using methods similar to those used by independent appraisers, which may involve estimated cash flows that are based on a number of factors, including capitalization rates and discount rates, among others. In some circumstances, we engage independent real estate appraisal firms to provide market information and evaluations which are relevant to our purchase price allocations and determinations of depreciable useful lives; however, we are ultimately responsible for the purchase price allocations and determinations of useful lives. We allocate a portion of the purchase price to above market and below market leases based on the present value (using an interest rate which reflects the risks associated with acquired in place leases at the time each property was acquired by us) of the difference, if any, between (i) the contractual amounts to be paid pursuant to the acquired in place leases and (ii) our estimates of fair market lease rates for the corresponding leases, measured over a period equal to the terms of the respective leases. The terms of below market leases that include bargain renewal options, if any, are further adjusted if we determine that renewal is probable. We allocate a portion of the purchase price to acquired in place leases and tenant relationships based upon market estimates to lease up the property based on the leases in place at the time of purchase. In making these allocations, we consider factors such as estimated carrying costs during the expected lease up periods, including real estate taxes, insurance and other operating income and expenses and costs, such as leasing commissions, legal and other related expenses, to execute similar leases in current market conditions at the time a property was acquired by us. We allocate this aggregate value between acquired in place lease values and tenant relationships based on our evaluation of the specific characteristics of each tenant's lease. However, we have not separated the value of tenant relationships from the value of acquired in place leases because such value and related amortization expense is immaterial to our consolidated financial statements. If the value of tenant relationships becomes material in the future, we may separately allocate those amounts and amortize the allocated amount over the estimated life of the relationships. We amortize capitalized above market lease values (included in acquired real estate leases and other intangible assets, net in our consolidated balance sheets) as a reduction to rental income over the remaining non-cancelable terms of the respective leases. We amortize capitalized below market lease values (presented as assumed real estate lease obligations in our consolidated balance sheets) as an increase to rental income over the non-cancelable periods of the respective leases. For the years ended December 31, 2020, 2019 and 2018, such amortization resulted in a net increase in rental income of $7,405, $6,791 and $5,787, respectively. We amortize the value of in place leases exclusive of the value of above market and below market in place leases to expense over the remaining non-cancelable periods of the respective leases. During the years ended December 31, 2020, 2019 and 2018, such amortization included in depreciation totaled $48,669, $64,203 and $72,925, respectively. If a lease is terminated prior to its stated expiration, the unamortized amount relating to that lease is written off.
CASH AND CASH EQUIVALENTS	CASH AND CASH EQUIVALENTS. We consider highly liquid investments with original maturities of three months or less at the date of purchase to be cash equivalents.
RESTRICTED CASH	RESTRICTED CASH. Restricted cash consists of amounts escrowed for real estate taxes, insurance and capital expenditures at certain of our mortgaged properties and cash held for the operations of one of our life science properties that is owned in a joint venture arrangement in which we own a 55% equity interest.
INVESTMENTS IN EQUITY SECURITIES	INVESTMENTS IN EQUITY SECURITIES. We previously owned 2,637,408 shares of class A common stock of The RMR Group Inc., or RMR Inc., that we sold on July 1, 2019. Prior to July 1, 2019, our equity securities were recorded at fair value based on their quoted market price at the end of each reporting period. We classify the common shares we own of Five Star Senior Living Inc., or Five Star, as equity securities and carry them at fair value in investments of equity securities in our consolidated balance sheets. Effective January 1, 2018, changes in the fair value of our equity securities were recorded through earnings in accordance with FASB Accounting Standards Update, or ASU, No. 2016-01, Recognition and Measurement of Financial Assets and Financial Liabilities. Prior to the adoption of ASU No. 2016-01, unrealized gains and losses were recorded as a component of cumulative other comprehensive income (loss) in shareholder's equity.
EQUITY METHOD INVESTMENTS	EQUITY METHOD INVESTMENTS. We account for our investment in Affiliates Insurance Company, or AIC, until AIC was dissolved as described in Note 8, using the equity method of accounting. Significant influence was present through common representation on our Board of Trustees and the board of directors of AIC until February 13, 2020. The Chair of our Board of Trustees and one of our Managing Trustees, Adam D. Portnoy, as the sole trustee of ABP Trust, is the controlling shareholder of RMR Inc. He is also a managing director and an executive officer of RMR Inc. Substantially all of the business of RMR Inc. is conducted by its majority owned subsidiary, The RMR Group LLC, or RMR LLC, which is our manager and provided management and administrative services to AIC. Most of our Trustees were directors of AIC. See Note 8 for more information about our investment in AIC. Refer to the above section for further details on our equity method investment in Five Star. We periodically evaluate our equity method investments for possible indicators of other than temporary impairment whenever events or changes in circumstances indicate the carrying amount of the investment might not be recoverable. These indicators may include the length of time and the extent to which the market value of our investment is below our carrying value, the financial condition of our investees, our intent and ability to be a long term holder of the investment and other considerations. If the decline in fair value is judged to be other than temporary, we record an impairment charge to adjust the basis of the investment to its estimated fair value.
DEBT ISSUANCE COSTS	DEBT ISSUANCE COSTS. Debt issuance costs include issuance or assumption costs related to borrowings and we amortize those costs as interest expense over the terms of the respective loans.
DEFERRED LEASING COSTS	DEFERRED LEASING COSTS. Deferred leasing costs include capitalized brokerage and other fees associated with the successful negotiation of leases, which are amortized to depreciation and amortization expense on a straight line basis over the terms of the respective leases. Deferred leasing costs are included in other assets in our consolidated balance sheets.
REVENUE RECOGNITION	REVENUE RECOGNITION. We are a lessor of medical office and life science properties, senior living communities and other healthcare related properties. Our leases provide our tenants with the contractual right to use and economically benefit from all of the premises demised under the leases; therefore, we have determined to evaluate our leases as lease arrangements.
PER COMMON SHARE AMOUNTS	PER COMMON SHARE AMOUNTS. We calculate basic earnings per common share by dividing net income (loss) by the weighted average number of our common shares of beneficial interest, $.01 par value, or our common shares, outstanding during the period. We calculate diluted earnings per common share using the more dilutive of the two class method or the treasury stock method. Unvested share awards and other potentially dilutive common shares and the related impact on earnings, are considered when calculating diluted earnings per share.
INCOME TAXES	INCOME TAXES. We have elected to be taxed as a REIT under the United States Internal Revenue Code of 1986, as amended, and as such are generally not subject to federal and most state income taxation on our operating income provided we distribute our taxable income to our shareholders and meet certain organization and operating requirements. We do, however, lease our managed senior living communities to our wholly owned TRSs that, unlike most of our subsidiaries, file a separate consolidated federal corporate income tax return and are subject to federal and state income taxes. Our consolidated income tax provision includes the income tax provision related to the operations of our TRSs and certain state income taxes we incur despite our taxation as a REIT. The Income Taxes Topic of the Codification prescribes how we should recognize, measure and present in our financial statements uncertain tax positions that have been taken or are expected to be taken in a tax return. Tax benefits are recognized to the extent that it is “more likely than not” that a particular tax position will be sustained upon examination or audit. To the extent the “more likely than not” standard has been satisfied, the benefit associated with a tax position is measured as the largest amount that has a greater than 50% likelihood of being realized upon settlement. We classify interest and penalties related to uncertain tax positions, if any, in our financial statements as a component of general and administrative expense.
SEGMENT REPORTING	SEGMENT REPORTING. As of December 31, 2020, we operate in, and report financial information for, the following two segments: our portfolio of medical office and life science properties, or our Office Portfolio, and our senior housing operating portfolio, or SHOP. We aggregate each of these two reporting segments based on their similar operating and economic characteristics.
NEW ACCOUNTING PRONOUNCEMENTS	NEW ACCOUNTING PRONOUNCEMENTS. In June 2016, the FASB issued ASU No. 2016-13, Financial Instruments - Credit Losses (Topic 326): Measurement of Credit Losses on Financial Instruments, which requires that entities use a new forward-looking “expected loss” model that generally will result in the earlier recognition of allowance for credit losses. The measurement of expected credit losses is based upon historical experience, current conditions, and reasonable and supportable forecasts that affect the collectability of the reported amount. We adopted this standard on January 1, 2020 using the modified retrospective approach. The implementation of this standard did not have a material impact on our consolidated financial statements.

