Summary of Significant Accounting Policies (Policies)	12 Months Ended
Dec. 31, 2021
Accounting Policies [Abstract]
Basis of Presentation	Basis of Presentation. The consolidated financial statements include our investments in 100% owned subsidiaries and majority owned subsidiaries that are controlled by us. References to we, us, our and the Company, refer to Equity Commonwealth and its consolidated subsidiaries as of December 31, 2021, unless the context indicates otherwise. All intercompany transactions and balances have been eliminated. Dollar amounts presented may be approximate. Share amounts are presented in whole numbers, except where noted.
Real Estate Properties	Real Estate Properties. We record real estate properties at cost. We depreciate real estate investments on a straight-line basis over estimated useful lives of up to 40 years for buildings and improvements, and up to 12 years for personal property. Each time we enter into a new lease, or materially modify an existing lease, we evaluate its classification as either a finance or operating lease. The classification of a lease as finance or operating affects the carrying value of a property, as well as our recognition of rental payments as revenue. These evaluations require us to make estimates of, among other things, the remaining useful life and fair market value of a leased property, appropriate discount rates and future cash flows. We allocate the consideration paid for our properties among land, buildings and improvements and, for properties that qualify as acquired businesses under the Business Combinations Topic of the Financial Accounting Standards Board, or FASB, Accounting Standards Codification, or ASC, to identified intangible assets and liabilities, consisting of the value of above market and below market leases, the value of acquired in place leases and the value of tenant relationships. Purchase price allocations and the determination of useful lives are based on our estimates and, under some circumstances, studies from independent real estate appraisal firms to provide market information and evaluations that are relevant to our purchase price allocations and determinations of useful lives; however, we are ultimately responsible for the purchase price allocations and determination of useful lives. We allocate the consideration to land, buildings and improvements based on a determination of the fair values of these assets assuming the property is vacant. We determine the fair value of a property using methods that we believe are similar to those used by independent appraisers. Purchase price allocations for above market and below market leases are based on the estimated present value (using an interest rate which reflects our assessment of the risks associated with the leases acquired) of the difference between (1) the contractual amounts to be paid pursuant to the acquired in place leases and (2) our estimate of fair market lease rates for the corresponding leases, measured over a period equal to the remaining non-cancelable terms of the respective leases. Purchase price allocations to acquired in place leases and tenant relationships are determined as the excess of (1) the purchase price paid for a property after adjusting existing in place leases to estimated market rental rates over (2) the estimated fair value of the property as if vacant. We aggregate this value between acquired in place lease values and tenant relationships based on our evaluation of the specific characteristics of each tenant's lease; however, the value of tenant relationships has not been separated from acquired in place lease value for our properties because we believe such value and related amortization expense is immaterial for acquisitions reflected in our historical financial statements. We consider certain factors in performing these analyses including estimates of carrying costs during the expected lease up periods, including real estate taxes, insurance and other operating income and expenses and costs to execute similar leases in current market conditions, such as leasing commissions, legal and other related costs. If we believe the value of tenant relationships is material in the future, those amounts will be separately allocated and amortized over the estimated lives of the relationships. We recognize the excess, if any, of the consideration paid over amounts allocated to land, buildings and improvements and identified intangible assets and liabilities as goodwill and we recognize gains if amounts allocated exceed the consideration paid. We amortize capitalized above market lease values as a reduction to rental income over the remaining terms of the respective leases. We amortize capitalized below market lease values as an increase to rental income over the remaining terms of the respective leases. We amortize the value of acquired in place leases exclusive of the value of above market and below market acquired in place leases to expense over the remaining terms of the respective leases. If a lease is terminated prior to its stated expiration, the unamortized lease intangibles relating to that lease is written off. We review our properties for impairment quarterly, or whenever events or changes in circumstances indicate that their carrying amounts may not be recoverable. Impairment indicators may include our decision to dispose of an asset before the end of its estimated useful life, declining tenant occupancy, lack of progress releasing vacant space, tenant bankruptcies, low long-term prospects for improvement in property performance, weak or declining tenant profitability, and cash flow or liquidity. When indicators of potential impairment are present that suggest that the carrying amounts of real estate assets may not be recoverable, we assess the recoverability of these assets by determining whether the respective carrying values will be recovered through the estimated undiscounted future operating cash flows expected from the use of the assets and their eventual disposition. The determination of undiscounted cash flow includes consideration of many factors including income to be earned from the investment over our anticipated hold period, holding costs (exclusive of interest), estimated selling prices, and prevailing economic and market conditions. In the event that such expected undiscounted future cash flows do not exceed the carrying values, we estimate the fair value of the assets and record an impairment charge equal to the amount by which the carrying value exceeds the estimated fair value. Estimated fair values are calculated based on the following information, (i) recent third party estimates of market value, (ii) market prices for comparable properties, or (iii) the