Equity Commonwealth (EQC) 10-K 14 Feb 19 2018 FY Annual report Financial data

Document and Entity Information

Document and Entity Information - USD ($) $ in Billions	12 Months Ended
	Dec. 31, 2018	Feb. 07, 2019	Jun. 29, 2018
Document and Entity Information
Entity Registrant Name	Equity Commonwealth
Entity Central Index Key	803,649
Document Type	10-K
Document Period End Date	Dec. 31, 2018
Amendment Flag	false
Current Fiscal Year End Date	--12-31
Document Fiscal Year Focus	2,018
Document Fiscal Period Focus	FY
Entity Well-known Seasoned Issuer	Yes
Entity Current Reporting Status	Yes
Entity Shell Company	false
Entity Voluntary Filers	No
Entity Filer Category	Large Accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	false
Entity Common Stock, Shares Outstanding (in shares)		121,684,514
Entity Public Float			$ 3.8

CONSOLIDATED BALANCE SHEETS

CONSOLIDATED BALANCE SHEETS - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Real estate properties:
Land	$ 135,142	$ 191,775
Buildings and improvements	1,004,500	1,555,836
Total real estate properties, at cost, gross	1,139,642	1,747,611
Accumulated depreciation	(375,968)	(450,718)
Total real estate properties, at cost, net	763,674	1,296,893
Assets held for sale	0	97,688
Acquired real estate leases, net	275	23,847
Cash and cash equivalents	2,400,803	2,351,693
Marketable securities	249,602	276,928
Restricted cash	3,298	8,897
Rents receivable, net of allowance for doubtful accounts of $4,974 and $4,771, respectively	51,089	93,436
Other assets, net	62,031	87,563
Total assets	3,530,772	4,236,945
LIABILITIES AND EQUITY
Senior unsecured debt, net	248,473	815,984
Mortgage notes payable, net	26,482	32,594
Liabilities related to properties held for sale	0	1,840
Accounts payable, accrued expenses and other	62,368	74,956
Rent collected in advance	9,451	11,076
Total liabilities	346,774	936,450
Commitments and contingencies
Shareholders’ equity:
Common shares of beneficial interest, $0.01 par value: 350,000,000 shares authorized; 121,572,155 and 124,217,616 shares issued and outstanding, respectively	1,216	1,242
Additional paid in capital	4,305,974	4,380,313
Cumulative net income	2,870,974	2,596,259
Cumulative other comprehensive loss	(342)	(95)
Cumulative common distributions	(3,420,548)	(3,111,868)
Cumulative preferred distributions	(693,736)	(685,748)
Total shareholders’ equity	3,182,801	3,299,366
Noncontrolling interest	1,197	1,129
Total equity	3,183,998	3,300,495
Total liabilities and equity	3,530,772	4,236,945
Series D
Shareholders’ equity:
Series D preferred shares; 6 1/2% cumulative convertible; 4,915,196 shares issued and outstanding, aggregate liquidation preference of $122,880	119,263	119,263
Total equity	$ 119,263	$ 119,263

CONSOLIDATED BALANCE SHEETS (Pa

CONSOLIDATED BALANCE SHEETS (Parenthetical) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Rents receivable, allowance for doubtful accounts	$ 4,974	$ 4,771
Common shares of beneficial interest, par value (in dollars per share)	$ 0.01	$ 0.01
Common shares of beneficial interest, shares authorized (in shares)	350,000,000	350,000,000
Common shares of beneficial interest, shares issued (in shares)	121,572,155	124,217,616
Common shares of beneficial interest, shares outstanding (in shares)	121,572,155	124,217,616
Series D
Preferred shares of beneficial interest, par value (in dollars per share)	$ 0.01	$ 0.01
Preferred shares of beneficial interest, shares authorized (in shares)	50,000,000	50,000,000
Preferred shares, dividend yield	6.50%	6.50%
Preferred shares of beneficial interest, shares issued (in shares)	4,915,196	4,915,196
Preferred shares, of beneficial interest, shares outstanding (in shares)	4,915,196	4,915,196
Preferred shares, aggregate liquidation preference	$ 122,880	$ 122,880
Common shares of beneficial interest, shares issued (in shares)	4,915,196	4,915,196

CONSOLIDATED STATEMENTS OF OPER

CONSOLIDATED STATEMENTS OF OPERATIONS - USD ($) shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Revenues:
Rental income	$ 144,425	$ 270,320	$ 409,071
Total revenues	197,022	340,571	500,680
Expenses:
Operating expenses	79,916	141,425	200,706
Depreciation and amortization	49,041	90,708	131,806
General and administrative	44,439	47,760	50,256
Loss on asset impairment	12,087	19,714	58,476
Total expenses	185,483	299,607	441,244
Operating income	11,539	40,964	59,436
Interest and other income, net	46,815	26,380	10,331
Interest expense (including net amortization of debt discounts, premiums and deferred financing fees of $2,553, $3,135, and $3,725, respectively)	(26,585)	(52,183)	(84,329)
Loss on early extinguishment of debt	(7,122)	(493)	(2,680)
Foreign currency exchange loss	0	0	(5)
Gain on sale of properties, net	251,417	15,498	250,886
Income before income taxes	276,064	30,166	233,639
Income tax expense	(3,156)	(500)	(745)
Net income	272,908	29,666	232,894
Net income attributable to noncontrolling interest	(95)	(10)	0
Net income attributable to Equity Commonwealth	272,813	29,656	232,894
Preferred distributions	(7,988)	(7,988)	(17,956)
Excess fair value of consideration paid over carrying value of preferred shares	0	0	(9,609)
Net income attributable to Equity Commonwealth common shareholders	$ 264,825	$ 21,668	$ 205,329
Weighted average common shares outstanding — basic (in shares)	122,314	124,125	125,474
Weighted average common shares outstanding — diluted (in shares)	123,385	125,129	126,768
Earnings per common share attributable to Equity Commonwealth common shareholders:
Basic (in dollars per share)	$ 2.17	$ 0.17	$ 1.64
Diluted (in dollars per share)	$ 2.15	$ 0.17	$ 1.62
Tenant reimbursements and other income
Revenues:
Revenues	$ 52,597	$ 70,251	$ 91,609

CONSOLIDATED STATEMENTS OF OP_2

CONSOLIDATED STATEMENTS OF OPERATIONS (Parenthetical) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Statement [Abstract]
Debt discounts, premiums and deferred financing fees	$ 2,553	$ 3,135	$ 3,725

CONSOLIDATED STATEMENTS OF COMP

CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Statement of Comprehensive Income [Abstract]
Net income	$ 272,908	$ 29,666	$ 232,894
Other comprehensive income (loss), net of tax:
Unrealized gain (loss) on derivative instruments and other assets	456	(248)	3,479
Unrealized gain, net on marketable securities	1,199	361	0
Total comprehensive income	274,563	29,779	236,373
Comprehensive income attributable to the noncontrolling interest	(95)	(10)	0
Total comprehensive income attributable to Equity Commonwealth	$ 274,468	$ 29,769	$ 236,373

CONSOLIDATED STATEMENTS OF SHAR

CONSOLIDATED STATEMENTS OF SHAREHOLDERS' EQUITY - USD ($) $ in Thousands	Total	Series D	Series E	Cumulative Preferred Distributions	Common Shares	Cumulative Common Distributions	Additional Paid in Capital	Cumulative Net Income	Cumulative Other Comprehensive Loss	Noncontrolling Interest
Balance (in shares) at Dec. 31, 2015		4,915,196	11,000,000		126,349,914
Balance at Dec. 31, 2015	$ 3,368,487	$ 119,263	$ 265,391	$ (650,195)	$ 1,263	$ (3,111,868)	$ 4,414,611	$ 2,333,709	$ (3,687)	$ 0
Comprehensive income:
Net income	232,894							232,894		0
Unrealized gain (loss) on derivative instrument	3,479								3,479
Unrealized gain on marketable securities	0
Total comprehensive income	236,373
Repurchase of shares (in shares)					(2,491,675)
Repurchase of shares	(69,987)				$ (25)		(69,962)			0
Redemption of shares (in shares)		0	(11,000,000)		0
Redemption of shares	(275,000)	$ 0	$ (275,000)	0	$ 0		0
Excess fair value of consideration paid over carrying value of preferred shares	(9,609)		$ 9,609	(9,609)
Share-based compensation (in shares)					136,226
Share-based compensation	18,530				$ 2		18,528			0
Distributions	(17,956)			(17,956)		0				0
Balance (in shares) at Dec. 31, 2016		4,915,196	0		123,994,465
Balance at Dec. 31, 2016	3,260,447	$ 119,263	$ 0	(677,760)	$ 1,240	(3,111,868)	4,363,177	2,566,603	(208)	0
Comprehensive income:
Net income	29,666							29,656		10
Unrealized gain (loss) on derivative instrument	(248)								(248)
Unrealized gain on marketable securities	361								361	0
Total comprehensive income	29,779
Repurchase of shares (in shares)					(43,329)
Repurchase of shares	(3,188)				$ 0		(3,188)			0
Excess fair value of consideration paid over carrying value of preferred shares	0
Share-based compensation (in shares)					266,480
Share-based compensation	21,414				$ 2		20,156			1,256
Contributions	31									31
Distributions	(7,988)			(7,988)		0				0
Adjustment for noncontrolling interest	$ 0						168			(168)
Balance (in shares) at Dec. 31, 2017	124,217,616	4,915,196	0		124,217,616
Balance at Dec. 31, 2017	$ 3,300,495	$ 119,263	$ 0	(685,748)	$ 1,242	(3,111,868)	4,380,313	2,596,259	(95)	1,129
Comprehensive income:
Reclassification pursuant to change in accounting principle	0
Reclassification pursuant to change in accounting principle | Accounting Standards Update 2016-01								1,902	(1,902)
Net income	272,908							272,813		95
Unrealized gain (loss) on derivative instrument	456								456
Unrealized gain on marketable securities	1,199								1,199
Total comprehensive income	274,563
Repurchase of shares (in shares)					(3,163,730)
Repurchase of shares	(93,976)				$ (31)		(93,945)
Excess fair value of consideration paid over carrying value of preferred shares	0
Share-based compensation (in shares)					518,269
Share-based compensation	19,697				$ 5		18,371			1,321
Contributions	1									1
Distributions	(316,782)			(7,988)		(308,680)				(114)
Adjustment for noncontrolling interest	$ 0						1,235			(1,235)
Balance (in shares) at Dec. 31, 2018	121,572,155	4,915,196	0		121,572,155
Balance at Dec. 31, 2018	$ 3,183,998	$ 119,263	$ 0	$ (693,736)	$ 1,216	$ (3,420,548)	$ 4,305,974	$ 2,870,974	$ (342)	$ 1,197

CONSOLIDATED STATEMENTS OF CASH

CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
CASH FLOWS FROM OPERATING ACTIVITIES:
Net income	$ 272,908	$ 29,666	$ 232,894
Adjustments to reconcile net income to cash provided by operating activities:
Depreciation	40,386	73,169	102,695
Net amortization of debt discounts, premiums and deferred financing fees	2,553	3,135	3,725
Straight line rental income	(4,971)	(14,425)	(14,083)
Amortization of acquired real estate leases	2,187	8,994	21,129
Other amortization	6,127	9,989	14,513
Share-based compensation	19,697	21,414	18,530
Loss on asset impairment	12,087	19,714	58,476
Loss on early extinguishment of debt	7,122	493	2,680
Loss on sale of marketable securities	4,987	0	0
Foreign currency exchange loss	0	0	5
Net gain on sale of properties	(251,417)	(15,498)	(250,886)
Loss on sale of real estate mortgage receivable	2,117	0	0
Change in assets and liabilities:
Rents receivable and other assets	(19,886)	(23,708)	(23,921)
Accounts payable, accrued expenses and other	(704)	(9,384)	4,927
Rent collected in advance	(3,657)	(3,563)	(6,981)
Cash provided by operating activities	89,536	99,996	163,703
CASH FLOWS FROM INVESTING ACTIVITIES:
Real estate acquisitions	0	0	(2,802)
Real estate improvements	(49,930)	(64,813)	(121,450)
Insurance proceeds received	1,443	4,000	500
Principal payments received from real estate mortgages receivable	0	313	0
Proceeds from sale of properties, net	961,079	802,324	1,149,471
Proceeds from sale of real estate mortgage receivable	5,599	0	0
Purchase of marketable securities	0	(276,238)	0
Proceeds from sale of marketable securities	23,933	0	0
Cash provided by investing activities	942,124	465,586	1,025,719
CASH FLOWS FROM FINANCING ACTIVITIES:
Repurchase and retirement of common shares	(93,976)	(3,188)	(69,987)
Redemption of preferred shares	0	0	(275,000)
Payments on borrowings	(581,460)	(295,053)	(560,187)
Deferred financing fees	0	0	(52)
Contributions from holders of noncontrolling interest	1	31	0
Distributions to common shareholders	(304,612)	0	0
Distributions to preferred shareholders	(7,988)	(7,988)	(17,956)
Distributions to holders of noncontrolling interest	(114)	0	0
Cash used in financing activities	(988,149)	(306,198)	(923,182)
Effect of exchange rate changes on cash	0	0	(8)
Increase in cash, cash equivalents, and restricted cash	43,511	259,384	266,232
Cash, cash equivalents, and restricted cash at beginning of year	2,360,590	2,101,206	1,834,974
Cash, cash equivalents, and restricted cash at end of year	2,404,101	2,360,590	2,101,206
SUPPLEMENTAL CASH FLOW INFORMATION:
Interest paid	27,117	56,796	85,310
Taxes paid	2,264	910	327
NON-CASH INVESTING AND FINANCING ACTIVITIES:
Increase (decrease) in accrued capital expenditures	6,372	(3,970)	(10,331)
Total cash, cash equivalents, and restricted cash shown in the statement of cash flows	$ 2,360,590	$ 2,101,206	$ 1,834,974

Organization

Organization	12 Months Ended
	Dec. 31, 2018
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Organization	Organization Equity Commonwealth (the Company) is a real estate investment trust, or REIT, formed in 1986 under the laws of the State of Maryland. Our business is primarily the ownership and operation of office buildings in the United States. On November 10, 2016, the Company converted to what is commonly referred to as an umbrella partnership real estate investment trust, or UPREIT. In connection with this conversion, the Company contributed substantially all of its assets to EQC Operating Trust, a Maryland real estate investment trust (the Operating Trust), and the Operating Trust assumed substantially all of the Company’s liabilities pursuant to a contribution and assignment agreement between the Company and the Operating Trust. The Company now conducts and intends to continue to conduct substantially all of its activities through the Operating Trust. The Company beneficially owned, 99.96% of the outstanding shares of beneficial interest, designated as units, in the Operating Trust (OP Units) as of December 31, 2018 , and the Company is the sole trustee of the Operating Trust. As the sole trustee, the Company generally has the power under the declaration of trust of the Operating Trust to manage and conduct the business of the Operating Trust, subject to certain limited approval and voting rights of other holders of OP Units. At December 31, 2018 , our portfolio, consisted of 10 properties ( 18 buildings), with a combined 5.1 million square feet. As of December 31, 2018 , we had $2.7 billion of cash and cash equivalents and marketable securities. All numbers of properties, numbers of buildings and square feet are unaudited.

