Summary of Significant Accounting Policies (Policies)	12 Months Ended
Dec. 31, 2021
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 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.
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, 2021, 2020 and 2019, such amortization resulted in a net increase in rental income of $7,211, $7,405 and $6,791, 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, 2021, 2020 and 2019, such amortization included in depreciation totaled $42,783, $48,669 and $64,203, 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 amounts held as collateral pursuant to our credit agreement.
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 AlerisLife Inc. (f/k/a Five Star Senior Living Inc.), or AlerisLife, as an equity method investment. This equity method investment is included in investments of equity securities in our consolidated balance sheets.
EQUITY METHOD INVESTMENTS	EQUITY METHOD INVESTMENTS. As of December 31, 2021, we owned a 20% equity interest in an unconsolidated joint venture which owns a life science property located in Boston, Massachusetts. The property owned by this joint venture is encumbered by an aggregate $620,000 of mortgage debts. We do not control the activities that are most significant to this joint venture and, as a result, we account for our investment in this joint venture under the equity method of accounting under the fair value option. See Notes 3, 10 and 11 for more information regarding this joint venture.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. As previously discussed, we also account for our investment in AlerisLife as an equity method investment under the fair value option.
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, 2021, 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 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 Financial Accounting Standards Board, or FASB, issued Accounting Standards Update, or 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. In October 2021, the FASB issued ASU No. 2021-08, Business Combinations (Topic 805): Accounting for Contract Assets and Contract Liabilities from Contracts with Customers, or ASU No. 2021-08, which requires that an acquirer account for the related revenue contracts in accordance with Topic 606 as if it had originated the contracts. The acquiring entity shall recognize and measure the acquired contract assets and contract liabilities consistent with how they were recognized and measured in the acquiree’s financial statements, rather than at fair value at the acquisition date. ASU No. 2021-08 is effective for fiscal years beginning after December 15, 2022, with early adoption permitted. On October 1, 2021, we early adopted ASU No. 2021-08. The adoption of ASU No. 2021-08 did not have an impact on our consolidated financial statements. In November 2021, the FASB issued ASU No. 2021-10, Government Assistance (Topic 832): Disclosures by Business Entities about Government Assistance, or ASU No. 2021-10, which requires business entities to disclose government assistance accounted for by applying a grant or contribution model by analogy. ASU No. 2021-10 states that an entity shall disclose the nature of the transactions, the related accounting policies used, the effect of the transactions on an entity's financial statements and significant terms and conditions of the transactions. ASU No. 2021-10 is effective for annual periods beginning after December 15, 2021, with early adoption permitted. On November 17, 2021, we early adopted ASU No. 2021-10. The adoption of ASU No. 2021-10 did not have a material impact on our consolidated financial statements.

Summary of Significant Accounting Policies (Policies)

12 Months Ended

Dec. 31, 2021

BASIS OF PRESENTATION. Our consolidated financial statements include the accounts of Diversified Healthcare Trust, we, us or our, and our subsidiaries, all of which 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.

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, 2021, 2020 and 2019, such amortization resulted in a net increase in rental income of $7,211, $7,405 and $6,791, 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, 2021, 2020 and 2019, such amortization included in depreciation totaled $42,783, $48,669 and $64,203, 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 amounts held as collateral pursuant to our credit agreement.

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 AlerisLife Inc. (f/k/a Five Star Senior Living Inc.), or AlerisLife, as an equity method investment. This equity method investment is included in investments of equity securities in our consolidated balance sheets.

EQUITY METHOD INVESTMENTS

EQUITY METHOD INVESTMENTS. As of December 31, 2021, we owned a 20% equity interest in an unconsolidated joint venture which owns a life science property located in Boston, Massachusetts. The property owned by this joint venture is encumbered by an aggregate $620,000 of mortgage debts. We do not control the activities that are most significant to this joint venture and, as a result, we account for our investment in this joint venture under the equity method of accounting under the fair value option. See Notes 3, 10 and 11 for more information regarding this joint venture.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. As previously discussed, we also account for our investment in AlerisLife as an equity method investment under the fair value option.

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, 2021, 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 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 Financial Accounting Standards Board, or FASB, issued Accounting Standards Update, or 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.

In October 2021, the FASB issued ASU No. 2021-08, Business Combinations (Topic 805): Accounting for Contract Assets and Contract Liabilities from Contracts with Customers, or ASU No. 2021-08, which requires that an acquirer account for the related revenue contracts in accordance with Topic 606 as if it had originated the contracts. The acquiring entity shall recognize and measure the acquired contract assets and contract liabilities consistent with how they were recognized and measured in the acquiree’s financial statements, rather than at fair value at the acquisition date. ASU No. 2021-08 is effective for fiscal years beginning after December 15, 2022, with early adoption permitted. On October 1, 2021, we early adopted ASU No. 2021-08. The adoption of ASU No. 2021-08 did not have an impact on our consolidated financial statements.

In November 2021, the FASB issued ASU No. 2021-10, Government Assistance (Topic 832): Disclosures by Business Entities about Government Assistance, or ASU No. 2021-10, which requires business entities to disclose government assistance accounted for by applying a grant or contribution model by analogy. ASU No. 2021-10 states that an entity shall disclose the nature of the transactions, the related accounting policies used, the effect of the transactions on an entity's financial statements and significant terms and conditions of the transactions. ASU No. 2021-10 is effective for annual periods beginning after December 15, 2021, with early adoption permitted. On November 17, 2021, we early adopted ASU No. 2021-10. The adoption of ASU No. 2021-10 did not have a material impact on our consolidated financial statements.