Summary of Significant Accounting Policies (Policies)

12 Months Ended

Dec. 31, 2020

BASIS OF PRESENTATION. Our consolidated financial statements include the accounts of Diversified Healthcare Trust, we, us or our, and our subsidiaries, all of which, except for the joint venture discussed below, are 100% owned directly or indirectly by us. All intercompany transactions and balances with or among our consolidated subsidiaries have been eliminated. Accounting principles generally accepted in the United States, or GAAP, require us to make estimates and assumptions that may affect the amounts reported in these financial statements and related notes. The actual results could differ from these estimates. In March 2017, we entered a joint venture arrangement with an institutional investor for one of our life science properties located in Boston, Massachusetts. We have determined that this joint venture is a variable interest entity, or VIE, as defined under the Consolidation Topic of the Financial Accounting Standards Board, or FASB, Accounting Standards Codification, or the Codification. We concluded that we must consolidate this VIE because we are the entity with the power to direct the activities that most significantly impact the VIE's economic performance and we have the obligation to absorb losses of, and the right to receive benefits from, the VIE that could be significant to the VIE, and therefore are the primary beneficiary of the VIE. The assets of this VIE were $970,142 and $1,015,661 as of December 31, 2020 and 2019, respectively, and consist primarily of the net real estate owned by the joint venture. The liabilities of this VIE were $697,129 and $704,344 as of December 31, 2020 and 2019, respectively, and consist primarily of mortgage debts secured by the property. The investor's interest in this consolidated entity is reflected as a noncontrolling interest in our consolidated financial statements.