Summary of Significant Accounti

Summary of Significant Accounting Policies	12 Months Ended
	Dec. 31, 2018
Accounting Policies [Abstract]
Summary of Significant Accounting Policies	Summary of Significant Accounting Policies 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, 2018 , unless the context indicates otherwise. All intercompany transactions and balances have been eliminated. 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 capital or operating lease. The classification of a lease as capital 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 (FASB) Accounting Standards Codification (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 to 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 (presented in our consolidated balance sheets as acquired real estate leases) 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, 2018 , 2017 , and 2016 we recorded a loss on asset impairment in continuing operations totaling $12.1 million , $19.7 million and $58.5 million respectively, to reduce the carrying value of properties to their estimated fair values (see Note 15). 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. 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. As of December 31, 2018 and 2017 , we did not have any accrued environmental remediation costs. Cash and Cash Equivalents. Our cash and cash equivalents consist of cash maintained in time deposits, depository accounts and money market accounts. We continually 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. Marketable Securities. All of our marketable securities are classified as available-for-sale and consisted of United States Treasury notes and common stock. Available-for-sale securities are presented on our consolidated balance sheets at fair value. Changes in values of the United States Treasury notes are recognized in cumulative other comprehensive loss. Realized gains and losses are recognized in earnings only upon the sale of the United States Treasury notes. Changes in values of common stock prior to their sale in March 2018, were recognized in interest and other income, net on the consolidated statements of operations. We evaluate our marketable securities for impairment each reporting period. For securities with unrealized losses, we review the underlying cause of the decline in value and the estimated recovery period, as well as the severity and duration of the decline. In our evaluation, we consider our ability and intent to hold these investments for a reasonable period of time sufficient for us to recover our cost basis. To the extent an other-than-temporary impairment is deemed to have occurred, an impairment charge is recorded and a new cost basis is established. Restricted Cash. Restricted cash consists 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 consist principally of deferred financing fees, deferred leasing costs, capitalized lease incentives and prepaid property operating expenses. Deferred financing fees include issuance costs related to borrowings and are capitalized and amortized over the terms of the respective loans. Deferred leasing costs include brokerage, legal and other fees associated with the successful negotiation of leases and 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. Derivative Instruments. Derivative instruments are recognized as either assets or liabilities in the consolidated balance sheets at fair value. The accounting for changes in the fair value of a derivative instrument depends on whether such instrument has been designated and qualifies as part of a hedging relationship and, further, on the type of hedging relationship. For those derivative instruments that are designated and qualify as hedging instruments, we must designate the hedging instrument, based upon the exposure being hedged, as a fair value hedge, cash flow hedge or a hedge of a net investment in a foreign operation. As of December 31, 2018 , we do not have any interest rate derivatives. We are exposed to certain risks relating to our ongoing business operations. The primary risk managed by using derivative instruments is interest rate risk. Interest rate swaps and caps may be entered into to manage interest rate risk associated with floating rate borrowings. We designate interest rate swaps and caps as cash flow hedges of floating rate borrowings. Revenue Recognition. Rental income 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. Tenant reimbursements and other income includes property level operating expenses reimbursed by our tenants, as well as other incidental revenues, which are recorded as expenses are incurred. Allowance for Doubtful Accounts. We maintain an allowance for doubtful accounts for estimated losses resulting from the inability or unwillingness of certain tenants to make payments required under their leases. The computation of the allowance is based on the tenants' payment histories and current credit profiles, as well as other considerations. Share-Based Compensation. All share-based compensation is measured at fair value on the grant date or date of modification, as applicable, and recognized in earnings over the requisite service period. Depending upon the settlement terms of the awards, all or a portion of the fair value of share-based awards may be presented as a liability or as equity in the consolidated balance sheets. 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 (RSUs) or beneficial interests in the Operating Trust (LTIP Units) were converted into our common shares, which could result in a lower EPS amount. The effect of our series D convertible preferred shares on net income attributable to common shareholders is anti-dilutive for all periods presented. Reclassifications. Reclassifications have been made to the prior years' financial statements and notes to conform to the current year's presentation. 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. 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. 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. In August 2018, the FASB issued Accounting Standards Update (ASU) 2018-13, Fair Value Measurement (Topic 820): Disclosure Framework - Changes to the Disclosure Requirements for Fair Value Measurement, which changes the fair value measurement disclosure requirements of ASC 820. This update is effective for fiscal years beginning after December 15, 2019, and for interim periods within those fiscal years. We do not expect the adoption of ASU 2018-13 to have a material impact on our consolidated financial statements. In June 2018, the FASB issued ASU 2018-07, Improvements to Nonemployee Share-Based Payment Accounting, which simplifies the accounting for share-based payments granted to nonemployees for goods and services. This update is effective for fiscal years beginning after December 15, 2018, and for interim periods within those fiscal years. We do not expect the adoption of ASU 2018-07 to have a material impact on our consolidated financial statements. In May 2017, the FASB issued ASU 2017-09 Compensation-Stock Compensation (Topic 718): Scope of Modification Accounting. ASU 2017-09 is designed to provide clarity and reduce both (1) diversity in practice and (2) cost and complexity when applying the guidance in Topic 718, to a change to the terms or conditions of a share-based payment award. ASU 2017-09 is effective for fiscal years beginning after December 15, 2017. We adopted ASU 2017-09 on January 1, 2018, and the adoption did not have a material impact on our consolidated financial statements. In February 2017, the FASB issued ASU 2017-05 Clarifying the Scope of Asset Derecognition Guidance and Accounting for Partial Sales of Nonfinancial Assets. ASU 2017-05 is designed to provide guidance on how to recognize gain and losses on sales, including partial sales, of nonfinancial assets to noncustomers. We adopted ASU 2017-05 on January 1, 2018 and the adoption did not have a material impact on our consolidated financial statements. In June 2016, the FASB issued ASU 2016-13, Financial Instruments-Credit Losses (Topic 326): Measurement of Credit Losses on Financial Instruments, which requires more timely recognition of credit losses associated with financial assets. This update is effective for fiscal years beginning after December 15, 2019, and for interim periods within those fiscal years. Early adoption is permitted for fiscal years, and interim periods within those years, beginning after December 15, 2018. We are currently evaluating the impact, if any, the adoption of ASU 2016-13 will have on our consolidated financial statements. In February 2016, the FASB issued ASU 2016-02, Leases, 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). ASU 2016-02 requires lessees to apply a dual approach, classifying leases as either finance or operating leases based on the principle of whether or not the lease is effectively a financed purchase of the leased asset by the lessee. This classification will determine whether the lease expense is recognized based on an effective interest method or on a straight-line basis over the term of the lease. A lessee is also required to record a right-of-use asset and a lease liability for all leases with a term of greater than 12 months regardless of their classification. Leases with a term of 12 months or less will be accounted for similar to existing guidance for operating leases today. ASU 2016-02 supersedes previous leasing standards. ASU 2016-02 is effective for us for reporting periods beginning after December 15, 2018, with early adoption permitted. For leases where we are the lessor, we expect to account for these leases using an approach that is substantially equivalent to current guidance. Additionally, under ASU 2016-02 lessors may only capitalize incremental direct leasing costs. For leases in which we are the lessee, we expect to recognize a right-of-use asset and a lease liability equal to the present value of the minimum lease payments with rent expense being recognized on a straight-line basis and the right of use asset being reduced when lease payments are made. In July 2018, the FASB issued ASU 2018-11 to provide entities with relief from the costs of implementing certain aspects of ASU 2016-02. The amendment to the new leases standard includes a practical expedient that provides lessors an option not to separate lease and non-lease components when certain criteria are met and instead account for those components as a single component under the new leases standard. The amendment also provides a transition option that permits the application of the new guidance as of the adoption date rather than to all periods presented. We anticipate electing the practical expedient to account for both our lease and non-lease components as a single component under the leases standard and electing the new transition option. We believe the adoption of these pronouncements will have an immaterial impact on our financial statements, both as a lessor and as a lessee. In January 2016, the FASB issued ASU 2016-01, Financial Instruments-Overall: Recognition and Measurement of Financial Assets and Financial Liabilities, related to certain aspects of recognition, measurement, presentation, and disclosure of financial instruments. ASU 2016-01 will require entities to measure their equity investments at fair value and recognize any changes in fair value in net income, with certain exceptions, rather than other comprehensive income. This update is effective for fiscal years beginning after December 15, 2017, including interim periods within those fiscal years. We adopted ASU 2016-01 on January 1, 2018 and reclassified a $1.9 million unrealized gain from cumulative other comprehensive loss to cumulative net income on our consolidated balance sheet (see Note 10). In May 2014, the FASB issued ASU 2014-09, Revenue from Contracts with Customers. The objective of ASU 2014-09, as amended, is to establish a single comprehensive model for entities to use in accounting for revenue arising from contracts with customers and will supersede most of the existing revenue recognition guidance, including industry-specific guidance. The core principle is that an entity should recognize revenue to depict the transfer of promised goods or services to customers in an amount that reflects the consideration to which the entity expects to be entitled in exchange for those goods or services. In applying ASU 2014-09, companies will perform a five-step analysis of transactions to determine when and how revenue is recognized. ASU 2014-09 applies to all contracts with customers except those that are within the scope of other topics in the FASB’s ASC, and more particularly lease contracts with customers, which are a scope exception. ASU 2014-09 is effective for the first interim period within annual reporting periods beginning after December 15, 2017, with early adoption permitted. We adopted ASU 2014-09 on January 1, 2018 and the adoption did not have a material impact on our consolidated financial statements.

Real Estate Properties

Real Estate Properties	12 Months Ended
	Dec. 31, 2018
Real Estate [Abstract]
Real Estate Properties	Real Estate Properties Acquisitions and Expenditures In October 2016, we purchased a parcel of land adjacent to our Research Park property in Austin, Texas for $2.8 million . We did not make any acquisitions during the years ended December 31, 2018 or 2017 . During the years ended December 31, 2018 , 2017 , and 2016 , we made improvements, excluding tenant-funded improvements, to our properties totaling $56.3 million , $ 55.0 million and $110.7 million , respectively. We committed $98.7 million for expenditures related to 1.1 million square feet of leases executed during 2018 . Committed but unspent tenant related obligations are leasing commissions and tenant improvements. Based on existing leases as of December 31, 2018 , committed but unspent tenant related obligations were $86.6 million . Properties Held For Sale: We classify all properties that meet the criteria outlined in the Property, Plant and Equipment Topic of the FASB ASC as held for sale on our consolidated balance sheets. As of December 31, 2018 , we did no t have any properties classified as held for sale. As of December 31, 2017 , we classified 1600 Market Street as held for sale. Summarized balance sheet information for 1600 Market Street is as follows (in thousands): December 31, 2017 Real estate properties $ 76,066 Rents receivable, net of allowance for doubtful accounts of $5 13,270 Other assets, net 8,352 Assets held for sale $ 97,688 Accounts payable and accrued expenses $ 1,021 Rent collected in advance 408 Security deposits 411 Liabilities related to properties held for sale $ 1,840 Property Dispositions: During the year ended December 31, 2018 , we sold the following properties (dollars in thousands): Asset Date Sold Number of Number of Square Footage Gross Sales Price Gain (Loss) on Sale Properties 1600 Market Street February 2018 1 1 825,968 $ 160,000 $ 54,599 600 West Chicago Avenue (1) February 2018 1 2 1,561,477 510,000 107,790 5073, 5075, & 5085 S. Syracuse Street March 2018 1 1 248,493 115,186 42,762 1601 Dry Creek Drive May 2018 1 1 552,865 68,500 26,979 777 East Eisenhower Parkway August 2018 1 1 290,530 29,500 5,308 8750 Bryn Mawr Avenue September 2018 1 2 636,078 141,000 15,194 97 Newberry Road December 2018 1 1 289,386 7,100 (1,174 ) 7 9 4,404,797 $ 1,031,286 $ 251,458 (1) The sale of this property did not represent a strategic shift under ASC Topic 205. However, the sale does represent an individually significant disposition. The operating results of this property are included in continued operations for all periods presented through the date of sale. Net income for this property was $110.6 million , $9.3 million and $12.2 million for the years ended December 31, 2018 , 2017 and 2016 , respectively. During the year ended December 31, 2017 , we sold the following properties (dollars in thousands): Asset Date Sold Number of Number of Square Footage Gross Sales Price Gain (Loss) on Sale Properties 111 Market Place January 2017 1 1 589,380 $ 60,100 $ (5,968 ) Cabot Business Park Land March 2017 — — — 575 (57 ) Parkshore Plaza April 2017 1 4 271,072 40,000 (2,460 ) 25 S. Charles Street April 2017 1 1 359,254 24,500 (3,487 ) 802 Delaware Avenue May 2017 1 1 240,780 34,000 9,099 1500 Market Street July 2017 1 1 1,759,193 328,000 38,585 6600 North Military Trail August 2017 1 3 639,825 132,050 (14,175 ) 789 East Eisenhower Parkway (1) December 2017 — 1 130,961 24,942 1,242 33 Stiles Lane December 2017 1 1 175,301 10,500 2,163 625 Crane Street (Land) December 2017 — — — 307 249 Mineral Rights December 2017 — — — 200 169 Portfolios of properties 4515 Seton Center Parkway March 2017 1 1 117,265 4516 Seton Center Parkway March 2017 1 1 120,559 Seton Center 2 2 237,824 $ 52,450 $ 22,479 Cherrington Corporate Center December 2017 1 7 454,700 Foster Plaza December 2017 1 8 727,743 Pittsburgh Portfolio 2 15 1,182,443 $ 71,000 $ (33,048 ) 820 W. Diamond July 2017 1 1 134,933 Danac Stiles Business Park July 2017 1 3 276,637 411 Farwell Avenue July 2017 1 1 422,727 2250 Pilot Knob Road July 2017 1 1 87,183 4700 Belleview Avenue July 2017 1 1 80,615 Five Property Portfolio 5 7 1,002,095 $ 84,000 $ 702 16 37 6,588,128 $ 862,624 $ 15,493 (1) We sold one building in a property containing two buildings. During the year ended December 31, 2016 , we sold 30 properties ( 62 buildings) with a combined 7,972,692 square feet for an aggregate gross sales price of $1.3 billion , excluding credits and closing costs. Lease Payments Our real estate properties are generally leased on gross lease and modified gross lease bases pursuant to non-cancelable, fixed term operating leases expiring between 2019 and 2037. These gross leases and modified gross leases require us to pay all or some property operating expenses and to provide all or some property management services. A portion of these property operating expenses are reimbursed by the tenants. The future minimum lease payments, excluding tenant reimbursement revenue, scheduled to be received by us during the current terms of our leases as of December 31, 2018 are as follows (in thousands): 2019 $ 109,888 2020 96,309 2021 104,279 2022 95,128 2023 86,299 Thereafter 567,547 $ 1,059,450 One of our real estate properties that we sold in 2016, 111 River Street in Hoboken, New Jersey, was subject to a ground lease. The amount of ground lease expense included in operating expenses during the year ended December 31, 2016 , totaled $0.9 million . Ground lease expense includes percentage rent.

Lease Intangibles

Lease Intangibles	12 Months Ended
	Dec. 31, 2018
Leases [Abstract]
Leases Intangibles	Lease Intangibles The following table summarizes the carrying amounts for our acquired real estate leases and assumed real estate lease obligations as of December 31, 2018 , and 2017 , and (in thousands): December 31, 2018 2017 Acquired in-place leases $ 3,691 $ 56,853 Acquired above market leases — 21,021 Acquired real estate leases 3,691 77,874 Accumulated amortization, acquired in-place leases (3,416 ) (41,978 ) Accumulated amortization, acquired above market leases — (12,049 ) Acquired real estate leases, net $ 275 $ 23,847 Acquired below market leases $ 1,570 $ 6,093 Accumulated amortization (1,453 ) (5,092 ) Assumed real estate lease obligations, net $ 117 $ 1,001 Amortization of the lease intangibles for the years ended December 31, 2018 , 2017 , and 2016 , is as follows (in thousands): December 31, Income Statement Location 2018 2017 2016 Amortization of acquired in-place leases Depreciation and amortization $ 2,133 $ 7,220 $ 14,598 Amortization of above and below market leases Increase (decrease) to rental income (54 ) (1,774 ) (6,531 ) Future amortization of net intangible lease assets and liabilities to be recognized by us during the current terms of our leases as of December 31, 2018 , are approximately (in thousands): 2019 $ 158 2020 — 2021 — 2022 — 2023 — Thereafter — $ 158

Marketable Securities

Marketable Securities	12 Months Ended
	Dec. 31, 2018
Investments, Debt and Equity Securities [Abstract]
Marketable Securities	Marketable Securities During the year ended December 31, 2018 , our marketable securities consisted of United States Treasury notes and common stock. The United States Treasury notes are classified as available-for-sale and mature in 2019. On January 1, 2018 we adopted ASU 2016-01 (see Note 2) and reclassified a $1.9 million unrealized gain from cumulative other comprehensive loss to cumulative net income on our consolidated balance sheet. In March 2018, we sold all common stock we held for total proceeds of $23.9 million and recognized a loss of $5.0 million in interest and other income, net during the year ended December 31, 2018 . Below is a summary of our marketable securities as of December 31, 2018 and 2017 (in thousands): December 31, 2018 2017 Amortized Cost Unrealized Loss Estimated Fair Value Cost or Amortized Cost Unrealized Gain, Net Estimated Fair Value Marketable securities $ 249,944 $ (342 ) $ 249,602 $ 276,567 $ 361 $ 276,928 The unrealized losses on our United States Treasury notes were caused by interest rate increases. The contractual terms of those investments do not permit the issuer to settle the securities at a price less than the amortized cost bases of the investments. Because we do not intend to sell the investments and it is not more likely than not that we will be required to sell the investments before recovery of their amortized cost bases, which may be maturity, we do not consider those investments to be other-than-temporarily impaired at December 31, 2018 .

Other Assets

Other Assets	12 Months Ended
	Dec. 31, 2018
Other Assets [Abstract]
Other Assets	Other Assets Real Estate Mortgages Receivable We provided mortgage financing totaling $7.7 million at 6.0% per annum in connection with our sale of three properties ( 18 buildings) in January 2013 in Dearborn, MI. In August 2018, we sold this real estate mortgage receivable for $5.7 million and recorded a loss of $2.1 million in interest and other income for the year ended December 31, 2018. We also provided mortgage financing totaling $0.4 million at 6.0% per annum in connection with our sale of a property in Salina, NY in April 2012. In September 2017, we received a $0.3 million repayment, representing a settlement of the obligation, related to this real estate mortgage receivable and recorded a loss of $0.1 million in interest and other income for the year ended December 31, 2017. As of December 31, 2017 , we had total real estate mortgages receivable with an aggregate carrying value of $7.7 million included in other assets in our consolidated balance sheet. We monitored the payment history of the borrowers and determined that no allowance for losses related to the real estate mortgage receivable was necessary at December 31, 2017 . As of December 31, 2018, we did no t have any real estate mortgages receivable. Deferred Financing Fees, Deferred Leasing Costs and Capitalized Lease Incentives The following table summarizes our deferred financing fees related to our unsecured revolving credit facility included in other assets, deferred leasing costs and capitalized lease incentives as of December 31, 2018 , and 2017 (in thousands): December 31, 2018 2017 Deferred financing fees $ — $ 14,458 Accumulated amortization — (12,709 ) Deferred financing fees, net $ — $ 1,749 Deferred leasing costs $ 69,930 $ 85,606 Accumulated amortization (18,807 ) (22,067 ) Deferred leasing costs, net $ 51,123 $ 63,539 Capitalized lease incentives $ 5,701 $ 8,562 Accumulated amortization (1,393 ) (2,054 ) Capitalized lease incentives, net $ 4,308 $ 6,508 Future amortization of deferred leasing costs and capitalized lease incentives to be recognized by us during the current terms of our leases as of December 31, 2018 are approximately (in thousands): Deferred Leasing Costs Capitalized Lease Incentives 2019 $ 5,550 $ 695 2020 5,812 583 2021 5,810 553 2022 5,126 425 2023 4,478 320 Thereafter 24,347 1,732 $ 51,123 $ 4,308

Indebtedness

Indebtedness	12 Months Ended
	Dec. 31, 2018
Debt Disclosure [Abstract]
Indebtedness	Indebtedness At December 31, 2018 and 2017 , our outstanding indebtedness included the following (in thousands): December 31, Interest Rate at December 31, 2018 Maturity Date 2018 2017 Unsecured revolving credit facility, at LIBOR plus a premium — % — $ — $ — 5-year unsecured term loan, at LIBOR plus a premium — % — — 200,000 7-year unsecured term loan, at LIBOR plus a premium — % — — 200,000 Unsecured floating rate debt — % $ — $ 400,000 5.875% Senior Unsecured Notes due 2020 5.875 % 9/15/2020 $ 250,000 $ 250,000 5.75% Senior Unsecured Notes due 2042 — % — — 175,000 Unsecured fixed rate debt 5.875 % $ 250,000 $ 425,000 206 East 9th Street 5.69 % 1/5/2021 $ 26,000 $ 26,536 97 Newberry Road — % — — 5,404 Secured fixed rate debt 5.69 % $ 26,000 $ 31,940 $ 276,000 $ 856,940 Unamortized net premiums, discounts and deferred financing fees (1,045 ) (8,362 ) $ 274,955 $ 848,578 Unsecured Revolving Credit Facility and Term Loan: On January 29, 2015, we entered into a new credit agreement, pursuant to which the lenders agreed to provide (i) a $750.0 million unsecured revolving credit facility, (ii) a $200.0 million 5 -year term loan facility and (iii) a $200.0 million 7 -year term loan facility. The new agreement replaced our prior credit agreement, dated as of August 9, 2010, and our prior term loan agreement, dated as of December 16, 2010. On November 10, 2016, in connection with our conversion to an UPREIT structure, the Operating Trust entered into an amended and restated credit agreement, replacing the Company’s prior credit agreement. Under the amended and restated credit agreement, the Operating Trust has assumed all obligations of the Company as borrower and the Company is released from such obligations. The economic terms of the amended and restated credit agreement are substantially the same as the terms of the Company’s prior credit agreement. On May 4, 2018, we redeemed at par the total $400.0 million outstanding under our 5 -year and 7 -year term loans and recognized a loss on early extinguishment of debt of $1.5 million from the write off of unamortized deferred financing fees. Prior to the redemption of the term loans, borrowings under the 5 -year term loan and 7 -year term loan, subject to certain exceptions, had interest rates of LIBOR rate plus a margin of 90 to 180 basis points for the 5 -year term loan and 140 to 235 basis points for the 7 -year term loan, in each case depending on our credit rating. On December 26, 2018, we terminated the credit agreement and recognized a loss on early extinguishment of debt of $0.2 million from the write off of unamortized deferred financing fees. We were required to pay a facility fee of 12.5 to 30 basis points, depending on our credit rating, on the borrowings available under the revolving credit facility, whether or not utilized. Debt Covenants: Our public debt indenture and related supplements contain a number of financial ratio covenants which generally restrict our ability to incur debts, in excess of calculated amounts, and require us to maintain other financial ratios. At December 31, 2018 , we believe we were in compliance with all of our respective covenants under our public debt indenture and related supplements. Senior Unsecured Notes: On March 7, 2018, we redeemed at par all $175.0 million of our 5.75% senior unsecured notes due 2042 and recognized a loss on early extinguishment of debt of $4.9 million from the write off of unamortized deferred financing fees. On July 15, 2017, we redeemed at par $250.0 million of our 6.65% senior unsecured notes due 2018 and recognized a loss on early extinguishment of debt of $0.2 million for the year ended December 31, 2017 from the write off of unamortized deferred financing fees and the write off of an unamortized discount. On December 15, 2016, we redeemed at par $250.0 million of our 6.25% senior unsecured notes due 2017 and recognized a loss on early extinguishment of debt of $0.1 million for the year ended December 31, 2016 from the write-off of an unamortized discount and unamortized deferred financing fees. On February 16, 2016, we redeemed at par $139.1 million of our 6.25% senior unsecured notes due 2016 and recognized a loss on early extinguishment of debt of $0.1 million for the year ended December 31, 2016 from the write-off of an unamortized discount and unamortized deferred financing fees. Mortgage Notes Payable: At December 31, 2018 , one of our properties with an aggregate net book value of $44.3 million had a secured mortgage note totaling $26.5 million (including a net premium and unamortized deferred financing fees) maturing in 2021. In December 2018, we repaid $4.9 million of mortgage debt at 97 Newberry Road and recognized a loss on early extinguishment of debt of $0.6 million for the year ended December 31, 2018 from prepayment fees and the write off of unamortized deferred financing fees. In December 2017, we repaid $2.0 million of mortgage debt at 33 Stiles Lane and recognized a loss on early extinguishment of debt of $0.2 million for the year ended December 31, 2017 from prepayment fees and the write off of unamortized deferred financing fees. In April 2017, we repaid at par $41.3 million of mortgage debt at Parkshore Plaza and recognized a loss on early extinguishment of debt of $0.1 million for the year ended December 31, 2017 from prepayment fees and the write off of unamortized deferred financing fees, net of the write off of an unamortized premium. On November 10, 2016, we repaid at par $167.8 million of mortgage debt at 1735 Market Street and recognized a loss on early extinguishment of debt of $2.4 million from the write-off of unamortized deferred financing fees and breakage costs for the year ended December 31, 2016. We also recognized $0.2 million of expense included in interest and other income related to an interest rate swap as a result of the early repayment of debt for the year ended December 31, 2016. Required Principal Payments: The required principal payments due during the next five years and thereafter under all of our outstanding debt at December 31, 2018 are as follows (in thousands): 2019 $ 567 2020 250,597 2021 24,836 2022 — 2023 — Thereafter — $ 276,000