present value of future cash flows. During the years ended December 31, 2021, 2020 and 2019 we did not record any loss on asset impairment. When we classify properties as held for sale, we discontinue the recording of depreciation expense and estimate their fair value less costs to sell. If we determine that the carrying value for these properties exceed their estimated fair value less costs to sell, we record a loss on asset impairment. As of December 31, 2021 and 2020, we did not have any properties classified as held for sale. Certain of our real estate assets contain hazardous substances, including asbestos. We believe any asbestos in our buildings is contained in accordance with current regulations. If we remove the asbestos or renovate or demolish these properties, certain environmental regulations govern the manner in which the asbestos must be handled and removed. We do not believe that there are other environmental conditions or issues at any of our properties that have had or will have a material adverse effect on us. However, no assurances can be given that conditions or issues are not present at our properties or that costs we may be required to incur in the future to remediate contamination or comply with environmental, health and safety laws will not have a material adverse effect on our business or financial condition.
Cash and Cash Equivalents	Cash and Cash Equivalents. Our cash and cash equivalents consist of cash maintained in time deposits, depository accounts and money market accounts. We regularly monitor the credit ratings of the financial institutions holding our deposits to minimize our exposure to credit risk. Throughout the year, we have cash balances in excess of federally insured limits deposited with various financial institutions. We do not believe we are exposed to any significant credit risk on cash and cash equivalents.
Restricted Cash	Restricted Cash. Restricted cash consisted of amounts escrowed for future real estate taxes, insurance, leasing costs, capital expenditures and debt service, as required by our mortgage debt, as well as security deposits paid to us by some of our tenants.
Other Assets, Net	Other Assets, Net. Other assets consist principally of deferred leasing costs, capitalized lease incentives and prepaid property operating expenses. Deferred leasing costs are amortized on a straight-line basis over the terms of the respective leases. Capitalized lease incentives are amortized on a straight-line basis against rental income over the terms of the respective leases.
Revenue Recognition	Revenue Recognition. Rental revenue from operating leases, which includes rent concessions (including free rent and other lease incentives) and scheduled increases in rental rates during the lease term, is recognized on a straight-line basis over the life of the lease agreements. We defer the recognition of contingent rental income, such as percentage rents, until the specific targets that trigger the contingent rental income are achieved. Rental revenue also includes property level operating expenses reimbursed by our tenants, as well as other incidental revenues, which are recorded as expenses are incurred.
Lessee Lease Classification	Lessee Lease Classification. We classify leases as either finance or operating in accordance with FASB Topic 842, Leases. This classification determines whether the related expense is recognized based on an effective interest method for finance leases or on a straight-line basis over its life for operating leases. Additionally, lessees are required to record a right-of-use asset and lease liability for all leases with a term greater than 12 months. We have made an accounting policy election as permitted under ASC 842 to forgo recognition of a right-of-use asset and lease liability for short-term leases of less than 12 months.
Earnings Per Common Share	Earnings Per Common Share. Earnings per common share, or EPS, is computed using the weighted average number of common shares outstanding during the period. Diluted EPS reflects the potential dilution that could occur if our series D convertible preferred shares, our restricted share units, or RSUs, or beneficial interests in the Operating Trust, or LTIP Units, were converted into our common shares, which could result in a lower EPS amount.
Reclassifications	Reclassifications. Reclassifications have been made to the prior years' financial statements and notes to conform to the current year's presentation.
Legal Matters	Legal Matters. We are or may become a party to various legal proceedings. We are not currently involved in any litigation nor, to our knowledge, is any litigation threatened against us where the outcome would, in our judgment based on information currently available to us, have a material adverse effect on the Company.
Income Taxes	Income Taxes. We are a REIT under the Internal Revenue Code of 1986, as amended, and are generally not subject to federal and state income taxes provided we distribute our taxable income to our shareholders and meet other requirements for qualifying as a REIT. We are also subject to certain state and local taxes without regard to our REIT status. The Income Taxes Topic of the FASB ASC 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. Deferred tax assets 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.
Use of Estimates	Use of Estimates. Preparation of these financial statements in conformity with U.S. generally accepted accounting principles, or GAAP, requires 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.
New Accounting Pronouncements	New Accounting Pronouncements. In February 2016, the FASB issued ASU 2016-02, Leases (Topic 842), which sets out the principles for the recognition, measurement, presentation and disclosure of leases for both parties to a contract (i.e., lessees and lessors). We adopted ASU 2016-02 and the related amendments on January 1, 2019, and the adoption did not have a material impact to our financial statements. Certain reclassifications were made to conform the prior period to our presentation of the condensed consolidated statements of operations as a result of adopting ASU 2016-02. Amounts that were previously disclosed as "Tenant reimbursements and other income" are now included in "Rental revenue" and are no longer presented as a separate line item. Parking revenues that do not represent components of leases and were previously disclosed as "Rental income" are now included in "Other revenue." Subsequent to January 1, 2019, provisions for credit losses are included in "Rental revenue." Provisions for credit losses prior to January 1, 2019 were disclosed as "Operating expenses" and were not reclassified to conform prior periods to the current presentation.