REAL ESTATE PROPERTIES

REAL ESTATE PROPERTIES. We record properties at our cost and calculate depreciation on real estate investments on a straight line basis over estimated useful lives generally up to 40 years.

We allocate the purchase prices of our properties to land, building and improvements based on determinations of the fair values of these assets assuming the properties are vacant. We determine the fair value of each property using methods similar to those used by independent appraisers, which may involve estimated cash flows that are based on a number of factors, including capitalization rates and discount rates, among others. In some circumstances, we engage independent real estate appraisal firms to provide market information and evaluations which are relevant to our purchase price allocations and determinations of depreciable useful lives; however, we are ultimately responsible for the purchase price allocations and determinations of useful lives. We allocate a portion of the purchase price to above market and below market leases based on the present value (using an interest rate which reflects the risks associated with acquired in place leases at the time each property was acquired by us) of the difference, if any, between (i) the contractual amounts to be paid pursuant to the acquired in place leases and (ii) our estimates of fair market lease rates for the corresponding leases, measured over a period equal to the terms of the respective leases. The terms of below market leases that include bargain renewal options, if any, are further adjusted if we determine that renewal is probable. We allocate a portion of the purchase price to acquired in place leases and tenant relationships based upon market estimates to lease up the property based on the leases in place at the time of purchase. In making these allocations, we consider factors such as estimated carrying costs during the expected lease up periods, including real estate taxes, insurance and other operating income and expenses and costs, such as leasing commissions, legal and other related expenses, to execute similar leases in current market conditions at the time a property was acquired by us. We allocate this aggregate value between acquired in place lease values and tenant relationships based on our evaluation of the specific characteristics of each tenant's lease. However, we have not separated the value of tenant relationships from the value of acquired in place leases because such value and related amortization expense is immaterial to our consolidated financial statements. If the value of tenant relationships becomes material in the future, we may separately allocate those amounts and amortize the allocated amount over the estimated life of the relationships.

We amortize capitalized above market lease values (included in acquired real estate leases and other intangible assets, net in our consolidated balance sheets) as a reduction to rental income over the remaining non-cancelable terms of the respective leases. We amortize capitalized below market lease values (presented as assumed real estate lease obligations in our

consolidated balance sheets) as an increase to rental income over the non-cancelable periods of the respective leases. For the years ended December 31, 2020, 2019 and 2018, such amortization resulted in a net increase in rental income of $7,405, $6,791 and $5,787, respectively. We amortize the value of in place leases exclusive of the value of above market and below market in place leases to expense over the remaining non-cancelable periods of the respective leases. During the years ended December 31, 2020, 2019 and 2018, such amortization included in depreciation totaled $48,669, $64,203 and $72,925, respectively. If a lease is terminated prior to its stated expiration, the unamortized amount relating to that lease is written off.

CASH AND CASH EQUIVALENTS

CASH AND CASH EQUIVALENTS. We consider highly liquid investments with original maturities of three months or less at the date of purchase to be cash equivalents.

RESTRICTED CASH

RESTRICTED CASH. Restricted cash consists of amounts escrowed for real estate taxes, insurance and capital expenditures at certain of our mortgaged properties and cash held for the operations of one of our life science properties that is owned in a joint venture arrangement in which we own a 55% equity interest.

INVESTMENTS IN EQUITY SECURITIES

INVESTMENTS IN EQUITY SECURITIES. We previously owned 2,637,408 shares of class A common stock of The RMR Group Inc., or RMR Inc., that we sold on July 1, 2019. Prior to July 1, 2019, our equity securities were recorded at fair value based on their quoted market price at the end of each reporting period. We classify the common shares we own of Five Star Senior Living Inc., or Five Star, as equity securities and carry them at fair value in investments of equity securities in our consolidated balance sheets. Effective January 1, 2018, changes in the fair value of our equity securities were recorded through earnings in accordance with FASB Accounting Standards Update, or ASU, No. 2016-01, Recognition and Measurement of Financial Assets and Financial Liabilities. Prior to the adoption of ASU No. 2016-01, unrealized gains and losses were recorded as a component of cumulative other comprehensive income (loss) in shareholder's equity.