Shareholders' Equity

Shareholders' Equity	12 Months Ended
	Dec. 31, 2018
Stockholders' Equity Note [Abstract]
Shareholders' Equity	Shareholders’ Equity Common Share Issuances: See Note 13 for information regarding equity issuances related to share-based compensation. Common Share Repurchases: On August 24, 2015, our Board of Trustees approved a common share repurchase program. On March 17, 2016, our Board of Trustees authorized the repurchase of up to $150.0 million of our outstanding common shares over the twelve month period following the date of authorization. In March 2017, this share repurchase authorization, of which $106.6 million was not utilized, expired. On March 15, 2017, our Board of Trustees authorized the repurchase of up to an additional $150.0 million of our outstanding common shares over the twelve month period following the date of authorization. In March 2018, this share repurchase authorization, of which $81.0 million was not utilized, expired. On March 14, 2018, our Board of Trustees authorized the repurchase of up to an additional $150.0 million of our outstanding common shares over the twelve month period following the date of authorization. During the year ended December 31, 2018, we purchased and retired 2,970,209 of our common shares at a weighted average price of $29.67 per share for a total investment of $88.1 million , of which $69.0 million was under the March 2017 authorization and $19.1 million was under the March 2018 authorization. During the year ended December 31, 2017, we did no t purchase any common shares under our common share repurchase program. During the year ended December 31, 2016, we purchased and retired 2,491,675 of our common shares at a weighted average price of $27.68 per share, for a total investment of $69.0 million . The $130.9 million of remaining authorization available under our share repurchase program as of December 31, 2018 is scheduled to expire on March 14, 2019. During the years ended December 31, 2018 , 2017 and 2016 , certain of our employees surrendered 193,521 , 43,329 and 31,025 common shares owned by them, respectively, to satisfy their statutory tax withholding obligations in connection with the vesting of such common shares. Common Share and Unit Distributions: On September 26, 2018 , our Board of Trustees declared a special, one-time cash distribution of $2.50 per common share/unit to shareholders/unitholders of record on October 9, 2018. On October 23, 2018, we paid this distribution to such shareholders/unitholders in the aggregate amount of $304.7 million . We did no t pay any cash distributions to our common shareholders in 2017 or 2016 . Our credit agreement contains a number of financial and other covenants, including a covenant which restricts our ability to make distributions under certain circumstances. See Note 7 for additional information regarding our credit agreement. The following characterizes distributions paid per common share for the years ended December 31, 2018 , 2017 , and 2016 : Year Ended December 31, 2018 2017 2016 Ordinary income 100.0 % — % — % Return of capital — % — % — % Capital gain — % — % — % Unrecaptured Section 1250 gain — % — % — % 100.0 % — % — % Series D Preferred Shares: Each of our 4,915,196 series D cumulative convertible preferred shares accrue dividends of $1.625 , or 6.50% per annum of the liquidation amount, payable in equal quarterly payments. Our series D preferred shares are convertible, at the holder's option, into our common shares at a conversion rate of 0.5215 common shares per series D preferred share, which is equivalent to a conversion price of $47.94 per common share, or 2,563,275 additional common shares at December 31, 2018 . The conversion rate changed from 0.480775 to 0.5215 common shares per series D preferred share effective October 10, 2018 as a result of the common share distribution declared by our Board of Trustees. On or after November 20, 2011, if our common shares trade at or above the then applicable conversion price, we may, at our option, convert some or all of the series D preferred shares into common shares at the then applicable conversion rate. If a fundamental change occurs, which generally will be deemed to occur upon a change in control or a termination of trading of our common shares (or other equity securities into which our series D preferred shares are then convertible), holders of our series D preferred shares will have a special right to convert their series D preferred shares into a number of our common shares per $25.00 liquidation preference, plus accrued and unpaid distributions, divided by 98% of the average closing market price of our common shares for a specified period before such event is effective, unless we exercise our right to repurchase these series D preferred shares for cash, at a purchase price equal to 100% of their liquidation preference, plus accrued and unpaid distributions. The issuance of a large number of common shares as a result of the exercise of this conversion right after a fundamental change may have a dilutive effect on net income attributable to Equity Commonwealth common shareholders per share for future periods. As of December 31, 2018 , we had 4,915,196 outstanding series D preferred shares that were convertible into 2,563,275 of our common shares. Series E Preferred Shares: On May 15, 2016, we redeemed all of our 11,000,000 outstanding series E preferred shares at a price of $25.00 per share, for a total of $275.0 million , plus any accrued and unpaid dividends. The redemption payment occurred on May 16, 2016 (the first business day following the redemption date). We recorded $9.6 million related to the excess fair value of consideration paid over the carrying value of the preferred shares as a reduction to net income attributable to common shareholders for the year ended December 31, 2016. Preferred Share Distributions: Under our governing documents and Maryland law, distributions to our shareholders are to be authorized and declared by our Board of Trustees. In 2018 , our Board of Trustees declared distributions on our series D preferred shares to date as follows: Declaration Date Record Date Payment Date Dividend Per Share January 12, 2018 January 30, 2018 February 15, 2018 $ 0.40625 April 11, 2018 April 27, 2018 May 15, 2018 $ 0.40625 July 12, 2018 July 30, 2018 August 15, 2018 $ 0.40625 September 26, 2018 October 29, 2018 November 15, 2018 $ 0.40625 Alternative Minimum Tax: Alternative minimum tax adjustments are to be apportioned between a REIT and its shareholders under Internal Revenue Code Section 59(d). Although regulations have not yet been issued under that provision, based on regulations issued pursuant to a similar provision of prior law and the legislative history of the current provision, it appears that such alternative minimum tax adjustments are to be apportioned to a REIT’s shareholders to the extent that the REIT distributes its regular taxable income. It is our policy to distribute all of our regular taxable income and accordingly, all of our alternative minimum tax adjustments are being apportioned to our shareholders. We have determined that 0% and 96.31% of each distribution to our shareholders for the tax years ended December 31, 2017 and 2016 , respectively, consists of an alternative minimum tax adjustment. Our taxable earnings in 2018 are no longer subject to an alternative minimum tax adjustment as a result of H.R. 1, the Tax Cuts and Jobs Act, which generally takes effect for taxable years beginning on or after January 1, 2018 (subject to certain exceptions).

Noncontrolling Interest

Noncontrolling Interest	12 Months Ended
	Dec. 31, 2018
Noncontrolling Interest [Abstract]
Noncontrolling Interest	Noncontrolling Interest Noncontrolling interest represents the portion of the units in the Operating Trust not beneficially owned by the Company. An OP Unit and a share of our common stock have essentially the same economic characteristics. Distributions with respect to OP Units will generally mirror distributions with respect to the Company’s common shares. Unitholders (other than the Company) generally have the right, commencing six months from the date of issuance of such OP Units, to cause the Operating Trust to redeem their OP Units in exchange for cash or, at the option of the Company, common shares of the Company on a one -for-one basis. As sole trustee, the Company will have the sole discretion to elect whether the redemption right will be satisfied by the Company in cash or the Company’s common shares. As a result, the Noncontrolling interest is classified as permanent equity. As of December 31, 2018 , the portion of the Operating Trust not beneficially owned by the Company is in the form of OP Units and LTIP Units (see Note 13 for a description of LTIP Units). LTIP Units may be subject to additional vesting requirements. The following table presents the changes in Equity Commonwealth’s issued and outstanding common shares and units for the year ended December 31, 2018 : Common Shares OP Units and LTIP Units Total Outstanding at January 1, 2018 124,217,616 42,520 124,260,136 Repurchase of shares (2,970,209 ) — (2,970,209 ) Restricted share, time-based LTIP Unit grants and vested restricted stock units, net of forfeitures 324,748 3,200 327,948 Outstanding at December 31, 2018 121,572,155 45,720 121,617,875 Noncontrolling ownership interest in the Operating Trust 0.04 % The carrying value of the Noncontrolling interest is allocated based on the number of OP Units and LTIP Units in proportion to the number of OP Units and LTIP Units plus the number of common shares. We adjust the noncontrolling interest balance at the end of each period to reflect the noncontrolling partners’ interest in the net assets of the Operating Trust. Net income is allocated to the Noncontrolling interest in the Operating Trust based on the weighted average ownership percentage during the period. Equity Commonwealth’s weighted average ownership interest in the Operating Trust was 99.96% and 99.97% , respectively, for the years ended December 31, 2018 and 2017 .

Cumulative Other Comprehensive

Cumulative Other Comprehensive Loss	12 Months Ended
	Dec. 31, 2018
Accumulated Other Comprehensive Income (Loss), Net of Tax [Abstract]
Cumulative Other Comprehensive Loss	Cumulative Other Comprehensive Loss The following tables present the amounts recognized in cumulative other comprehensive loss for the years ended December 31, 2018 and 2017 (in thousands): Unrealized Loss on Derivative Instruments Unrealized Gain (Loss) on Marketable Securities Total Balance as of January 1, 2018 $ (456 ) $ 361 $ (95 ) Amounts reclassified from cumulative other comprehensive loss to cumulative net income pursuant to a change in accounting principle — (1,902 ) (1,902 ) Other comprehensive income before reclassifications 84 1,199 1,283 Amounts reclassified from cumulative other comprehensive loss to net income 372 — 372 Net current period other comprehensive income 456 1,199 1,655 Balance as of December 31, 2018 $ — $ (342 ) $ (342 ) Unrealized Loss Unrealized Gain, Net on Marketable Securities Total Balance as of January 1, 2017 $ (208 ) $ — $ (208 ) Other comprehensive (loss) income before reclassifications (297 ) 361 64 Amounts reclassified from cumulative other comprehensive loss to net income 49 — 49 Net current period other comprehensive (loss) income (248 ) 361 113 Balance as of December 31, 2017 $ (456 ) $ 361 $ (95 ) The following table presents reclassifications out of cumulative other comprehensive loss for the years ended December 31, 2018 and 2017 (in thousands): Amounts Reclassified from Cumulative Other Comprehensive Loss to Net Income Year Ended December 31, Details about Cumulative Other Comprehensive Loss Components 2018 2017 Affected Line Items in the Statement of Operations Interest rate cap contract $ 293 $ — Interest and other income Interest rate cap contract 79 49 Interest expense $ 372 $ 49

Income Taxes

Income Taxes	12 Months Ended
	Dec. 31, 2018
Income Tax Disclosure [Abstract]
Income Taxes	Income Taxes Our provision for income taxes consists of the following (in thousands): Year Ended December 31, 2018 2017 2016 Current: State and local $ (3,156 ) $ (500 ) $ (745 ) Federal — — — Income tax expense $ (3,156 ) $ (500 ) $ (745 ) The tax expense recorded in the current period is primarily the result of the taxable gains from sales of properties during the year ended December 31, 2018. A reconciliation of our effective tax rate and the U.S. Federal statutory income tax rate is as follows: Year Ended December 31, 2018 2017 2016 Taxes at statutory U.S. federal income tax rate 21.00 % 35.00 % 35.00 % Dividends paid deduction and net operating loss utilization (21.00 )% (35.00 )% (35.00 )% Federal taxes on built-in gain — % — % — % State and local income taxes 1.14 % 1.66 % 0.32 % Effective tax rate 1.14 % 1.66 % 0.32 % At December 31, 2018 and 2017 , we had federal net operating loss (NOL) carryforwards of approximately $ 90 million and $120 million , respectively. These amounts can be used to offset future taxable income, if any. The REIT will be entitled to utilize NOL carryforwards only to the extent that REIT taxable income exceeds our deduction for dividends paid. NOLs arising in taxable years ending before January 1, 2018 can generally be carried forward 20 years, with no carryforward limitation on NOLs generated after that date. Our NOL carryforwards expire in years 2035 to 2037.

Derivative Instruments

Derivative Instruments	12 Months Ended
	Dec. 31, 2018
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Derivative Instruments	Derivative Instruments Risk Management Objective of Using Derivatives We are exposed to certain risks relating to our ongoing business operations, including the effect of changes in interest rates. We may use derivative financial instruments, including interest rate swaps, caps, options, floors and other interest rate derivative contracts, to hedge all or a portion of any interest rate risk associated with any borrowings. The principal objective of such arrangements would reduce the risks and/or costs associated with our operating and financial structure as well as hedge specific anticipated transactions. We do not intend to utilize derivatives for speculative or other purposes other than interest rate risk management. The use of derivative financial instruments carries certain risks, including the risk that the counterparties to these contractual arrangements are not able to perform under the agreements. To reduce this risk, we only enter into derivative financial instruments with counterparties with high credit ratings and with major financial institutions with which we and our affiliates may also have other financial relationships. Cash Flow Hedges of Interest Rate Risk Our objectives in using interest rate derivatives are to add stability to interest expense and to manage any exposure to interest rate movements. To accomplish this objective, we may use interest rate swaps, caps, or other similar instruments as part of our interest rate risk management strategy. Interest rate swaps designated as cash flow hedges involve the receipt of variable-rate amounts from a counterparty in exchange for us making fixed-rate payments over the life of the agreements without exchange of the underlying notional amount. Interest rate caps designated as cash flow hedges involve the receipt of variable-rate amounts if interest rates rise above the cap strike rate. The effective portion of changes in the fair value of derivatives designated and that qualify as cash flow hedges is recorded in cumulative other comprehensive loss and is subsequently reclassified into earnings in the period that the hedged forecasted transaction affects earnings. During 2018 , such derivatives were used to hedge the variable cash flows associated with variable-rate debt. The ineffective portion of the change in fair value of the derivatives is recognized directly in earnings. We previously had interest rate swap agreements to manage our interest rate risk exposure on $167.8 million of mortgage debt at 1735 Market Street, which required interest at a spread over LIBOR. The interest rate swap agreements utilized by us qualified as cash flow hedges and effectively modified our exposure to interest rate risk by converting our floating interest rate debt to a fixed interest rate basis for this loan through December 1, 2016, thus reducing the impact of interest rate changes on future interest expense. On November 10, 2016 we repaid at par the mortgage debt at 1735 Market Street and terminated the related interest rate swap agreement. We recognized $0.2 million of expense in interest and other income on the consolidated statement of operations for the year ended December 31, 2016 related to the early termination of the interest rate swap agreement. On March 8, 2018, we terminated an interest rate cap that had a LIBOR strike price of 2.50% , a notional amount of $400.0 million and a maturity date of March 1, 2019. We recognized $0.3 million of expense in interest and other income, net on the consolidated statement of operations for the year ended December 31, 2018 related to the early termination of the interest rate cap agreement. As of December 31, 2018, we do not have any outstanding interest rate derivatives designated as cash flow hedges of interest rate risk. The table below presents the fair value of derivative financial instruments as well as classification on the consolidated balance sheets as of December 31, 2018 , and 2017 (amounts in thousands): Fair Value as of December 31, Interest Rate Derivative Designated as Hedging Instrument Balance Sheet Location 2018 2017 Interest rate cap Other assets $ — $ 17 The table below details the location in the financial statements of the gain or loss recognized on interest rate derivatives designated as cash flow hedges for the years ended December 31, 2018 , 2017 , and 2016 (amounts in thousands): Year Ended December 31, 2018 2017 2016 Amount of gain (loss) recognized in cumulative other comprehensive loss (effective portion) $ 84 $ (297 ) $ (554 ) Amount of loss reclassified from cumulative other comprehensive loss into interest expense (effective portion) 79 49 3,792 Amount of loss recognized in income (ineffective portion and amount excluded from effectiveness testing) 293 — 241