Summary of Significant Accounting Policies (Policies)

12 Months Ended

Dec. 31, 2021

Basis of Presentation. The consolidated financial statements include our investments in 100% owned subsidiaries and majority owned subsidiaries that are controlled by us. References to we, us, our and the Company, refer to Equity Commonwealth and its consolidated subsidiaries as of December 31, 2021, unless the context indicates otherwise. All intercompany transactions and balances have been eliminated.

Dollar amounts presented may be approximate. Share amounts are presented in whole numbers, except where noted.

Real Estate Properties

Real Estate Properties. We record real estate properties at cost. We depreciate real estate investments on a straight-line basis over estimated useful lives of up to 40 years for buildings and improvements, and up to 12 years for personal property.

Each time we enter into a new lease, or materially modify an existing lease, we evaluate its classification as either a finance or operating lease. The classification of a lease as finance or operating affects the carrying value of a property, as well as our recognition of rental payments as revenue. These evaluations require us to make estimates of, among other things, the remaining useful life and fair market value of a leased property, appropriate discount rates and future cash flows.

We allocate the consideration paid for our properties among land, buildings and improvements and, for properties that qualify as acquired businesses under the Business Combinations Topic of the Financial Accounting Standards Board, or FASB, Accounting Standards Codification, or ASC, to identified intangible assets and liabilities, consisting of the value of above market and below market leases, the value of acquired in place leases and the value of tenant relationships. Purchase price allocations and the determination of useful lives are based on our estimates and, under some circumstances, studies from independent real estate appraisal firms to provide market information and evaluations that are relevant to our purchase price allocations and determinations of useful lives; however, we are ultimately responsible for the purchase price allocations and determination of useful lives.