EQUITY METHOD INVESTMENTS

EQUITY METHOD INVESTMENTS. We account for our investment in Affiliates Insurance Company, or AIC, until AIC was dissolved as described in Note 8, using the equity method of accounting. Significant influence was present through common representation on our Board of Trustees and the board of directors of AIC until February 13, 2020. The Chair of our Board of Trustees and one of our Managing Trustees, Adam D. Portnoy, as the sole trustee of ABP Trust, is the controlling shareholder of RMR Inc. He is also a managing director and an executive officer of RMR Inc. Substantially all of the business of RMR Inc. is conducted by its majority owned subsidiary, The RMR Group LLC, or RMR LLC, which is our manager and provided management and administrative services to AIC. Most of our Trustees were directors of AIC. See Note 8 for more information about our investment in AIC. Refer to the above section for further details on our equity method investment in Five Star.

We periodically evaluate our equity method investments for possible indicators of other than temporary impairment whenever events or changes in circumstances indicate the carrying amount of the investment might not be recoverable. These indicators may include the length of time and the extent to which the market value of our investment is below our carrying value, the financial condition of our investees, our intent and ability to be a long term holder of the investment and other considerations. If the decline in fair value is judged to be other than temporary, we record an impairment charge to adjust the basis of the investment to its estimated fair value.

DEBT ISSUANCE COSTS

DEBT ISSUANCE COSTS. Debt issuance costs include issuance or assumption costs related to borrowings and we amortize those costs as interest expense over the terms of the respective loans.

DEFERRED LEASING COSTS

DEFERRED LEASING COSTS. Deferred leasing costs include capitalized brokerage and other fees associated with the successful negotiation of leases, which are amortized to depreciation and amortization expense on a straight line basis over the terms of the respective leases. Deferred leasing costs are included in other assets in our consolidated balance sheets.

REVENUE RECOGNITION

REVENUE RECOGNITION. We are a lessor of medical office and life science properties, senior living communities and other healthcare related properties. Our leases provide our tenants with the contractual right to use and economically benefit from all of the premises demised under the leases; therefore, we have determined to evaluate our leases as lease arrangements.

PER COMMON SHARE AMOUNTS

PER COMMON SHARE AMOUNTS. We calculate basic earnings per common share by dividing net income (loss) by the weighted average number of our common shares of beneficial interest, $.01 par value, or our common shares, outstanding during the period. We calculate diluted earnings per common share using the more dilutive of the two class method or the treasury stock method. Unvested share awards and other potentially dilutive common shares and the related impact on earnings, are considered when calculating diluted earnings per share.

INCOME TAXES

INCOME TAXES. We have elected to be taxed as a REIT under the United States Internal Revenue Code of 1986, as amended, and as such are generally not subject to federal and most state income taxation on our operating income provided we distribute our taxable income to our shareholders and meet certain organization and operating requirements. We do, however, lease our managed senior living communities to our wholly owned TRSs that, unlike most of our subsidiaries, file a separate consolidated federal corporate income tax return and are subject to federal and state income taxes. Our consolidated income tax provision includes the income tax provision related to the operations of our TRSs and certain state income taxes we incur despite our taxation as a REIT.

The Income Taxes Topic of the Codification prescribes how we should recognize, measure and present in our financial statements uncertain tax positions that have been taken or are expected to be taken in a tax return. Tax benefits are recognized to the extent that it is “more likely than not” that a particular tax position will be sustained upon examination or audit. To the extent the “more likely than not” standard has been satisfied, the benefit associated with a tax position is measured as the largest amount that has a greater than 50% likelihood of being realized upon settlement. We classify interest and penalties related to uncertain tax positions, if any, in our financial statements as a component of general and administrative expense.

SEGMENT REPORTING

SEGMENT REPORTING. As of December 31, 2020, we operate in, and report financial information for, the following two segments: our portfolio of medical office and life science properties, or our Office Portfolio, and our senior housing operating portfolio, or SHOP. We aggregate each of these two reporting segments based on their similar operating and economic characteristics.

NEW ACCOUNTING PRONOUNCEMENTS

NEW ACCOUNTING PRONOUNCEMENTS. In June 2016, the FASB issued ASU No. 2016-13, Financial Instruments - Credit Losses (Topic 326): Measurement of Credit Losses on Financial Instruments, which requires that entities use a new forward-looking “expected loss” model that generally will result in the earlier recognition of allowance for credit losses. The measurement of expected credit losses is based upon historical experience, current conditions, and reasonable and supportable forecasts that affect the collectability of the reported amount. We adopted this standard on January 1, 2020 using the modified retrospective approach. The implementation of this standard did not have a material impact on our consolidated financial statements.