Share-Based Compensation

Share-Based Compensation	12 Months Ended
	Dec. 31, 2018
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Share-Based Compensation	Share-Based Compensation Equity Commonwealth 2015 Omnibus Incentive Plan On June 16, 2015, at our 2015 annual meeting of shareholders, our shareholders approved the 2015 Incentive Plan. The 2015 Incentive Plan replaced the Equity Commonwealth 2012 Equity Compensation Plan (as amended, the 2012 Plan). The Board of Trustees approved the 2015 Incentive Plan, subject to shareholder approval, on March 18, 2015 (the Effective Date). On January 26, 2016, the Board of Trustees approved an amendment to the 2015 Incentive Plan to allow the Compensation Committee (Committee) to authorize in an award agreement a transfer of all or a part of certain equity awards not for value to a “family member” (as defined in the 2015 Incentive Plan). The following description of certain terms of the 2015 Incentive Plan is qualified in all respects by the terms of the 2015 Incentive Plan. Eligibility. Awards may be granted under the 2015 Incentive Plan to employees, officers and non-employee directors of the Company, its subsidiaries or its affiliates, or consultants and advisors (who are natural persons) providing services to the Company, its subsidiaries or its affiliates, or any other person whose participation in the 2015 Incentive Plan is determined by the Committee to be in the best interests of the Company. Term . The 2015 Incentive Plan terminates automatically ten years after the Effective Date, unless it is terminated earlier by the Board of Trustees. Shares Available for Issuance. Subject to adjustment as provided in the 2015 Incentive Plan, the maximum number of common shares of the Company that are available for issuance under the 2015 Incentive Plan is 3,250,000 shares. Awards. The following types of awards may be made under the 2015 Incentive Plan, subject to limitations set forth in the 2015 Incentive Plan: · Stock options; · Stock appreciation rights; · Restricted stock; · Restricted stock units; · Unrestricted stock; · Dividend equivalent rights; · Performance shares and other performance-based awards; · Limited partnership interests in any partnership entity through which the Company may conduct its business in the future; · Other equity-based awards; and · Cash bonus awards. Recipients of the Company’s restricted shares have the same voting rights as any other common shareholder. During the period of restriction, the Company’s unvested restricted shareholders are eligible to receive dividend payments on their shares at the same rate and on the same date as any other common shareholder. The restricted shares are service based awards and vest over a four -year period. Recipients of the Company’s restricted stock units (RSUs) are entitled to receive dividends with respect to the common shares underlying the RSUs if and when the RSUs are earned, at which time the recipient will be entitled to receive an amount in cash equal to the aggregate amount of cash dividends that would have been paid in respect of the common shares underlying the recipient’s earned RSUs had such common shares been issued to the recipient on the first day of the performance period. To the extent that an award does not vest, the dividends related to unvested RSUs will be forfeited. The RSUs are market-based awards with a service condition and recipients may earn RSUs based on the Company’s total shareholder return (TSR) relative to the TSRs of the companies that comprise the NAREIT Office Index over a three -year performance period. Following the end of the three -year performance period, the number of earned awards will be determined. The earned awards vest in two tranches with 50% of the earned award vesting following the end of the performance period on the date the Committee determines the level of achievement of the performance metric and the remaining 50% of the earned award vesting approximately one year thereafter, subject to the grant recipient’s continued employment. Compensation expense for the RSUs is determined using a Monte Carlo simulation model and is recognized ratably from the grant date to the vesting date of each tranche. LTIP Units are a class of beneficial interests in the Operating Trust that may be issued to employees, officers or trustees of the Operating Trust, the Company or their subsidiaries (LTIP Units). Time-based LTIP Units have the same general characteristics as restricted shares and market-based LTIP Units have the same general characteristics as RSUs. Each LTIP Unit will convert automatically into an OP Unit on a one -for-one basis when the LTIP Unit becomes vested and its capital account is equalized with the per-unit capital account of the OP Units. Holders of LTIP Units generally will be entitled to receive the same per-unit distributions as the other outstanding OP Units in the Operating Trust, except that market-based LTIP Units will not participate in distributions until expiration of the applicable performance period, at which time any earned market-based LTIP Units generally will become entitled to receive a catch-up distribution for the periods prior to such time. Administration. The 2015 Incentive Plan will be administered by the Committee, which will determine all terms and recipients of awards under the 2015 Incentive Plan. 2018 Equity Award Activity On October 28, 2018, 225,655 RSUs vested and we issued 225,655 common shares, prior to certain employees surrendering their common shares to satisfy statutory tax withholding obligations (see Note 8). On June 20, 2018, in accordance with the Company’s compensation plan for independent Trustees, the Committee awarded each of the nine independent Trustees $0.1 million in restricted shares or time-based LTIP Units as part of their compensation for the 2018-2019 year of service on the Board of Trustees. These awards equated to 3,200 shares or time-based LTIP Units per Trustee, for a total of 25,600 shares and 3,200 time-based LTIP Units, valued at $31.25 per share and unit, the closing price of our common shares on the New York Stock Exchange (NYSE) on that day. These shares and time-based LTIP Units vest one year after the date of the award. On January 29, 2018, the Committee approved a grant of 125,409 restricted shares and 254,615 RSUs at target ( 634,628 RSUs at maximum) to the Company’s officers, certain employees and to Mr. Zell, the Chairman of our Board of Trustees, as part of their compensation for fiscal year 2017. The restricted shares granted on January 29, 2018 were valued at $29.78 per share, the closing price of our common shares on the NYSE on that day. 2017 Equity Award Activity On November 8, 2017, 226,258 RSUs vested and we issued 226,258 common shares, prior to certain employees surrendering their common shares to satisfy statutory tax withholding obligations (see Note 8). On June 20, 2017, in accordance with the Company’s compensation plan for independent Trustees, the Committee awarded each of the nine independent Trustees $0.1 million in restricted shares or time-based LTIP Units as part of their compensation for the 2017-2018 year of service on the Board of Trustees. These awards equated to 3,156 shares or time-based LTIP Units per Trustee, for a total of 25,248 shares and 3,156 time-based LTIP Units, valued at $31.69 per share and unit, the closing price of our common shares on the NYSE on the grant date. These shares and time-based LTIP Units vested on June 20, 2018. On January 24, 2017, the Committee approved a grant of 39,364 time-based LTIP Units, 79,924 market-based LTIP Units at target ( 199,211 market-based LTIP Units at maximum), 76,424 restricted shares and 155,168 RSUs at target ( 386,756 RSUs at maximum) to the Company’s officers, certain employees and to Mr. Zell, the Chairman of our Board of Trustees, as part of their compensation for fiscal year 2016. The restricted shares and time-based LTIP Units were valued at $31.47 per share and per unit, the closing price of our common shares on the NYSE on the grant date. 2016 Equity Award Activity On June 15, 2016, in accordance with the Company’s compensation plan for independent Trustees, the Committee awarded each of the nine independent Trustees $0.1 million in restricted shares as part of their compensation for the 2016-2017 year of service on the Board of Trustees. These awards equated to 3,463 shares per Trustee, for a total of 31,167 shares, valued at $28.88 per share, the closing price of our common shares on the NYSE on that day. These shares vested on June 15, 2017. On January 26, 2016, the Committee approved a grant of 136,623 restricted shares and 277,386 RSUs at target ( 691,385 RSUs at maximum) to the Company’s officers, certain employees and to Mr. Zell, the Chairman of our Board of Trustees, as part of their compensation for fiscal year 2015. The restricted shares were granted on January 26, 2016 and were valued at $26.93 per share, the closing price of our common shares on the NYSE on the grant date. Outstanding Equity Awards The table below presents a summary of restricted share, RSU and LTIP Unit activity for the years ended December 31, 2018 , 2017 and 2016 : Number Weighted Number Weighted Outstanding at December 31, 2015 776,456 $ 26.62 1,884,110 $ 15.82 Granted 167,790 27.29 691,385 15.57 Vested (191,498 ) 26.54 — — Forfeited (539 ) 26.82 (2,727 ) 15.54 Outstanding at December 31, 2016 752,209 $ 26.79 2,572,768 $ 15.75 Granted 144,192 31.51 585,967 15.97 Vested (217,449 ) 27.97 (226,258 ) 15.99 Not earned (1) — — (800,530 ) 15.99 Outstanding at December 31, 2017 678,952 $ 27.41 2,131,947 $ 15.69 Granted 154,209 30.05 634,628 14.90 Vested (407,273 ) 27.06 (367,260 ) 15.79 Not earned (1) — — (352,671 ) 15.47 Outstanding at December 31, 2018 425,888 $ 28.70 2,046,644 $ 15.47 (1) The table presents the maximum number of shares issued or issuable from outstanding equity awards. RSUs and market-based LTIP Units not earned are the shares market-based award recipients do not receive based on the performance measurement completed at the end of the performance period. The 425,888 unvested restricted shares and time-based LTIP Units as of December 31, 2018 are scheduled to vest as follows: 151,272 shares/units in 2019 , 124,959 shares/units in 2020 , 87,772 shares/units in 2021 and 61,885 shares/units in 2022 . As of December 31, 2018 , the estimated future compensation expense for all unvested restricted shares and time-based LTIP Units was $6.1 million . Compensation expense for the restricted share and time-based LTIP Units is being recognized on a straight-line basis over the requisite service period for each separately vesting portion of the award. The weighted average period over which the future compensation expense will be recorded for the restricted shares and time-based LTIP Units is approximately 2.3 years. As of December 31, 2018 , the estimated future compensation expense for all unvested RSUs and market-based LTIP Units was $12.4 million . The weighted average period over which the future compensation expense will be recorded for the RSUs and market-based LTIP Units is approximately 2.1 years. The assumptions and fair values for the RSUs and market-based LTIP Units granted for the years ended December 31, 2018 , 2017 and 2016 are included in the following table on a per share and unit basis. 2018 2017 2016 Fair value of RSUs and market-based LTIP Units granted at the target amount $ 37.13 $ 39.81 $ 38.80 Fair value of RSUs and market-based LTIP Units granted at the maximum amount $ 14.90 $ 15.97 $ 15.57 Expected term (years) 4 4 4 Expected volatility — — — Expected dividend yield 1.68 % 1.59 % 1.86 % Risk-free rate 2.26 % 1.49 % 1.07 % During the years ended December 31, 2018 , 2017 and 2016 , we recorded $19.7 million , $21.4 million and $18.5 million , respectively, of compensation expense, net of forfeitures, in general and administrative expense for grants to our Trustees, officers and employees related to our equity compensation plans. Forfeitures are recognized as they occur. At December 31, 2018 , 839,470 shares/units remain available for issuance under the 2015 Incentive Plan.

Defined Contribution Plan

Defined Contribution Plan	12 Months Ended
	Dec. 31, 2018
Retirement Benefits [Abstract]
Defined Contribution Plan	Defined Contribution Plan We have a defined contribution plan that covers employees meeting eligibility requirements. We match 100% of the first 3% of compensation that an employee elects to defer plus 50% of compensation that an employee elects to defer exceeding 3% but not exceeding 5% , subject to a maximum of $8,000 . The Company’s matching contribution vests immediately. The Company's contributions were $0.3 million , $0.3 million and $0.4 million for the years ended December 31, 2018 , 2017 and 2016 , respectively.

Fair Value of Assets and Liabil

Fair Value of Assets and Liabilities	12 Months Ended
	Dec. 31, 2018
Fair Value Disclosures [Abstract]
Fair Value of Assets and Liabilities	Fair Value of Assets and Liabilities The table below presents certain of our assets and liabilities measured at fair value during 2018 and 2017 , categorized by the level of inputs used in the valuation of each asset and liability (dollars in thousands): Fair Value at December 31, 2018 Using Quoted Prices in Active Markets for Identical Assets Significant Other Observable Inputs Significant Unobservable Inputs Description Total (Level 1) (Level 2) (Level 3) Recurring Fair Value Measurements: Marketable securities $ 249,602 $ 249,602 $ — $ — Fair Value at December 31, 2017 Using Quoted Prices in Active Markets for Identical Assets Significant Other Observable Inputs Significant Unobservable Inputs Description Total (Level 1) (Level 2) (Level 3) Recurring Fair Value Measurements: Effective portion of interest rate cap contract $ 17 $ — $ 17 $ — Marketable securities $ 276,928 $ 276,928 $ — $ — Effective Portion of Interest Rate Cap Contract The fair value of our interest rate cap contract was determined using the net discounted cash flows of the derivative based on the market based interest rate curve (level 2 inputs) and adjusted for our credit spread and the actual and estimated credit spreads of the counterparties (level 3 inputs). Although we have determined that the majority of the inputs used to value our derivative fall within level 2 of the fair value hierarchy, the credit valuation adjustments associated with our derivative utilize level 3 inputs, such as estimates of current credit spreads, to evaluate the likelihood of default by us and the counterparties. As of December 31, 2017 , we have assessed the significance of the impact of the credit valuation adjustments on the overall valuation of our derivative position and have determined that the credit valuation adjustments were not significant to the overall valuation of our derivative. As a result, we have determined that our derivative valuation in its entirety was classified as level 2 inputs in the fair value hierarchy. Properties Held and Used and Properties Held for Sale As part of our office repositioning strategy adopted by our Board of Trustees, and pursuant to our accounting policy, in 2018, we evaluated the recoverability of the carrying values of each of the real estate assets that comprised our portfolio and determined that due to the shortening of the expected periods of ownership as a result of the office repositioning strategy and current estimates of market value less estimated costs to sell, it was necessary to reduce the net book value of a portion of the real estate assets in our portfolio to their estimated fair values. We anticipated the potential disposition of certain properties prior to the end of their remaining useful lives. As a result, in the first quarter of 2018, we recorded an impairment charge related to 777 East Eisenhower Parkway and 97 Newberry Road of $12.1 million in accordance with our impairment analysis procedures. We determined this impairment based on independent third party broker information, which are level 3 inputs according to the fair value hierarchy established in ASC 820. We reduced the aggregate carrying value of these properties from $41.8 million to their estimated fair value less estimated costs to sell of $29.7 million . These properties were sold in 2018 (see Note 3 for additional information). We evaluated each of our properties and determined there were no additional valuation adjustments necessary at December 31, 2018 . As part of our office repositioning strategy adopted by our Board of Trustees, and pursuant to our accounting policy, in 2017, we evaluated the recoverability of the carrying values of each of the real estate assets that comprised our portfolio and determined that due to the shortening of the expected periods of ownership as a result of the office repositioning strategy and current estimates of market value less estimated costs to sell, it was necessary to reduce the net book value of a portion of the real estate assets in our portfolio to their estimated fair values. As a result, in the first quarter of 2017, we recorded an impairment charge related to 25 S. Charles Street of $1.3 million in accordance with our impairment analysis procedures. We determined this impairment based on third party offer prices, which are level 2 inputs according to the fair value hierarchy established in ASC 820. We reduced the aggregate carrying value of this property from $24.6 million to its estimated fair value less estimated costs to sell of $23.3 million . This property was sold in April 2017 (see Note 3 for additional information). In the second quarter of 2017, we recorded an impairment charge related to the Five Property Portfolio held for sale as of June 30, 2017 of $18.4 million in accordance with our impairment analysis procedures. We determined this impairment based on third party offer prices, which are level 2 inputs according to the fair value hierarchy established in ASC 820. We reduced the aggregate carrying value of these properties from $99.0 million to their estimated fair value less estimated costs to sell of $80.6 million . This portfolio of properties was sold in July 2017 (see Note 3 for additional information).We evaluated each of our properties and determined there were no additional valuation adjustments necessary at December 31, 2017 . Financial Instruments In addition to the assets described in the above table, our financial instruments include our cash and cash equivalents, real estate mortgage receivable, restricted cash, senior unsecured debt and mortgage notes payable. At December 31, 2018 and 2017 , the fair value of these additional financial instruments were not materially different from their carrying values, except as follows (in thousands): December 31, 2018 December 31, 2017 Principal Balance Fair Value Principal Balance Fair Value Senior unsecured debt and mortgage notes payable $ 276,000 $ 283,214 $ 856,940 $ 874,280 The fair values of our senior notes are based on quoted market prices (level 2 inputs) and the fair values of our mortgage notes payable are based on estimates using discounted cash flow analyses and currently prevailing interest rates adjusted by credit risk spreads (level 3 inputs). Other financial instruments that potentially subject us to concentrations of credit risk consist principally of rents receivable; however, as of December 31, 2018 , no single tenant of ours is responsible for more than 7.5% of our total annualized rents, other than one tenant, Expedia, Inc., that is responsible for 13.9% of our total annualized rents (see Note 17).

Earnings Per Common Share

Earnings Per Common Share	12 Months Ended
	Dec. 31, 2018
Earnings Per Share [Abstract]
Earnings Per Common Share	Earnings Per Common Share The following table sets forth the computation of basic and diluted earnings per share (amounts in thousands except per share amounts): Year Ended December 31, 2018 2017 2016 Numerator for earnings per common share - basic: Net income $ 272,908 $ 29,666 $ 232,894 Net income attributable to noncontrolling interest (95 ) (10 ) — Preferred distributions (7,988 ) (7,988 ) (17,956 ) Excess fair value of consideration paid over carrying value of preferred shares — — (9,609 ) Numerator for net income per share - basic $ 264,825 $ 21,668 $ 205,329 Numerator for earnings per common share - diluted: Net income $ 272,908 $ 29,666 $ 232,894 Net income attributable to noncontrolling interest (95 ) — — Preferred distributions (7,988 ) (7,988 ) (17,956 ) Excess fair value of consideration paid over carrying value of preferred shares — — (9,609 ) Numerator for net income per share - diluted $ 264,825 $ 21,678 $ 205,329 Denominator for earnings per common share - basic and diluted: Weighted average number of common shares outstanding - basic (1) 122,314 124,125 125,474 RSUs (2) 956 912 1,294 LTIP Units (3) 115 92 — Weighted average number of common shares outstanding - diluted 123,385 125,129 126,768 Net income per common share attributable to Equity Commonwealth common shareholders: Basic $ 2.17 $ 0.17 $ 1.64 Diluted $ 2.15 $ 0.17 $ 1.62 Anti-dilutive securities: Effect of Series D preferred shares; 6 1/2% cumulative convertible (4) 2,563 2,363 2,363 Effect of LTIP Units 43 — — Effect of OP Units (5) 1 — — (1) The years ended December 31, 2018 , 2017 and 2016 , include 308 , 33 and 0 weighted-average, unvested, earned RSUs, respectively. (2) Represents weighted-average number of common shares that would have been issued if the year-end was the measurement date for RSUs. (3) Represents the weighted-average dilutive shares issuable from LTIP Units if the year-end was the measurement date for the periods shown. (4) The Series D preferred shares are excluded from the diluted earnings per share calculation because including the Series D preferred shares would also require that the preferred distributions be added back to net income, resulting in anti-dilution. (5) Beneficial interests in the Operating Trust.

Segment Information

Segment Information	12 Months Ended
	Dec. 31, 2018
Segment Reporting [Abstract]
Segment Information	Segment Information Our primary business is the ownership and operation of office properties, and we currently have one reportable segment. More than 90% of our revenues for the year ended December 31, 2018 are from office properties. For the year ended December 31, 2018, Expedia, Inc. individually accounted for 10.3% of our consolidated revenues.

Related Person Transactions

Related Person Transactions	12 Months Ended
	Dec. 31, 2018
Related Party Transactions [Abstract]
Related Person Transactions	Related Person Transactions The following discussion includes a description of our related person transactions for the years ended December 31, 2018 , 2017 and 2016 . Two North Riverside Plaza Joint Venture Limited Partnership: Effective July 20, 2015, we entered into a lease with Two North Riverside Plaza Joint Venture Limited Partnership, an entity associated with Mr. Zell, our Chairman, to occupy office space on the twentieth and twenty-first floors of Two North Riverside Plaza in Chicago, Illinois (20th/21st Floor Office Lease). The initial term of the lease is approximately five years, with one 5 -year renewal option. We made improvements to the office space utilizing the $0.7 million tenant improvement allowance pursuant to the lease. In connection with the 20th/21st Floor Office Lease, we also have a storage lease with Two North Riverside Plaza Joint Venture Limited Partnership for storage space in the basement of Two North Riverside Plaza. The storage lease expires December 31, 2020; however, each party has the right to terminate on 30 days' prior written notice. During the years ended December 31, 2018 , 2017 and 2016 , we recognized expense of $0.8 million , $0.8 million and $0.8 million , respectively, pursuant to the 20th/21st Floor Office Lease and the related storage lease. The future minimum lease payments scheduled to be paid by us during the current terms of this lease as of December 31, 2018 are as follows: $0.9 million in 2019 and $0.9 million in 2020 . As of December 31, 2018 and 2017 , we did no t have any amounts due to Two North Riverside Plaza Joint Venture Limited Partnership pursuant to the 20th/21st Floor Office Lease and the related storage space. Related/Corvex: On July 31, 2014, at the reconvened session of our 2014 annual meeting of shareholders, our shareholders voted to approve the reimbursement of verified expenses incurred by Related Fund Management, LLC and Corvex Management LP (Related/Corvex) in connection with their consent solicitations to remove our former Trustees and elect the new Board of Trustees and to engage in related litigation. In August 2016, we paid a final $8.2 million payment of the $33.3 million in total reimbursements to Related/Corvex.