We allocate the consideration to land, buildings and improvements based on a determination of the fair values of these assets assuming the property is vacant. We determine the fair value of a property using methods that we believe are similar to those used by independent appraisers. Purchase price allocations for above market and below market leases are based on the estimated present value (using an interest rate which reflects our assessment of the risks associated with the leases acquired) of the difference between (1) the contractual amounts to be paid pursuant to the acquired in place leases and (2) our estimate of fair market lease rates for the corresponding leases, measured over a period equal to the remaining non-cancelable terms of the respective leases. Purchase price allocations to acquired in place leases and tenant relationships are determined as the excess of (1) the purchase price paid for a property after adjusting existing in place leases to estimated market rental rates over (2) the

estimated fair value of the property as if vacant. We aggregate this value between acquired in place lease values and tenant relationships based on our evaluation of the specific characteristics of each tenant's lease; however, the value of tenant relationships has not been separated from acquired in place lease value for our properties because we believe such value and related amortization expense is immaterial for acquisitions reflected in our historical financial statements. We consider certain factors in performing these analyses including estimates of carrying costs during the expected lease up periods, including real estate taxes, insurance and other operating income and expenses and costs to execute similar leases in current market conditions, such as leasing commissions, legal and other related costs. If we believe the value of tenant relationships is material in the future, those amounts will be separately allocated and amortized over the estimated lives of the relationships. We recognize the excess, if any, of the consideration paid over amounts allocated to land, buildings and improvements and identified intangible assets and liabilities as goodwill and we recognize gains if amounts allocated exceed the consideration paid.

We amortize capitalized above market lease values as a reduction to rental income over the remaining terms of the respective leases. We amortize capitalized below market lease values as an increase to rental income over the remaining terms of the respective leases. We amortize the value of acquired in place leases exclusive of the value of above market and below market acquired in place leases to expense over the remaining terms of the respective leases. If a lease is terminated prior to its stated expiration, the unamortized lease intangibles relating to that lease is written off.

We review our properties for impairment quarterly, or whenever events or changes in circumstances indicate that their carrying amounts may not be recoverable. Impairment indicators may include our decision to dispose of an asset before the end of its estimated useful life, declining tenant occupancy, lack of progress releasing vacant space, tenant bankruptcies, low long-term prospects for improvement in property performance, weak or declining tenant profitability, and cash flow or liquidity. When indicators of potential impairment are present that suggest that the carrying amounts of real estate assets may not be recoverable, we assess the recoverability of these assets by determining whether the respective carrying values will be recovered through the estimated undiscounted future operating cash flows expected from the use of the assets and their eventual disposition. The determination of undiscounted cash flow includes consideration of many factors including income to be earned from the investment over our anticipated hold period, holding costs (exclusive of interest), estimated selling prices, and prevailing economic and market conditions. In the event that such expected undiscounted future cash flows do not exceed the carrying values, we estimate the fair value of the assets and record an impairment charge equal to the amount by which the carrying value exceeds the estimated fair value. Estimated fair values are calculated based on the following information, (i) recent third party estimates of market value, (ii) market prices for comparable properties, or (iii) the present value of future cash flows. During the years ended December 31, 2021, 2020 and 2019 we did not record any loss on asset impairment.

When we classify properties as held for sale, we discontinue the recording of depreciation expense and estimate their fair value less costs to sell. If we determine that the carrying value for these properties exceed their estimated fair value less costs to sell, we record a loss on asset impairment. As of December 31, 2021 and 2020, we did not have any properties classified as held for sale.

Certain of our real estate assets contain hazardous substances, including asbestos. We believe any asbestos in our buildings is contained in accordance with current regulations. If we remove the asbestos or renovate or demolish these properties, certain environmental regulations govern the manner in which the asbestos must be handled and removed. We do not believe that there are other environmental conditions or issues at any of our properties that have had or will have a material adverse effect on us. However, no assurances can be given that conditions or issues are not present at our properties or that costs we may be required to incur in the future to remediate contamination or comply with environmental, health and safety laws will not have a material adverse effect on our business or financial condition.