Selected Quarterly Financial Da

Selected Quarterly Financial Data (Unaudited)	12 Months Ended
	Dec. 31, 2018
Quarterly Financial Information Disclosure [Abstract]
Selected Quarterly Financial Data (Unaudited)	Selected Quarterly Financial Data (Unaudited) The following is a summary of our unaudited quarterly results of operations for 2018 and 2017 (dollars in thousands): 2018 First Quarter Second Quarter Third Quarter Fourth Quarter Total revenues $ 58,588 $ 48,636 $ 46,873 $ 42,925 Net income attributable to Equity Commonwealth common shareholders 185,602 35,036 30,767 13,420 Net income attributable to Equity Commonwealth common shareholders per share—basic 1.50 0.29 0.25 0.11 Net income attributable to Equity Commonwealth common shareholders per share—diluted 1.48 0.29 0.25 0.11 The first quarter 2018 net income attributable to Equity Commonwealth common shareholders was primarily attributable to the gain on sale of properties of $205.2 million . The decrease in the second, third and fourth quarter 2018 total revenues is primarily attributable to properties sold in 2018. 2017 First Quarter Second Quarter Third Quarter Fourth Quarter Total revenues $ 99,551 $ 91,599 $ 77,798 $ 71,623 Net income (loss) attributable to Equity Commonwealth common shareholders 21,817 (7,806 ) 31,215 (23,558 ) Net income (loss) attributable to Equity Commonwealth common shareholders per share—basic 0.18 (0.06 ) 0.25 (0.19 ) Net income (loss) attributable to Equity Commonwealth common shareholders per share—diluted 0.17 (0.06 ) 0.25 (0.19 ) The decrease in the third and fourth quarter 2017 total revenues is primarily attributable to properties sold in 2017. The second quarter 2017 net loss attributable to Equity Commonwealth common shareholders was primarily attributable to the loss on asset impairment of $18.4 million . The fourth quarter 2017 net loss attributable to Equity Commonwealth common shareholders was primarily attributable to the loss on sale of properties of $29.2 million .

Subsequent Events

Subsequent Events	12 Months Ended
	Dec. 31, 2018
Subsequent Events [Abstract]
Subsequent Events	Subsequent Events On January 11, 2019, we announced that our Board of Trustees declared a dividend of $0.40625 per series D preferred share, which will be paid on February 15, 2019 to shareholders of record on January 30, 2019. On January 29, 2019, certain of our subsidiaries entered into a contract to sell 100% of the equity interests in the fee simple owner of our 1,286,936 square foot property at 1735 Market Street, for a sales price of $451.6 million , excluding credits and closing costs. This transaction is subject to customary closing extensions and conditions, and there is no certainty that this transaction will close.

SCHEDULE II VALUATION AND QUALI

SCHEDULE II VALUATION AND QUALIFYING ACCOUNTS	12 Months Ended
	Dec. 31, 2018
SEC Schedule, 12-09, Valuation and Qualifying Accounts [Abstract]
Schedule II Valuation and Qualifying Accounts	Description Balance at Charged to Deductions Balance at Year Ended December 31, 2016: Allowance for doubtful accounts $ 7,715 $ 1,056 $ (3,666 ) $ 5,105 Year Ended December 31, 2017: Allowance for doubtful accounts $ 5,105 $ 935 $ (1,269 ) $ 4,771 Year Ended December 31, 2018: Allowance for doubtful accounts $ 4,771 $ 1,273 $ (1,070 ) $ 4,974

SCHEDULE III REAL ESTATE AND AC

SCHEDULE III REAL ESTATE AND ACCUMULATED DEPRECIATION	12 Months Ended
	Dec. 31, 2018
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation Disclosure [Abstract]
Schedule III Real Estate and Accumulated Depreciation	Initial Cost to Company Cost Amount Carried at Close of Period Property City State Encumbrances(1) Land Buildings and Improvements Costs Impairment/Write Downs Land Buildings and Improvements Total(2) Accumulated Depreciation(3) Date Acquired Original Construction Date 1225 Seventeenth Street Denver CO $ — $ 22,400 $ 110,090 $ 33,108 $ (2,750 ) $ 22,400 $ 140,448 $ 162,848 $ 35,066 6/24/2009 1982 1250 H Street, NW Washington DC — 5,975 53,778 15,895 (579 ) 5,975 69,094 75,069 33,383 6/23/1998 1992 Georgetown-Green and Harris Buildings Washington DC — 24,000 35,979 1,076 — 24,000 37,055 61,055 8,402 9/3/2009 1960;1975 109 Brookline Avenue Boston MA — 3,168 30,397 15,636 (1,397 ) 3,168 44,636 47,804 21,853 9/28/1995 1915 1735 Market Street Philadelphia PA — 24,753 222,775 95,918 (14,927 ) 24,748 303,771 328,519 136,425 6/30/1998 1990 206 East 9th Street Austin TX 26,000 7,900 38,533 5,362 (444 ) 7,900 43,451 51,351 7,073 5/31/2012 1984 Bridgepoint Square Austin TX — 7,784 70,526 20,637 (1,172 ) 7,784 89,991 97,775 45,146 12/5/1997 1986;1996;1997 Research Park Austin TX — 15,859 60,175 33,383 — 21,212 88,205 109,417 38,117 10/7/1998; 10/11/2016 1968;1998; 2001 333 108th Avenue NE Bellevue WA — 14,400 136,412 2,746 (31 ) 14,400 139,127 153,527 33,117 11/12/2009 2008 600 108th Avenue NE Bellevue WA — 3,555 30,244 19,677 (1,199 ) 3,555 48,722 52,277 17,386 7/16/2004 1980 $ 26,000 $ 129,794 $ 788,909 $ 243,438 $ (22,499 ) $ 135,142 $ 1,004,500 $ 1,139,642 $ 375,968 Analysis of the carrying amount of real estate properties and accumulated depreciation: Real Estate Accumulated Balance at January 1, 2016 $ 3,887,352 $ 898,939 Additions 113,258 101,647 Loss on asset impairment (58,595 ) (119 ) Disposals (1,085,125 ) (245,212 ) Balance at December 31, 2016 2,856,890 755,255 Additions 55,041 71,979 Loss on asset impairment (6,711 ) — Properties classified to held for sale (138,130 ) (62,064 ) Disposals (1,019,479 ) (314,452 ) Balance at December 31, 2017 1,747,611 450,718 Additions 58,618 39,161 Loss on asset impairment (12,032 ) — Disposals (654,555 ) (113,911 ) Balance at December 31, 2018 $ 1,139,642 $ 375,968 (1) Excludes net unamortized premiums and net unamortized deferred financing costs. (2) Excludes value of real estate intangibles. Aggregate cost for federal income tax purposes is approximately $1,129,227 . (3) Depreciation is calculated using the straight line method over estimated useful lives of up to 40 years for buildings and improvements and up to 12 years for personal property.

Summary of Significant Accoun_2

Summary of Significant Accounting Policies (Policies)	12 Months Ended
	Dec. 31, 2018
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, 2018 , unless the context indicates otherwise. All intercompany transactions and balances have been eliminated.
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 capital or operating lease. The classification of a lease as capital 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 (FASB) Accounting Standards Codification (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 to 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 (presented in our consolidated balance sheets as acquired real estate leases) 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, 2018 , 2017 , and 2016 we recorded a loss on asset impairment in continuing operations totaling $12.1 million , $19.7 million and $58.5 million respectively, to reduce the carrying value of properties to their estimated fair values (see Note 15). 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. 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 continually 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.
Marketable Securities	Marketable Securities. All of our marketable securities are classified as available-for-sale and consisted of United States Treasury notes and common stock. Available-for-sale securities are presented on our consolidated balance sheets at fair value. Changes in values of the United States Treasury notes are recognized in cumulative other comprehensive loss. Realized gains and losses are recognized in earnings only upon the sale of the United States Treasury notes. Changes in values of common stock prior to their sale in March 2018, were recognized in interest and other income, net on the consolidated statements of operations. We evaluate our marketable securities for impairment each reporting period. For securities with unrealized losses, we review the underlying cause of the decline in value and the estimated recovery period, as well as the severity and duration of the decline. In our evaluation, we consider our ability and intent to hold these investments for a reasonable period of time sufficient for us to recover our cost basis. To the extent an other-than-temporary impairment is deemed to have occurred, an impairment charge is recorded and a new cost basis is established.
Restricted Cash	Restricted Cash. Restricted cash consists 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 financing fees, deferred leasing costs, capitalized lease incentives and prepaid property operating expenses. Deferred financing fees include issuance costs related to borrowings and are capitalized and amortized over the terms of the respective loans. Deferred leasing costs include brokerage, legal and other fees associated with the successful negotiation of leases and 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.
Accounting Policy for Derivative Instruments	Derivative Instruments. Derivative instruments are recognized as either assets or liabilities in the consolidated balance sheets at fair value. The accounting for changes in the fair value of a derivative instrument depends on whether such instrument has been designated and qualifies as part of a hedging relationship and, further, on the type of hedging relationship. For those derivative instruments that are designated and qualify as hedging instruments, we must designate the hedging instrument, based upon the exposure being hedged, as a fair value hedge, cash flow hedge or a hedge of a net investment in a foreign operation. As of December 31, 2018 , we do not have any interest rate derivatives. We are exposed to certain risks relating to our ongoing business operations. The primary risk managed by using derivative instruments is interest rate risk. Interest rate swaps and caps may be entered into to manage interest rate risk associated with floating rate borrowings. We designate interest rate swaps and caps as cash flow hedges of floating rate borrowings.
Revenue Recognition	Revenue Recognition. Rental income 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. Tenant reimbursements and other income includes property level operating expenses reimbursed by our tenants, as well as other incidental revenues, which are recorded as expenses are incurred.
Allowance for Doubtful Accounts	Allowance for Doubtful Accounts. We maintain an allowance for doubtful accounts for estimated losses resulting from the inability or unwillingness of certain tenants to make payments required under their leases. The computation of the allowance is based on the tenants' payment histories and current credit profiles, as well as other considerations.
Share-Based Compensation	Share-Based Compensation. All share-based compensation is measured at fair value on the grant date or date of modification, as applicable, and recognized in earnings over the requisite service period. Depending upon the settlement terms of the awards, all or a portion of the fair value of share-based awards may be presented as a liability or as equity in the consolidated balance sheets.
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 (RSUs) or beneficial interests in the Operating Trust (LTIP Units) were converted into our common shares, which could result in a lower EPS amount. The effect of our series D convertible preferred shares on net income attributable to common shareholders is anti-dilutive for all periods presented.
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 August 2018, the FASB issued Accounting Standards Update (ASU) 2018-13, Fair Value Measurement (Topic 820): Disclosure Framework - Changes to the Disclosure Requirements for Fair Value Measurement, which changes the fair value measurement disclosure requirements of ASC 820. This update is effective for fiscal years beginning after December 15, 2019, and for interim periods within those fiscal years. We do not expect the adoption of ASU 2018-13 to have a material impact on our consolidated financial statements. In June 2018, the FASB issued ASU 2018-07, Improvements to Nonemployee Share-Based Payment Accounting, which simplifies the accounting for share-based payments granted to nonemployees for goods and services. This update is effective for fiscal years beginning after December 15, 2018, and for interim periods within those fiscal years. We do not expect the adoption of ASU 2018-07 to have a material impact on our consolidated financial statements. In May 2017, the FASB issued ASU 2017-09 Compensation-Stock Compensation (Topic 718): Scope of Modification Accounting. ASU 2017-09 is designed to provide clarity and reduce both (1) diversity in practice and (2) cost and complexity when applying the guidance in Topic 718, to a change to the terms or conditions of a share-based payment award. ASU 2017-09 is effective for fiscal years beginning after December 15, 2017. We adopted ASU 2017-09 on January 1, 2018, and the adoption did not have a material impact on our consolidated financial statements. In February 2017, the FASB issued ASU 2017-05 Clarifying the Scope of Asset Derecognition Guidance and Accounting for Partial Sales of Nonfinancial Assets. ASU 2017-05 is designed to provide guidance on how to recognize gain and losses on sales, including partial sales, of nonfinancial assets to noncustomers. We adopted ASU 2017-05 on January 1, 2018 and the adoption did not have a material impact on our consolidated financial statements. In June 2016, the FASB issued ASU 2016-13, Financial Instruments-Credit Losses (Topic 326): Measurement of Credit Losses on Financial Instruments, which requires more timely recognition of credit losses associated with financial assets. This update is effective for fiscal years beginning after December 15, 2019, and for interim periods within those fiscal years. Early adoption is permitted for fiscal years, and interim periods within those years, beginning after December 15, 2018. We are currently evaluating the impact, if any, the adoption of ASU 2016-13 will have on our consolidated financial statements. In February 2016, the FASB issued ASU 2016-02, Leases, 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). ASU 2016-02 requires lessees to apply a dual approach, classifying leases as either finance or operating leases based on the principle of whether or not the lease is effectively a financed purchase of the leased asset by the lessee. This classification will determine whether the lease expense is recognized based on an effective interest method or on a straight-line basis over the term of the lease. A lessee is also required to record a right-of-use asset and a lease liability for all leases with a term of greater than 12 months regardless of their classification. Leases with a term of 12 months or less will be accounted for similar to existing guidance for operating leases today. ASU 2016-02 supersedes previous leasing standards. ASU 2016-02 is effective for us for reporting periods beginning after December 15, 2018, with early adoption permitted. For leases where we are the lessor, we expect to account for these leases using an approach that is substantially equivalent to current guidance. Additionally, under ASU 2016-02 lessors may only capitalize incremental direct leasing costs. For leases in which we are the lessee, we expect to recognize a right-of-use asset and a lease liability equal to the present value of the minimum lease payments with rent expense being recognized on a straight-line basis and the right of use asset being reduced when lease payments are made. In July 2018, the FASB issued ASU 2018-11 to provide entities with relief from the costs of implementing certain aspects of ASU 2016-02. The amendment to the new leases standard includes a practical expedient that provides lessors an option not to separate lease and non-lease components when certain criteria are met and instead account for those components as a single component under the new leases standard. The amendment also provides a transition option that permits the application of the new guidance as of the adoption date rather than to all periods presented. We anticipate electing the practical expedient to account for both our lease and non-lease components as a single component under the leases standard and electing the new transition option. We believe the adoption of these pronouncements will have an immaterial impact on our financial statements, both as a lessor and as a lessee. In January 2016, the FASB issued ASU 2016-01, Financial Instruments-Overall: Recognition and Measurement of Financial Assets and Financial Liabilities, related to certain aspects of recognition, measurement, presentation, and disclosure of financial instruments. ASU 2016-01 will require entities to measure their equity investments at fair value and recognize any changes in fair value in net income, with certain exceptions, rather than other comprehensive income. This update is effective for fiscal years beginning after December 15, 2017, including interim periods within those fiscal years. We adopted ASU 2016-01 on January 1, 2018 and reclassified a $1.9 million unrealized gain from cumulative other comprehensive loss to cumulative net income on our consolidated balance sheet (see Note 10). In May 2014, the FASB issued ASU 2014-09, Revenue from Contracts with Customers. The objective of ASU 2014-09, as amended, is to establish a single comprehensive model for entities to use in accounting for revenue arising from contracts with customers and will supersede most of the existing revenue recognition guidance, including industry-specific guidance. The core principle is that an entity should recognize revenue to depict the transfer of promised goods or services to customers in an amount that reflects the consideration to which the entity expects to be entitled in exchange for those goods or services. In applying ASU 2014-09, companies will perform a five-step analysis of transactions to determine when and how revenue is recognized. ASU 2014-09 applies to all contracts with customers except those that are within the scope of other topics in the FASB’s ASC, and more particularly lease contracts with customers, which are a scope exception. ASU 2014-09 is effective for the first interim period within annual reporting periods beginning after December 15, 2017, with early adoption permitted. We adopted ASU 2014-09 on January 1, 2018 and the adoption did not have a material impact on our consolidated financial statements.
Properties Held For Sale	We classify all properties that meet the criteria outlined in the Property, Plant and Equipment Topic of the FASB ASC as held for sale on our consolidated balance sheets.

Real Estate Properties (Tables)

Real Estate Properties (Tables)	12 Months Ended
	Dec. 31, 2018
Real Estate [Abstract]
Summary of Properties Held-for-sale	As of December 31, 2017 , we classified 1600 Market Street as held for sale. Summarized balance sheet information for 1600 Market Street is as follows (in thousands): December 31, 2017 Real estate properties $ 76,066 Rents receivable, net of allowance for doubtful accounts of $5 13,270 Other assets, net 8,352 Assets held for sale $ 97,688 Accounts payable and accrued expenses $ 1,021 Rent collected in advance 408 Security deposits 411 Liabilities related to properties held for sale $ 1,840
Summary of Properties Disposed and Income Statement Information for Properties Included in Discontinued Operations	During the year ended December 31, 2018 , we sold the following properties (dollars in thousands): Asset Date Sold Number of Number of Square Footage Gross Sales Price Gain (Loss) on Sale Properties 1600 Market Street February 2018 1 1 825,968 $ 160,000 $ 54,599 600 West Chicago Avenue (1) February 2018 1 2 1,561,477 510,000 107,790 5073, 5075, & 5085 S. Syracuse Street March 2018 1 1 248,493 115,186 42,762 1601 Dry Creek Drive May 2018 1 1 552,865 68,500 26,979 777 East Eisenhower Parkway August 2018 1 1 290,530 29,500 5,308 8750 Bryn Mawr Avenue September 2018 1 2 636,078 141,000 15,194 97 Newberry Road December 2018 1 1 289,386 7,100 (1,174 ) 7 9 4,404,797 $ 1,031,286 $ 251,458 (1) The sale of this property did not represent a strategic shift under ASC Topic 205. However, the sale does represent an individually significant disposition. The operating results of this property are included in continued operations for all periods presented through the date of sale. Net income for this property was $110.6 million , $9.3 million and $12.2 million for the years ended December 31, 2018 , 2017 and 2016 , respectively. During the year ended December 31, 2017 , we sold the following properties (dollars in thousands): Asset Date Sold Number of Number of Square Footage Gross Sales Price Gain (Loss) on Sale Properties 111 Market Place January 2017 1 1 589,380 $ 60,100 $ (5,968 ) Cabot Business Park Land March 2017 — — — 575 (57 ) Parkshore Plaza April 2017 1 4 271,072 40,000 (2,460 ) 25 S. Charles Street April 2017 1 1 359,254 24,500 (3,487 ) 802 Delaware Avenue May 2017 1 1 240,780 34,000 9,099 1500 Market Street July 2017 1 1 1,759,193 328,000 38,585 6600 North Military Trail August 2017 1 3 639,825 132,050 (14,175 ) 789 East Eisenhower Parkway (1) December 2017 — 1 130,961 24,942 1,242 33 Stiles Lane December 2017 1 1 175,301 10,500 2,163 625 Crane Street (Land) December 2017 — — — 307 249 Mineral Rights December 2017 — — — 200 169 Portfolios of properties 4515 Seton Center Parkway March 2017 1 1 117,265 4516 Seton Center Parkway March 2017 1 1 120,559 Seton Center 2 2 237,824 $ 52,450 $ 22,479 Cherrington Corporate Center December 2017 1 7 454,700 Foster Plaza December 2017 1 8 727,743 Pittsburgh Portfolio 2 15 1,182,443 $ 71,000 $ (33,048 ) 820 W. Diamond July 2017 1 1 134,933 Danac Stiles Business Park July 2017 1 3 276,637 411 Farwell Avenue July 2017 1 1 422,727 2250 Pilot Knob Road July 2017 1 1 87,183 4700 Belleview Avenue July 2017 1 1 80,615 Five Property Portfolio 5 7 1,002,095 $ 84,000 $ 702 16 37 6,588,128 $ 862,624 $ 15,493 (1) We sold one building in a property containing two buildings.
Schedule of Future Minimum Lease Payments	The future minimum lease payments, excluding tenant reimbursement revenue, scheduled to be received by us during the current terms of our leases as of December 31, 2018 are as follows (in thousands): 2019 $ 109,888 2020 96,309 2021 104,279 2022 95,128 2023 86,299 Thereafter 567,547 $ 1,059,450

Lease Intangibles (Tables)

Lease Intangibles (Tables)	12 Months Ended
	Dec. 31, 2018
Leases [Abstract]
Summary of the Carrying Amounts for Acquired Real Estate Leases and Assumed Real Estate Lease Obligations	The following table summarizes the carrying amounts for our acquired real estate leases and assumed real estate lease obligations as of December 31, 2018 , and 2017 , and (in thousands): December 31, 2018 2017 Acquired in-place leases $ 3,691 $ 56,853 Acquired above market leases — 21,021 Acquired real estate leases 3,691 77,874 Accumulated amortization, acquired in-place leases (3,416 ) (41,978 ) Accumulated amortization, acquired above market leases — (12,049 ) Acquired real estate leases, net $ 275 $ 23,847 Acquired below market leases $ 1,570 $ 6,093 Accumulated amortization (1,453 ) (5,092 ) Assumed real estate lease obligations, net $ 117 $ 1,001
Schedule of Amortization of Lease Intangibles	Amortization of the lease intangibles for the years ended December 31, 2018 , 2017 , and 2016 , is as follows (in thousands): December 31, Income Statement Location 2018 2017 2016 Amortization of acquired in-place leases Depreciation and amortization $ 2,133 $ 7,220 $ 14,598 Amortization of above and below market leases Increase (decrease) to rental income (54 ) (1,774 ) (6,531 )
Schedule of Future Amortization of Net Intangible Lease Assets and Liabilities	Future amortization of net intangible lease assets and liabilities to be recognized by us during the current terms of our leases as of December 31, 2018 , are approximately (in thousands): 2019 $ 158 2020 — 2021 — 2022 — 2023 — Thereafter — $ 158

Marketable Securities (Tables)

Marketable Securities (Tables)	12 Months Ended
	Dec. 31, 2018
Investments, Debt and Equity Securities [Abstract]
Summary of Marketable Securities	Below is a summary of our marketable securities as of December 31, 2018 and 2017 (in thousands): December 31, 2018 2017 Amortized Cost Unrealized Loss Estimated Fair Value Cost or Amortized Cost Unrealized Gain, Net Estimated Fair Value Marketable securities $ 249,944 $ (342 ) $ 249,602 $ 276,567 $ 361 $ 276,928

Other Assets (Tables)

Other Assets (Tables)	12 Months Ended
	Dec. 31, 2018
Other Assets [Abstract]
Summary of Deferred Financing Fees, Deferred Leasing Costs and Capitalized Lease Incentives	The following table summarizes our deferred financing fees related to our unsecured revolving credit facility included in other assets, deferred leasing costs and capitalized lease incentives as of December 31, 2018 , and 2017 (in thousands): December 31, 2018 2017 Deferred financing fees $ — $ 14,458 Accumulated amortization — (12,709 ) Deferred financing fees, net $ — $ 1,749 Deferred leasing costs $ 69,930 $ 85,606 Accumulated amortization (18,807 ) (22,067 ) Deferred leasing costs, net $ 51,123 $ 63,539 Capitalized lease incentives $ 5,701 $ 8,562 Accumulated amortization (1,393 ) (2,054 ) Capitalized lease incentives, net $ 4,308 $ 6,508
Summary of Future Amortization of Deferred Leasing Costs and Capitalized Lease Incentives	Future amortization of deferred leasing costs and capitalized lease incentives to be recognized by us during the current terms of our leases as of December 31, 2018 are approximately (in thousands): Deferred Leasing Costs Capitalized Lease Incentives 2019 $ 5,550 $ 695 2020 5,812 583 2021 5,810 553 2022 5,126 425 2023 4,478 320 Thereafter 24,347 1,732 $ 51,123 $ 4,308

Indebtedness (Tables)

Indebtedness (Tables)	12 Months Ended
	Dec. 31, 2018
Debt Disclosure [Abstract]
Schedule of Outstanding Indebtedness	At December 31, 2018 and 2017 , our outstanding indebtedness included the following (in thousands): December 31, Interest Rate at December 31, 2018 Maturity Date 2018 2017 Unsecured revolving credit facility, at LIBOR plus a premium — % — $ — $ — 5-year unsecured term loan, at LIBOR plus a premium — % — — 200,000 7-year unsecured term loan, at LIBOR plus a premium — % — — 200,000 Unsecured floating rate debt — % $ — $ 400,000 5.875% Senior Unsecured Notes due 2020 5.875 % 9/15/2020 $ 250,000 $ 250,000 5.75% Senior Unsecured Notes due 2042 — % — — 175,000 Unsecured fixed rate debt 5.875 % $ 250,000 $ 425,000 206 East 9th Street 5.69 % 1/5/2021 $ 26,000 $ 26,536 97 Newberry Road — % — — 5,404 Secured fixed rate debt 5.69 % $ 26,000 $ 31,940 $ 276,000 $ 856,940 Unamortized net premiums, discounts and deferred financing fees (1,045 ) (8,362 ) $ 274,955 $ 848,578
Schedule of Required Principal Payments on Outstanding Debt	The required principal payments due during the next five years and thereafter under all of our outstanding debt at December 31, 2018 are as follows (in thousands): 2019 $ 567 2020 250,597 2021 24,836 2022 — 2023 — Thereafter — $ 276,000

Shareholders' Equity (Tables)

Shareholders' Equity (Tables)	12 Months Ended
	Dec. 31, 2018
Stockholders' Equity Note [Abstract]
Schedule of Paid Distributions and Declared Distributions	The following characterizes distributions paid per common share for the years ended December 31, 2018 , 2017 , and 2016 : Year Ended December 31, 2018 2017 2016 Ordinary income 100.0 % — % — % Return of capital — % — % — % Capital gain — % — % — % Unrecaptured Section 1250 gain — % — % — % 100.0 % — % — % In 2018 , our Board of Trustees declared distributions on our series D preferred shares to date as follows: Declaration Date Record Date Payment Date Dividend Per Share January 12, 2018 January 30, 2018 February 15, 2018 $ 0.40625 April 11, 2018 April 27, 2018 May 15, 2018 $ 0.40625 July 12, 2018 July 30, 2018 August 15, 2018 $ 0.40625 September 26, 2018 October 29, 2018 November 15, 2018 $ 0.40625

Noncontrolling Interest (Tables

Noncontrolling Interest (Tables)	12 Months Ended
	Dec. 31, 2018
Noncontrolling Interest [Abstract]
Schedule of Issued and Outstanding Common Shares	The following table presents the changes in Equity Commonwealth’s issued and outstanding common shares and units for the year ended December 31, 2018 : Common Shares OP Units and LTIP Units Total Outstanding at January 1, 2018 124,217,616 42,520 124,260,136 Repurchase of shares (2,970,209 ) — (2,970,209 ) Restricted share, time-based LTIP Unit grants and vested restricted stock units, net of forfeitures 324,748 3,200 327,948 Outstanding at December 31, 2018 121,572,155 45,720 121,617,875 Noncontrolling ownership interest in the Operating Trust 0.04 %
Schedule of Issued and Outstanding Units	The following table presents the changes in Equity Commonwealth’s issued and outstanding common shares and units for the year ended December 31, 2018 : Common Shares OP Units and LTIP Units Total Outstanding at January 1, 2018 124,217,616 42,520 124,260,136 Repurchase of shares (2,970,209 ) — (2,970,209 ) Restricted share, time-based LTIP Unit grants and vested restricted stock units, net of forfeitures 324,748 3,200 327,948 Outstanding at December 31, 2018 121,572,155 45,720 121,617,875 Noncontrolling ownership interest in the Operating Trust 0.04 % The table below presents a summary of restricted share, RSU and LTIP Unit activity for the years ended December 31, 2018 , 2017 and 2016 : Number Weighted Number Weighted Outstanding at December 31, 2015 776,456 $ 26.62 1,884,110 $ 15.82 Granted 167,790 27.29 691,385 15.57 Vested (191,498 ) 26.54 — — Forfeited (539 ) 26.82 (2,727 ) 15.54 Outstanding at December 31, 2016 752,209 $ 26.79 2,572,768 $ 15.75 Granted 144,192 31.51 585,967 15.97 Vested (217,449 ) 27.97 (226,258 ) 15.99 Not earned (1) — — (800,530 ) 15.99 Outstanding at December 31, 2017 678,952 $ 27.41 2,131,947 $ 15.69 Granted 154,209 30.05 634,628 14.90 Vested (407,273 ) 27.06 (367,260 ) 15.79 Not earned (1) — — (352,671 ) 15.47 Outstanding at December 31, 2018 425,888 $ 28.70 2,046,644 $ 15.47 (1) The table presents the maximum number of shares issued or issuable from outstanding equity awards. RSUs and market-based LTIP Units not earned are the shares market-based award recipients do not receive based on the performance measurement completed at the end of the performance period.

Cumulative Other Comprehensiv_2

Cumulative Other Comprehensive Loss (Tables)	12 Months Ended
	Dec. 31, 2018
Accumulated Other Comprehensive Income (Loss), Net of Tax [Abstract]
Schedule of Amounts Recognized in Cumulative Other Comprehensive Loss	The following tables present the amounts recognized in cumulative other comprehensive loss for the years ended December 31, 2018 and 2017 (in thousands): Unrealized Loss on Derivative Instruments Unrealized Gain (Loss) on Marketable Securities Total Balance as of January 1, 2018 $ (456 ) $ 361 $ (95 ) Amounts reclassified from cumulative other comprehensive loss to cumulative net income pursuant to a change in accounting principle — (1,902 ) (1,902 ) Other comprehensive income before reclassifications 84 1,199 1,283 Amounts reclassified from cumulative other comprehensive loss to net income 372 — 372 Net current period other comprehensive income 456 1,199 1,655 Balance as of December 31, 2018 $ — $ (342 ) $ (342 ) Unrealized Loss Unrealized Gain, Net on Marketable Securities Total Balance as of January 1, 2017 $ (208 ) $ — $ (208 ) Other comprehensive (loss) income before reclassifications (297 ) 361 64 Amounts reclassified from cumulative other comprehensive loss to net income 49 — 49 Net current period other comprehensive (loss) income (248 ) 361 113 Balance as of December 31, 2017 $ (456 ) $ 361 $ (95 )
Schedule of Reclassifications Out of Cumulative Other Comprehensive Loss	The following table presents reclassifications out of cumulative other comprehensive loss for the years ended December 31, 2018 and 2017 (in thousands): Amounts Reclassified from Cumulative Other Comprehensive Loss to Net Income Year Ended December 31, Details about Cumulative Other Comprehensive Loss Components 2018 2017 Affected Line Items in the Statement of Operations Interest rate cap contract $ 293 $ — Interest and other income Interest rate cap contract 79 49 Interest expense $ 372 $ 49

Income Taxes (Tables)

Income Taxes (Tables)	12 Months Ended
	Dec. 31, 2018
Income Tax Disclosure [Abstract]
Schedule of Provision for Income Taxes	Our provision for income taxes consists of the following (in thousands): Year Ended December 31, 2018 2017 2016 Current: State and local $ (3,156 ) $ (500 ) $ (745 ) Federal — — — Income tax expense $ (3,156 ) $ (500 ) $ (745 )
Schedule of Reconciliation of Effective Tax Rate and the U.S. Federal Statutory Income Tax Rate	A reconciliation of our effective tax rate and the U.S. Federal statutory income tax rate is as follows: Year Ended December 31, 2018 2017 2016 Taxes at statutory U.S. federal income tax rate 21.00 % 35.00 % 35.00 % Dividends paid deduction and net operating loss utilization (21.00 )% (35.00 )% (35.00 )% Federal taxes on built-in gain — % — % — % State and local income taxes 1.14 % 1.66 % 0.32 % Effective tax rate 1.14 % 1.66 % 0.32 %

Derivative Instruments (Tables)

Derivative Instruments (Tables)	12 Months Ended
	Dec. 31, 2018
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Schedule of Fair Value of Derivative Financial Instruments	The table below presents the fair value of derivative financial instruments as well as classification on the consolidated balance sheets as of December 31, 2018 , and 2017 (amounts in thousands): Fair Value as of December 31, Interest Rate Derivative Designated as Hedging Instrument Balance Sheet Location 2018 2017 Interest rate cap Other assets $ — $ 17
Schedule of Gain or Loss Recognized on Interest Rate Derivatives Designated as Cash Flow Hedges	The table below details the location in the financial statements of the gain or loss recognized on interest rate derivatives designated as cash flow hedges for the years ended December 31, 2018 , 2017 , and 2016 (amounts in thousands): Year Ended December 31, 2018 2017 2016 Amount of gain (loss) recognized in cumulative other comprehensive loss (effective portion) $ 84 $ (297 ) $ (554 ) Amount of loss reclassified from cumulative other comprehensive loss into interest expense (effective portion) 79 49 3,792 Amount of loss recognized in income (ineffective portion and amount excluded from effectiveness testing) 293 — 241

Share-Based Compensation (Table

Share-Based Compensation (Tables)	12 Months Ended
	Dec. 31, 2018
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Summary of Restricted Share and Restricted Stock Unit Activity	The following table presents the changes in Equity Commonwealth’s issued and outstanding common shares and units for the year ended December 31, 2018 : Common Shares OP Units and LTIP Units Total Outstanding at January 1, 2018 124,217,616 42,520 124,260,136 Repurchase of shares (2,970,209 ) — (2,970,209 ) Restricted share, time-based LTIP Unit grants and vested restricted stock units, net of forfeitures 324,748 3,200 327,948 Outstanding at December 31, 2018 121,572,155 45,720 121,617,875 Noncontrolling ownership interest in the Operating Trust 0.04 % The table below presents a summary of restricted share, RSU and LTIP Unit activity for the years ended December 31, 2018 , 2017 and 2016 : Number Weighted Number Weighted Outstanding at December 31, 2015 776,456 $ 26.62 1,884,110 $ 15.82 Granted 167,790 27.29 691,385 15.57 Vested (191,498 ) 26.54 — — Forfeited (539 ) 26.82 (2,727 ) 15.54 Outstanding at December 31, 2016 752,209 $ 26.79 2,572,768 $ 15.75 Granted 144,192 31.51 585,967 15.97 Vested (217,449 ) 27.97 (226,258 ) 15.99 Not earned (1) — — (800,530 ) 15.99 Outstanding at December 31, 2017 678,952 $ 27.41 2,131,947 $ 15.69 Granted 154,209 30.05 634,628 14.90 Vested (407,273 ) 27.06 (367,260 ) 15.79 Not earned (1) — — (352,671 ) 15.47 Outstanding at December 31, 2018 425,888 $ 28.70 2,046,644 $ 15.47 (1) The table presents the maximum number of shares issued or issuable from outstanding equity awards. RSUs and market-based LTIP Units not earned are the shares market-based award recipients do not receive based on the performance measurement completed at the end of the performance period.
Summary of Assumptions and Fair Values for Restricted Stock Units Granted in the Period	The assumptions and fair values for the RSUs and market-based LTIP Units granted for the years ended December 31, 2018 , 2017 and 2016 are included in the following table on a per share and unit basis. 2018 2017 2016 Fair value of RSUs and market-based LTIP Units granted at the target amount $ 37.13 $ 39.81 $ 38.80 Fair value of RSUs and market-based LTIP Units granted at the maximum amount $ 14.90 $ 15.97 $ 15.57 Expected term (years) 4 4 4 Expected volatility — — — Expected dividend yield 1.68 % 1.59 % 1.86 % Risk-free rate 2.26 % 1.49 % 1.07 %

Fair Value of Assets and Liab_2

Fair Value of Assets and Liabilities (Tables)	12 Months Ended
	Dec. 31, 2018
Fair Value Disclosures [Abstract]
Schedule of Assets and Liabilities Measured at Fair Value	The table below presents certain of our assets and liabilities measured at fair value during 2018 and 2017 , categorized by the level of inputs used in the valuation of each asset and liability (dollars in thousands): Fair Value at December 31, 2018 Using Quoted Prices in Active Markets for Identical Assets Significant Other Observable Inputs Significant Unobservable Inputs Description Total (Level 1) (Level 2) (Level 3) Recurring Fair Value Measurements: Marketable securities $ 249,602 $ 249,602 $ — $ — Fair Value at December 31, 2017 Using Quoted Prices in Active Markets for Identical Assets Significant Other Observable Inputs Significant Unobservable Inputs Description Total (Level 1) (Level 2) (Level 3) Recurring Fair Value Measurements: Effective portion of interest rate cap contract $ 17 $ — $ 17 $ — Marketable securities $ 276,928 $ 276,928 $ — $ —
Schedule of Fair Value and Carrying Value of Financial Instruments	At December 31, 2018 and 2017 , the fair value of these additional financial instruments were not materially different from their carrying values, except as follows (in thousands): December 31, 2018 December 31, 2017 Principal Balance Fair Value Principal Balance Fair Value Senior unsecured debt and mortgage notes payable $ 276,000 $ 283,214 $ 856,940 $ 874,280

Earnings Per Common Share (Tabl

Earnings Per Common Share (Tables)	12 Months Ended
	Dec. 31, 2018
Earnings Per Share [Abstract]
Schedule of Computation of Basic and Diluted Earnings Per Share	The following table sets forth the computation of basic and diluted earnings per share (amounts in thousands except per share amounts): Year Ended December 31, 2018 2017 2016 Numerator for earnings per common share - basic: Net income $ 272,908 $ 29,666 $ 232,894 Net income attributable to noncontrolling interest (95 ) (10 ) — Preferred distributions (7,988 ) (7,988 ) (17,956 ) Excess fair value of consideration paid over carrying value of preferred shares — — (9,609 ) Numerator for net income per share - basic $ 264,825 $ 21,668 $ 205,329 Numerator for earnings per common share - diluted: Net income $ 272,908 $ 29,666 $ 232,894 Net income attributable to noncontrolling interest (95 ) — — Preferred distributions (7,988 ) (7,988 ) (17,956 ) Excess fair value of consideration paid over carrying value of preferred shares — — (9,609 ) Numerator for net income per share - diluted $ 264,825 $ 21,678 $ 205,329 Denominator for earnings per common share - basic and diluted: Weighted average number of common shares outstanding - basic (1) 122,314 124,125 125,474 RSUs (2) 956 912 1,294 LTIP Units (3) 115 92 — Weighted average number of common shares outstanding - diluted 123,385 125,129 126,768 Net income per common share attributable to Equity Commonwealth common shareholders: Basic $ 2.17 $ 0.17 $ 1.64 Diluted $ 2.15 $ 0.17 $ 1.62 Anti-dilutive securities: Effect of Series D preferred shares; 6 1/2% cumulative convertible (4) 2,563 2,363 2,363 Effect of LTIP Units 43 — — Effect of OP Units (5) 1 — — (1) The years ended December 31, 2018 , 2017 and 2016 , include 308 , 33 and 0 weighted-average, unvested, earned RSUs, respectively. (2) Represents weighted-average number of common shares that would have been issued if the year-end was the measurement date for RSUs. (3) Represents the weighted-average dilutive shares issuable from LTIP Units if the year-end was the measurement date for the periods shown. (4) The Series D preferred shares are excluded from the diluted earnings per share calculation because including the Series D preferred shares would also require that the preferred distributions be added back to net income, resulting in anti-dilution. (5) Beneficial interests in the Operating Trust.