Cash and Cash Equivalents

Cash and Cash Equivalents. Our cash and cash equivalents consist of cash maintained in time deposits, depository accounts and money market accounts. We regularly monitor the credit ratings of the financial institutions holding our deposits to minimize our exposure to credit risk. Throughout the year, we have cash balances in excess of federally insured limits deposited with various financial institutions. We do not believe we are exposed to any significant credit risk on cash and cash equivalents.

Restricted Cash

Restricted Cash. Restricted cash consisted of amounts escrowed for future real estate taxes, insurance, leasing costs, capital expenditures and debt service, as required by our mortgage debt, as well as security deposits paid to us by some of our tenants.

Other Assets, Net

Other Assets, Net. Other assets consist principally of deferred leasing costs, capitalized lease incentives and prepaid property operating expenses. Deferred leasing costs are amortized on a straight-line basis over the terms of the respective leases. Capitalized lease incentives are amortized on a straight-line basis against rental income over the terms of the respective leases.

Revenue Recognition

Revenue Recognition. Rental revenue from operating leases, which includes rent concessions (including free rent and other lease incentives) and scheduled increases in rental rates during the lease term, is recognized on a straight-line basis over the life of the lease agreements. We defer the recognition of contingent rental income, such as percentage rents, until the specific targets that trigger the contingent rental income are achieved. Rental revenue also includes property level operating expenses reimbursed by our tenants, as well as other incidental revenues, which are recorded as expenses are incurred.

Lessee Lease Classification

Lessee Lease Classification. We classify leases as either finance or operating in accordance with FASB Topic 842, Leases. This classification determines whether the related expense is recognized based on an effective interest method for finance leases or on a straight-line basis over its life for operating leases. Additionally, lessees are required to record a right-of-use asset and lease liability for all leases with a term greater than 12 months. We have made an accounting policy election as permitted under ASC 842 to forgo recognition of a right-of-use asset and lease liability for short-term leases of less than 12 months.

Earnings Per Common Share

Earnings Per Common Share. Earnings per common share, or EPS, is computed using the weighted average number of common shares outstanding during the period. Diluted EPS reflects the potential dilution that could occur if our series D convertible preferred shares, our restricted share units, or RSUs, or beneficial interests in the Operating Trust, or LTIP Units, were converted into our common shares, which could result in a lower EPS amount.

Reclassifications

Reclassifications. Reclassifications have been made to the prior years' financial statements and notes to conform to the current year's presentation.

Legal Matters

Legal Matters. We are or may become a party to various legal proceedings. We are not currently involved in any litigation nor, to our knowledge, is any litigation threatened against us where the outcome would, in our judgment based on information currently available to us, have a material adverse effect on the Company.

Income Taxes

Income Taxes. We are a REIT under the Internal Revenue Code of 1986, as amended, and are generally not subject to federal and state income taxes provided we distribute our taxable income to our shareholders and meet other requirements for qualifying as a REIT. We are also subject to certain state and local taxes without regard to our REIT status.

The Income Taxes Topic of the FASB ASC 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. Deferred tax assets 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.

Use of Estimates

Use of Estimates. Preparation of these financial statements in conformity with U.S. generally accepted accounting principles, or GAAP, requires 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.

New Accounting Pronouncements

New Accounting Pronouncements. In February 2016, the FASB issued ASU 2016-02, Leases (Topic 842), which sets out the principles for the recognition, measurement, presentation and disclosure of leases for both parties to a contract (i.e., lessees and lessors). We adopted ASU 2016-02 and the related amendments on January 1, 2019, and the adoption did not have a material impact to our financial statements. Certain reclassifications were made to conform the prior period to our presentation of the condensed consolidated statements of operations as a result of adopting ASU 2016-02. Amounts that were previously disclosed as "Tenant reimbursements and other income" are now included in "Rental revenue" and are no longer presented as a separate line item. Parking revenues that do not represent components of leases and were previously disclosed as "Rental income" are now included in "Other revenue." Subsequent to January 1, 2019, provisions for credit losses are included in "Rental revenue." Provisions for credit losses prior to January 1, 2019 were disclosed as "Operating expenses" and were not reclassified to conform prior periods to the current presentation.