Selected Quarterly Financial _2

Selected Quarterly Financial Data (Unaudited) (Tables)	12 Months Ended
	Dec. 31, 2018
Quarterly Financial Information Disclosure [Abstract]
Summary of Unaudited Quarterly Results of Operations	The following is a summary of our unaudited quarterly results of operations for 2018 and 2017 (dollars in thousands): 2018 First Quarter Second Quarter Third Quarter Fourth Quarter Total revenues $ 58,588 $ 48,636 $ 46,873 $ 42,925 Net income attributable to Equity Commonwealth common shareholders 185,602 35,036 30,767 13,420 Net income attributable to Equity Commonwealth common shareholders per share—basic 1.50 0.29 0.25 0.11 Net income attributable to Equity Commonwealth common shareholders per share—diluted 1.48 0.29 0.25 0.11 2017 First Quarter Second Quarter Third Quarter Fourth Quarter Total revenues $ 99,551 $ 91,599 $ 77,798 $ 71,623 Net income (loss) attributable to Equity Commonwealth common shareholders 21,817 (7,806 ) 31,215 (23,558 ) Net income (loss) attributable to Equity Commonwealth common shareholders per share—basic 0.18 (0.06 ) 0.25 (0.19 ) Net income (loss) attributable to Equity Commonwealth common shareholders per share—diluted 0.17 (0.06 ) 0.25 (0.19 )

Organization (Details)

Organization (Details) ft² in Millions, $ in Billions	Dec. 31, 2018USD ($)ft²buildingproperty
Segment Reporting Information [Line Items]
Cash, cash equivalents, and marketable securities | $	$ 2.7
Consolidated portfolio
Segment Reporting Information [Line Items]
Number of properties | property	10
Number of buildings | building	18
Property square feet (in sqft) | ft²	5.1
EQC Operating Trust
Segment Reporting Information [Line Items]
Noncontrolling interest, ownership percentage by parent	99.96%

Summary of Significant Accoun_3

Summary of Significant Accounting Policies - Narrative (Details) - USD ($) $ in Thousands	3 Months Ended			12 Months Ended
	Mar. 31, 2018	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Jan. 01, 2018
Property, Plant and Equipment [Line Items]
Percentage of investments in subsidiaries				100.00%
Asset impairment charges	$ 12,100	$ 18,400	$ 1,300	$ 12,087	$ 19,714	$ 58,476
Minimum percentage of likelihood of realization of tax benefits (greater than)				50.00%
Reclassification pursuant to change in accounting principle					0
Accounting Standards Update 2016-01 | Cumulative Other Comprehensive Loss
Property, Plant and Equipment [Line Items]
Reclassification pursuant to change in accounting principle					(1,902)		$ (1,900)
Accounting Standards Update 2016-01 | Retained Earnings
Property, Plant and Equipment [Line Items]
Reclassification pursuant to change in accounting principle					$ 1,902		$ 1,900
Buildings and improvements | Maximum
Property, Plant and Equipment [Line Items]
Estimated useful life				40 years
Personal property | Maximum
Property, Plant and Equipment [Line Items]
Estimated useful life				12 years

Real Estate Properties - Narrat

Real Estate Properties - Narrative (Details) $ in Thousands	1 Months Ended	12 Months Ended
	Oct. 31, 2016USD ($)	Dec. 31, 2018USD ($)ft²building	Dec. 31, 2017USD ($)building	Dec. 31, 2016USD ($)ft²buildingproperty
Real Estate Properties [Line Items]
Real estate property improvements		$ 56,300	$ 55,000	$ 110,700
Committed expenditures on leases executed during period		$ 98,700
Square feet of leases committed for expenditures during the period (in sqft) | ft²		1,100,000
Committed but unspent tenant related obligations based on existing leases		$ 86,600
Ground lease expenses				$ 900
Disposed of by Sale
Real Estate Properties [Line Items]
Number of properties sold | property				30
Number of buildings sold | building		9	37	62
Property square feet (in sqft) | ft²				7,972,692
Gross sales price		$ 1,031,286	$ 862,624	$ 1,300,000
Austin, Texas
Real Estate Properties [Line Items]
Payments to acquire land	$ 2,800

Real Estate Properties - Proper

Real Estate Properties - Properties Held For Sale (Details) $ in Thousands	Dec. 31, 2018USD ($)property	Dec. 31, 2017USD ($)
Long Lived Assets Held-for-sale [Line Items]
Liabilities related to properties held for sale	$ 0	$ 1,840
Disposal Group, Held-for-sale, Not Discontinued Operations
Long Lived Assets Held-for-sale [Line Items]
Number of properties | property	0
Rents receivable, allowance		5
Disposal Group, Held-for-sale, Not Discontinued Operations | 1600 Market Street
Long Lived Assets Held-for-sale [Line Items]
Real estate properties		76,066
Rents receivable, net of allowance for doubtful accounts of $5		13,270
Other assets, net		8,352
Assets held for sale		97,688
Accounts payable and accrued expenses		1,021
Rent collected in advance		408
Security deposits		411
Liabilities related to properties held for sale		$ 1,840

Real Estate Properties - Summar

Real Estate Properties - Summary of Properties Disposed (Details) $ in Thousands	1 Months Ended																12 Months Ended
	Dec. 31, 2018USD ($)ft²buildingproperty	Sep. 30, 2018USD ($)ft²buildingproperty	Aug. 31, 2018USD ($)ft²buildingproperty	May 31, 2018USD ($)ft²buildingproperty	Mar. 31, 2018USD ($)ft²buildingproperty	Feb. 28, 2018USD ($)ft²buildingproperty	Dec. 31, 2017building	Dec. 31, 2017buildingproperty	Dec. 31, 2017ft²building	Dec. 31, 2017USD ($)building	Aug. 31, 2017USD ($)ft²buildingproperty	Jul. 31, 2017USD ($)ft²buildingproperty	May 31, 2017USD ($)ft²buildingproperty	Apr. 30, 2017USD ($)ft²buildingproperty	Mar. 31, 2017USD ($)ft²buildingproperty	Jan. 31, 2017USD ($)ft²buildingproperty	Dec. 31, 2018USD ($)ft²buildingproperty	Dec. 31, 2017USD ($)ft²buildingproperty	Dec. 31, 2016USD ($)building
Real Estate Properties [Line Items]
Gain (Loss) on Sale																	$ 251,417	$ 15,498	$ 250,886
Disposed of by Sale
Real Estate Properties [Line Items]
Number of Properties | property																	7	16
Number of Buildings | building																	9	37	62
Square Footage (in sqft) | ft²																	4,404,797	6,588,128
Gross Sales Price																	$ 1,031,286	$ 862,624	$ 1,300,000
Gain (Loss) on Sale																	251,458	15,493
Net income included in continued operations																	$ 110,600	$ 9,300	$ 12,200
Disposed of by Sale | 1600 Market Street
Real Estate Properties [Line Items]
Number of Properties | property						1
Number of Buildings | building						1
Square Footage (in sqft) | ft²						825,968
Gross Sales Price						$ 160,000
Gain (Loss) on Sale						$ 54,599
Disposed of by Sale | 600 West Chicago Avenue
Real Estate Properties [Line Items]
Number of Properties | property						1
Number of Buildings | building						2
Square Footage (in sqft) | ft²						1,561,477
Gross Sales Price						$ 510,000
Gain (Loss) on Sale						$ 107,790
Disposed of by Sale | 5073, 5075, & 5085 S. Syracuse Street
Real Estate Properties [Line Items]
Number of Properties | property					1
Number of Buildings | building					1
Square Footage (in sqft) | ft²					248,493
Gross Sales Price					$ 115,186
Gain (Loss) on Sale					$ 42,762
Disposed of by Sale | 1601 Dry Creek Drive
Real Estate Properties [Line Items]
Number of Properties | property				1
Number of Buildings | building				1
Square Footage (in sqft) | ft²				552,865
Gross Sales Price				$ 68,500
Gain (Loss) on Sale				$ 26,979
Disposed of by Sale | 777 East Eisenhower Parkway
Real Estate Properties [Line Items]
Number of Properties | property			1
Number of Buildings | building			1
Square Footage (in sqft) | ft²			290,530
Gross Sales Price			$ 29,500
Gain (Loss) on Sale			$ 5,308
Disposed of by Sale | 8750 Bryn Mawr Avenue
Real Estate Properties [Line Items]
Number of Properties | property		1
Number of Buildings | building		2
Square Footage (in sqft) | ft²		636,078
Gross Sales Price		$ 141,000
Gain (Loss) on Sale		$ 15,194
Disposed of by Sale | 97 Newberry Road
Real Estate Properties [Line Items]
Number of Properties | property	1
Number of Buildings | building	1
Square Footage (in sqft) | ft²	289,386
Gross Sales Price	$ 7,100
Gain (Loss) on Sale	$ (1,174)
Disposed of by Sale | 111 Market Place
Real Estate Properties [Line Items]
Number of Properties | property																1
Number of Buildings | building																1
Square Footage (in sqft) | ft²																589,380
Gross Sales Price																$ 60,100
Gain (Loss) on Sale																$ (5,968)
Disposed of by Sale | Cabot Business Park Land
Real Estate Properties [Line Items]
Number of Properties | property															0
Number of Buildings | building															0
Square Footage (in sqft) | ft²															0
Gross Sales Price															$ 575
Gain (Loss) on Sale															$ (57)
Disposed of by Sale | Parkshore Plaza
Real Estate Properties [Line Items]
Number of Properties | property														1
Number of Buildings | building														4
Square Footage (in sqft) | ft²														271,072
Gross Sales Price														$ 40,000
Gain (Loss) on Sale														$ (2,460)
Disposed of by Sale | 25 S. Charles Street
Real Estate Properties [Line Items]
Number of Properties | property														1
Number of Buildings | building														1
Square Footage (in sqft) | ft²														359,254
Gross Sales Price														$ 24,500
Gain (Loss) on Sale														$ (3,487)
Disposed of by Sale | 802 Delaware Avenue
Real Estate Properties [Line Items]
Number of Properties | property													1
Number of Buildings | building													1
Square Footage (in sqft) | ft²													240,780
Gross Sales Price													$ 34,000
Gain (Loss) on Sale													$ 9,099
Disposed of by Sale | 1500 Market Street
Real Estate Properties [Line Items]
Number of Properties | property												1
Number of Buildings | building												1
Square Footage (in sqft) | ft²												1,759,193
Gross Sales Price												$ 328,000
Gain (Loss) on Sale												$ 38,585
Disposed of by Sale | 6600 North Military Trail
Real Estate Properties [Line Items]
Number of Properties | property											1
Number of Buildings | building											3
Square Footage (in sqft) | ft²											639,825
Gross Sales Price											$ 132,050
Gain (Loss) on Sale											$ (14,175)
Disposed of by Sale | 789 East Eisenhower Parkway
Real Estate Properties [Line Items]
Number of Properties							1	0
Number of Buildings | building							1
Square Footage (in sqft) | ft²									130,961
Gross Sales Price										$ 24,942
Gain (Loss) on Sale										$ 1,242
Number of buildings contained in property | building							2	2	2	2								2
Disposed of by Sale | 33 Stiles Lane
Real Estate Properties [Line Items]
Number of Properties | property								1
Number of Buildings | building							1
Square Footage (in sqft) | ft²									175,301
Gross Sales Price										$ 10,500
Gain (Loss) on Sale										2,163
Disposed of by Sale | 625 Crane Street (Land)
Real Estate Properties [Line Items]
Number of Properties | property								0
Number of Buildings | building							0
Square Footage (in sqft) | ft²									0
Gross Sales Price										307
Gain (Loss) on Sale										249
Disposed of by Sale | Mineral Rights
Real Estate Properties [Line Items]
Number of Properties | property								0
Number of Buildings | building							0
Square Footage (in sqft) | ft²									0
Gross Sales Price										200
Gain (Loss) on Sale										169
Disposed of by Sale | 4515 Seton Center Parkway
Real Estate Properties [Line Items]
Number of Properties | property															1
Number of Buildings | building															1
Square Footage (in sqft) | ft²															117,265
Disposed of by Sale | 4516 Seton Center Parkway
Real Estate Properties [Line Items]
Number of Properties | property															1
Number of Buildings | building															1
Square Footage (in sqft) | ft²															120,559
Disposed of by Sale | Seton Center
Real Estate Properties [Line Items]
Number of Properties | property															2
Number of Buildings | building															2
Square Footage (in sqft) | ft²															237,824
Gross Sales Price															$ 52,450
Gain (Loss) on Sale															$ 22,479
Disposed of by Sale | Cherrington Corporate Center
Real Estate Properties [Line Items]
Number of Properties | property								1
Number of Buildings | building							7
Square Footage (in sqft) | ft²									454,700
Disposed of by Sale | Foster Plaza
Real Estate Properties [Line Items]
Number of Properties | property								1
Number of Buildings | building							8
Square Footage (in sqft) | ft²									727,743
Disposed of by Sale | Pittsburgh Portfolio
Real Estate Properties [Line Items]
Number of Properties | property								2
Number of Buildings | building							15
Square Footage (in sqft) | ft²									1,182,443
Gross Sales Price										71,000
Gain (Loss) on Sale										$ (33,048)
Disposed of by Sale | 820 W. Diamond
Real Estate Properties [Line Items]
Number of Properties | property												1
Number of Buildings | building												1
Square Footage (in sqft) | ft²												134,933
Disposed of by Sale | Danac Stiles Business Park
Real Estate Properties [Line Items]
Number of Properties | property												1
Number of Buildings | building												3
Square Footage (in sqft) | ft²												276,637
Disposed of by Sale | 411 Farwell Avenue
Real Estate Properties [Line Items]
Number of Properties | property												1
Number of Buildings | building												1
Square Footage (in sqft) | ft²												422,727
Disposed of by Sale | 2250 Pilot Knob Road
Real Estate Properties [Line Items]
Number of Properties | property												1
Number of Buildings | building												1
Square Footage (in sqft) | ft²												87,183
Disposed of by Sale | 4700 Belleview Avenue
Real Estate Properties [Line Items]
Number of Properties | property												1
Number of Buildings | building												1
Square Footage (in sqft) | ft²												80,615
Disposed of by Sale | Five Property Portfolio
Real Estate Properties [Line Items]
Number of Properties | property												5
Number of Buildings | building												7
Square Footage (in sqft) | ft²												1,002,095
Gross Sales Price												$ 84,000
Gain (Loss) on Sale												$ 702

Real Estate Properties - Schedu

Real Estate Properties - Schedule of Future Minimum Lease Payment Receivables and Payments (Details) $ in Thousands	Dec. 31, 2018USD ($)
Future minimum lease payments receivable:
2,019	$ 109,888
2,020	96,309
2,021	104,279
2,022	95,128
2,023	86,299
Thereafter	567,547
Total	$ 1,059,450

Lease Intangibles - Summary of

Lease Intangibles - Summary of the Carrying Amounts for Acquired Real Estate Leases and Assumed Real Estate Lease Obligations (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Finite-Lived Intangible Assets [Line Items]
Acquired real estate leases	$ 3,691	$ 77,874
Acquired real estate leases, net	275	23,847
Acquired in-place leases
Finite-Lived Intangible Assets [Line Items]
Acquired in-place leases	3,691	56,853
Accumulated amortization	(3,416)	(41,978)
Acquired above market leases
Finite-Lived Intangible Assets [Line Items]
Acquired above market leases	0	21,021
Accumulated amortization	0	(12,049)
Acquired below market leases
Finite-Lived Intangible Assets [Line Items]
Acquired below market leases	1,570	6,093
Accumulated amortization	(1,453)	(5,092)
Assumed real estate lease obligations, net	$ 117	$ 1,001

Lease Intangibles - Schedule of

Lease Intangibles - Schedule of Amortization of Lease Intangibles (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Leases [Abstract]
Amortization of acquired in-place leases, Depreciation and amortization	$ 2,133	$ 7,220	$ 14,598
Amortization of above and below market leases, Increase (decrease) to rental income	$ (54)	$ (1,774)	$ (6,531)

Lease Intangibles - Schedule _2

Lease Intangibles - Schedule of Future Amortization of Net Intangible Lease Assets and Liabilities (Details) $ in Thousands	Dec. 31, 2018USD ($)
Leases [Abstract]
2,019	$ 158
2,020	0
2,021	0
2,022	0
2,023	0
Thereafter	0
Acquired real estate leases, net	$ 158

Marketable Securities (Details)

Marketable Securities (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Jan. 01, 2018
Debt Securities, Available-for-sale [Line Items]
Reclassification pursuant to change in accounting principle		$ 0
Proceeds from sale of common stock	$ 23,933	0	$ 0
Recognized loss on sale of common stock included in interest and other income	5,000
Cost or Amortized Cost	249,944	276,567
Unrealized Loss	(342)
Unrealized Gain, Net		361
Marketable securities	$ 249,602	276,928
Accounting Standards Update 2016-01 | Cumulative Other Comprehensive Loss
Debt Securities, Available-for-sale [Line Items]
Reclassification pursuant to change in accounting principle		(1,902)		$ (1,900)
Accounting Standards Update 2016-01 | Retained Earnings
Debt Securities, Available-for-sale [Line Items]
Reclassification pursuant to change in accounting principle		$ 1,902		$ 1,900

Other Assets - Narrative (Detai

Other Assets - Narrative (Details)	1 Months Ended				12 Months Ended
	Aug. 31, 2018USD ($)	Sep. 30, 2017USD ($)	Jan. 31, 2013USD ($)buildingproperty	Apr. 30, 2012USD ($)	Dec. 31, 2018USD ($)building	Dec. 31, 2017USD ($)building	Dec. 31, 2016USD ($)buildingproperty
Accounts, Notes, Loans and Financing Receivable [Line Items]
Proceeds from sale of real estate mortgage receivable					$ 5,599,000	$ 0	$ 0
Loss on sale of real estate mortgage receivable					2,117,000	0	0
Principal payments received from real estate mortgages receivable					0	313,000	$ 0
Total real estate mortgage receivable included in other assets, carrying value					$ 0	7,700,000
Real estate mortgages receivable
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for losses related to real estate mortgages receivable						$ 0
Sales of properties resulting in gains and losses | Properties Sold in December
Accounts, Notes, Loans and Financing Receivable [Line Items]
Mortgage financing related to property sold				$ 400,000
Interest rate on real estate mortgage receivable				6.00%
Disposed of by Sale
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of properties sold or agreed to be sold | property							30
Number of buildings sold | building					9	37	62
Principal payments received from real estate mortgages receivable		$ 300,000
Disposed of by Sale | Real estate mortgages receivable
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loss on sale of real estate mortgage receivable						$ 100,000
Disposed of by Sale | Sales of properties resulting in gains and losses
Accounts, Notes, Loans and Financing Receivable [Line Items]
Mortgage financing related to property sold			$ 7,700,000
Interest rate on real estate mortgage receivable			6.00%
Number of properties sold or agreed to be sold | property			3
Number of buildings sold | building			18
Proceeds from sale of real estate mortgage receivable	$ 5,700,000
Loss on sale of real estate mortgage receivable					$ 2,100,000

Other Assets - Summary of Defer

Other Assets - Summary of Deferred Financing Fees, Deferred Leasing Costs and Capitalized Lease Incentives (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Other Assets [Abstract]
Deferred financing fees	$ 0	$ 14,458
Accumulated amortization	0	(12,709)
Deferred financing fees, net	0	1,749
Deferred leasing costs	69,930	85,606
Accumulated amortization	(18,807)	(22,067)
Deferred leasing costs, net	51,123	63,539
Capitalized lease incentives	5,701	8,562
Accumulated amortization	(1,393)	(2,054)
Capitalized lease incentives, net	$ 4,308	$ 6,508

Other Assets - Summary of Futur

Other Assets - Summary of Future Amortization of Deferred Leasing Costs and Capitalized Lease Incentives (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Deferred Leasing Costs
2,019	$ 5,550
2,020	5,812
2,021	5,810
2,022	5,126
2,023	4,478
Thereafter	24,347
Deferred leasing costs, net	51,123	$ 63,539
Capitalized Lease Incentives
2,019	695
2,020	583
2,021	553
2,022	425
2,023	320
Thereafter	1,732
Capitalized lease incentives, net	$ 4,308	$ 6,508

Indebtedness - Schedule of Outs

Indebtedness - Schedule of Outstanding Indebtedness (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Debt Instrument [Line Items]
Outstanding indebtedness	$ 276,000	$ 856,940
Unamortized net premiums, discounts and deferred financing fees	(1,045)	(8,362)
Total	$ 274,955	848,578
Unsecured revolving credit facility, at LIBOR plus a premium
Debt Instrument [Line Items]
Interest rate stated percentage	0.00%
Outstanding indebtedness	$ 0	0
5-year unsecured term loan, at LIBOR plus a premium
Debt Instrument [Line Items]
Interest rate stated percentage	0.00%
Outstanding indebtedness	$ 0	200,000
7-year unsecured term loan, at LIBOR plus a premium
Debt Instrument [Line Items]
Interest rate stated percentage	0.00%
Outstanding indebtedness	$ 0	200,000
Unsecured floating rate debt
Debt Instrument [Line Items]
Interest rate stated percentage	0.00%
Outstanding indebtedness	$ 0	400,000
5.875% Senior Unsecured Notes due 2020
Debt Instrument [Line Items]
Interest rate stated percentage	5.875%
Outstanding indebtedness	$ 250,000	250,000
5.75% Senior Unsecured Notes due 2042
Debt Instrument [Line Items]
Interest rate stated percentage	0.00%
Outstanding indebtedness	$ 0	175,000
Unsecured fixed rate debt
Debt Instrument [Line Items]
Interest rate stated percentage	5.875%
Outstanding indebtedness	$ 250,000	425,000
Secured fixed rate debt
Debt Instrument [Line Items]
Interest rate stated percentage	5.69%
Outstanding indebtedness	$ 26,000	31,940
Secured fixed rate debt | 206 East 9th Street
Debt Instrument [Line Items]
Interest rate stated percentage	5.69%
Outstanding indebtedness	$ 26,000	26,536
Secured fixed rate debt | 97 Newberry Road
Debt Instrument [Line Items]
Interest rate stated percentage	0.00%
Outstanding indebtedness	$ 0	$ 5,404

Indebtedness - Narrative (Detai

Indebtedness - Narrative (Details)

Dec. 26, 2018USD ($)

May 04, 2018USD ($)

Mar. 07, 2018USD ($)

Jul. 15, 2017USD ($)

Dec. 15, 2016USD ($)

Nov. 10, 2016USD ($)

Feb. 16, 2016USD ($)

Jan. 29, 2015USD ($)

Dec. 31, 2018USD ($)property

Dec. 31, 2017USD ($)

Apr. 30, 2017USD ($)

Dec. 31, 2015

Dec. 31, 2018USD ($)property

Dec. 31, 2017USD ($)

Dec. 31, 2016USD ($)

Debt Instrument [Line Items]

Loss on early extinguishment of debt

$ 7,122,000

$ 493,000

$ 2,680,000

Real estate properties, net

$ 763,674,000

$ 1,296,893,000

763,674,000

1,296,893,000

Mortgage notes payable, net

26,482,000

32,594,000

26,482,000

32,594,000

Designated as hedging instrument | Cash flow hedging

Debt Instrument [Line Items]

Expense included in interest and other income

$ 293,000

0

241,000

Mortgage notes | 97 Newberry Road

Debt Instrument [Line Items]

Amount of debt, redeemed, defeased, or prepaid

4,900,000

Loss on early extinguishment of debt

$ 600,000

Mortgage notes | 33 Stiles Lane

Debt Instrument [Line Items]

Amount of debt, redeemed, defeased, or prepaid

$ 2,000,000

Loss on early extinguishment of debt

200,000

Mortgage notes | Parkshore Plaza

Debt Instrument [Line Items]

Amount of debt, redeemed, defeased, or prepaid

$ 41,300,000

Loss on early extinguishment of debt

$ 100,000

Mortgage notes | 1735 Market Street

Debt Instrument [Line Items]

Amount of debt, redeemed, defeased, or prepaid

$ 167,800,000

Loss on early extinguishment of debt

2,400,000

Mortgage notes | 1735 Market Street | Designated as hedging instrument | Interest rate swap contracts | Cash flow hedging

Debt Instrument [Line Items]

Expense included in interest and other income

$ 200,000

Loan facility, 5-year term

Debt Instrument [Line Items]

Interest rate stated percentage

0.00%

0.00%

Loan facility, 5-year term | LIBOR | Minimum

Debt Instrument [Line Items]

Basis points on variable rate

0.90%

Loan facility, 5-year term | LIBOR | Maximum

Debt Instrument [Line Items]

Basis points on variable rate

1.80%

Loan facility, 7-year term

Debt Instrument [Line Items]

Interest rate stated percentage

0.00%

0.00%

Loan facility, 7-year term | LIBOR | Minimum

Debt Instrument [Line Items]

Basis points on variable rate

1.40%

Loan facility, 7-year term | LIBOR | Maximum

Debt Instrument [Line Items]

Basis points on variable rate

2.35%

5.75% Senior Unsecured Notes due 2024

Debt Instrument [Line Items]

Amount of debt, redeemed, defeased, or prepaid

$ 175,000,000

Loss on early extinguishment of debt

$ 4,900,000

Interest rate stated percentage

5.75%

6.65% Senior Unsecured Notes due 2018

Debt Instrument [Line Items]

Amount of debt, redeemed, defeased, or prepaid

$ 250,000,000

Loss on early extinguishment of debt

$ 200,000

Interest rate stated percentage

6.65%

6.25% Senior Unsecured Notes due 2017

Debt Instrument [Line Items]

Amount of debt, redeemed, defeased, or prepaid

$ 250,000,000

Loss on early extinguishment of debt

$ 100,000

Interest rate stated percentage

6.25%

6.25% Senior Unsecured Notes due 2016

Debt Instrument [Line Items]

Amount of debt, redeemed, defeased, or prepaid

$ 139,100,000

Loss on early extinguishment of debt

$ 100,000

Interest rate stated percentage

6.25%

Mortgage notes

Debt Instrument [Line Items]

Number of real estate properties secured by mortgage | property

1

1

Real estate properties, net

$ 44,300,000

$ 44,300,000

Mortgage notes payable, net

$ 26,500,000

$ 26,500,000

Revolving credit facility | Minimum

Debt Instrument [Line Items]

Commitment fee percentage

0.125%

Revolving credit facility | Maximum

Debt Instrument [Line Items]

Commitment fee percentage

0.30%

Revolving credit facility | Unsecured revolving credit facility

Debt Instrument [Line Items]

Maximum borrowing capacity

$ 750,000,000

Revolving credit facility | Unsecured revolving credit facility | Line of credit

Debt Instrument [Line Items]

Loss on early extinguishment of debt

$ 200,000

Revolving credit facility | Loan facility, 5-year term

Debt Instrument [Line Items]

Maximum borrowing capacity

$ 200,000,000

Debt instrument, term

5 years

Revolving credit facility | Loan facility, 7-year term

Debt Instrument [Line Items]

Maximum borrowing capacity

$ 200,000,000

Debt instrument, term

7 years

Revolving credit facility | Loan facility, 5-year term and 7-year term

Debt Instrument [Line Items]

Amount of debt, redeemed, defeased, or prepaid

$ 400,000,000

Loss on early extinguishment of debt

$ 1,500,000

Indebtedness - Schedule of Requ

Indebtedness - Schedule of Required Principal Payments on Outstanding Debt (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Debt Disclosure [Abstract]
2,019	$ 567
2,020	250,597
2,021	24,836
2,022	0
2,023	0
Thereafter	0
Total	$ 276,000	$ 856,940

Shareholders' Equity - Narrativ

Shareholders' Equity - Narrative (Details)	Oct. 23, 2018USD ($)	Oct. 09, 2018$ / shares	Mar. 14, 2018USD ($)	Mar. 15, 2017USD ($)	May 15, 2016USD ($)$ / sharesshares	Mar. 17, 2016USD ($)	Mar. 31, 2018USD ($)	Mar. 31, 2017USD ($)	Dec. 31, 2018USD ($)$ / sharesshares	Dec. 31, 2017USD ($)shares	Dec. 31, 2016USD ($)$ / sharesshares	Oct. 10, 2018
Class of Stock [Line Items]
Redemption of shares											$ 275,000,000
Distributions declared per common share (in dollars per share) | $ / shares		$ 2.5
Cash distributions paid	$ 304,700,000									$ 0	0
Preferred stock redemption premium									$ 0	$ 0	$ 9,609,000
Distribution percentage to shareholders consisting of an alternative minimum tax adjustment										0.00%	96.31%
Common Shares
Class of Stock [Line Items]
Stock repurchase program, additional authorized amount			$ 150,000,000	$ 150,000,000		$ 150,000,000
Stock repurchase program, period in force			12 months	12 months		12 months
Stock repurchase program, amount not utilized							$ 81,000,000	$ 106,600,000
Number of shares repurchased (in shares) | shares									2,970,209	0	2,491,675
Stock repurchased and retired during period, price per share (in dollars per share) | $ / shares									$ 29.67		$ 27.68
Redemption of shares			$ 19,100,000	$ 69,000,000					$ 88,100,000		$ 69,000,000
Stock repurchase program, remaining authorized repurchase amount									$ 130,900,000
Stock repurchase program, number of shares surrendered (in shares) | shares									193,521	43,329	31,025
Common shares from conversion of preferred shares (in shares) | shares									2,563,275
Series D
Class of Stock [Line Items]
Number of shares repurchased (in shares) | shares											0
Redemption of shares											$ 0
Preferred shares, of beneficial interest, shares outstanding (in shares) | shares									4,915,196	4,915,196
Preferred shares dividend (in dollars per share) | $ / shares									$ 1.625
Preferred shares, dividend yield									6.50%	6.50%
Initial conversion rate		0.480775										0.5215
Initial conversion price per share (in dollars per share) | $ / shares									$ 47.9386385427
Additional common shares (in shares) | shares									2,563,275
Liquidation preference (in dollars per share) | $ / shares									$ 25
Percentage of average closing market price of the entity's common stock based on which ratio for conversion of preferred shares into common shares is determined									98.00%
Purchase price as a percentage of liquidation preference									100.00%
Common shares from conversion of preferred shares (in shares) | shares									4,915,196
Series E
Class of Stock [Line Items]
Number of shares repurchased (in shares) | shares											11,000,000
Redemption of shares											$ 275,000,000
Stock redeemed during period (in shares) | shares					11,000,000
Redemption price (in dollars per share) | $ / shares					$ 25
Preferred stock, redemption amount					$ 275,000,000
Preferred stock redemption premium											$ (9,609,000)

Shareholders' Equity - Schedule

Shareholders' Equity - Schedule of Distributions Paid Per Common Share (Details)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Stockholders' Equity Note [Abstract]
Ordinary income	100.00%	0.00%	0.00%
Return of capital	0.00%	0.00%	0.00%
Capital gain	0.00%	0.00%	0.00%
Unrecaptured Section 1250 gain	0.00%	0.00%	0.00%
Common share, distributions paid	100.00%	0.00%	0.00%

Shareholders' Equity - Schedu_2

Shareholders' Equity - Schedule of Declared Distributions (Details) - Series D - $ / shares	Nov. 15, 2018	Oct. 29, 2018	Aug. 15, 2018	Jul. 30, 2018	May 15, 2018	Apr. 27, 2018	Feb. 15, 2018	Jan. 30, 2018
Class of Stock [Line Items]
Dividend declared (in dollars per share)		$ 0.40625		$ 0.40625		$ 0.40625		$ 0.40625
Dividends paid (in dollars per share)	$ 0.40625		$ 0.40625		$ 0.40625		$ 0.40625

Noncontrolling Interest (Detail

Noncontrolling Interest (Details) - shares	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Noncontrolling Interest [Abstract]
Common stock, conversion term	6 months
Common stock, conversion basis (in shares)	1
EQC Operating Trust
Noncontrolling Interest [Line Items]
Noncontrolling interest, ownership percentage by parent	99.96%	99.97%

Noncontrolling Interest - Commo

Noncontrolling Interest - Common Shares and Units Activity (Details)	12 Months Ended
	Dec. 31, 2018shares
Increase (Decrease) in Stockholders' Equity
Ownership percentage by noncontrolling owners	0.04%
Common Shares, OP Units and LTIP Units
Increase (Decrease) in Stockholders' Equity
Outstanding, beginning balance (in shares)	124,260,136
Redemption of shares (in shares)	(2,970,209)
Restricted share, time-based LTIP Unit grants and vested restricted stock units, net of forfeitures (in shares)	327,948
Outstanding, ending balance (in shares)	121,617,875
Common Shares
Increase (Decrease) in Stockholders' Equity
Outstanding, beginning balance (in shares)	124,217,616
Redemption of shares (in shares)	(2,970,209)
Restricted share, time-based LTIP Unit grants and vested restricted stock units, net of forfeitures (in shares)	324,748
Outstanding, ending balance (in shares)	121,572,155
OP Units and LTIP Units | Noncontrolling Interest
Increase (Decrease) in Stockholders' Equity
Outstanding, beginning balance (in shares)	42,520
Redemption of shares (in shares)	0
Restricted share, time-based LTIP Unit grants and vested restricted stock units, net of forfeitures (in shares)	3,200
Outstanding, ending balance (in shares)	45,720

Cumulative Other Comprehensiv_3

Cumulative Other Comprehensive Loss - Schedule of Amounts Recognized in Cumulative Other Comprehensive Loss (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Amounts recognized in cumulative other comprehensive income (Loss) by component
Balance	$ 3,300,495	$ 3,260,447
Amounts reclassified from cumulative other comprehensive loss to cumulative net income pursuant to a change in accounting principle	(1,902)
Other comprehensive income before reclassifications	1,283	64
Amounts reclassified from cumulative other comprehensive loss to net income	372	49
Net current period other comprehensive income	1,655	113
Balance	3,183,998	3,300,495
AOCI Including Portion Attributable to Noncontrolling Interest
Amounts recognized in cumulative other comprehensive income (Loss) by component
Balance	(95)	(208)
Balance	(342)	(95)
Unrealized Loss on Derivative Instruments
Amounts recognized in cumulative other comprehensive income (Loss) by component
Balance	(456)	(208)
Amounts reclassified from cumulative other comprehensive loss to cumulative net income pursuant to a change in accounting principle	0
Other comprehensive income before reclassifications	84	(297)
Amounts reclassified from cumulative other comprehensive loss to net income	372	49
Net current period other comprehensive income	456	(248)
Balance	0	(456)
Unrealized Gain (Loss) on Marketable Securities
Amounts recognized in cumulative other comprehensive income (Loss) by component
Balance	361	0
Amounts reclassified from cumulative other comprehensive loss to cumulative net income pursuant to a change in accounting principle	(1,902)
Other comprehensive income before reclassifications	1,199	361
Amounts reclassified from cumulative other comprehensive loss to net income	0	0
Net current period other comprehensive income	1,199	361
Balance	$ (342)	$ 361

Cumulative Other Comprehensiv_4

Cumulative Other Comprehensive Loss - Schedule of Reclassifications Out of Cumulative Other Comprehensive Loss (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Reclassifications out of cumulative other comprehensive income (Loss)
Interest and other income, net	$ 46,815	$ 26,380	$ 10,331
Interest expense	26,585	52,183	$ 84,329
Amounts reclassified from cumulative other comprehensive loss to net income	372	49
Amounts Reclassified from Cumulative Other Comprehensive Loss to Net Income
Reclassifications out of cumulative other comprehensive income (Loss)
Amounts reclassified from cumulative other comprehensive loss to net income	372	49
Amounts Reclassified from Cumulative Other Comprehensive Loss to Net Income | Interest rate cap contract
Reclassifications out of cumulative other comprehensive income (Loss)
Interest and other income, net	293	0
Interest expense	$ 79	$ 49

Income Taxes - Schedule of Prov

Income Taxes - Schedule of Provision for Income Taxes (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Current:
State and local	$ (3,156)	$ (500)	$ (745)
Federal	0	0	0
Income tax expense	$ (3,156)	$ (500)	$ (745)

Income Taxes - Schedule of Reco

Income Taxes - Schedule of Reconciliation of Effective Tax Rate and the U.S. Federal Statutory Income Tax Rate (Details)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Tax Disclosure [Abstract]
Taxes at statutory U.S. federal income tax rate	21.00%	35.00%	35.00%
Dividends paid deduction and net operating loss utilization	(21.00%)	(35.00%)	(35.00%)
Federal taxes on built-in gain	0.00%	0.00%	0.00%
State and local income taxes	1.14%	1.66%	0.32%
Effective tax rate	1.14%	1.66%	0.32%

Income Taxes - Narrative (Detai

Income Taxes - Narrative (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Income Tax Disclosure [Abstract]
Net operating loss carryforwards	$ 90	$ 120

Derivative Instruments - Narrat

Derivative Instruments - Narrative (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Mar. 08, 2018
Derivative [Line Items]
Interest and other income, net	$ 46,815	$ 26,380	$ 10,331
Interest rate cap | LIBOR
Derivative [Line Items]
Interest and other income, net	300
Derivative, cap interest rate				2.50%
Cash flow hedging | Designated as hedging instrument | Interest rate swap contracts
Derivative [Line Items]
Amount of hedged item	$ 167,800
Cash flow hedging | Designated as hedging instrument | Interest rate swap contracts | 1735 Market Street | Mortgage notes
Derivative [Line Items]
Interest and other income, net			$ 200
Cash flow hedging | Designated as hedging instrument | Interest rate cap
Derivative [Line Items]
Derivative, notional amount				$ 400,000

Derivative Instruments - Schedu

Derivative Instruments - Schedule of Fair Value of Derivative Financial Instruments (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Interest rate cap | Cash flow hedging | Designated as hedging instrument | Other assets
Derivatives, Fair Value [Line Items]
Interest Rate Derivative Designated as Hedging Instrument	$ 0	$ 17

Derivative Instruments - Sche_2

Derivative Instruments - Schedule of Gain or Loss Recognized on Interest Rate Derivatives Designated as Cash Flow Hedges (Details) - Cash flow hedging - Designated as hedging instrument - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Derivative Instruments, Gain (Loss) [Line Items]
Amount of gain (loss) recognized in cumulative other comprehensive loss (effective portion)	$ 84	$ (297)	$ (554)
Amount of loss reclassified from cumulative other comprehensive loss into interest expense (effective portion)	79	49	3,792
Amount of loss recognized in income (ineffective portion and amount excluded from effectiveness testing)	$ 293	$ 0	$ 241

Share-Based Compensation - Narr

Share-Based Compensation - Narrative (Details) $ / shares in Units, $ in Millions	Oct. 28, 2018shares	Jun. 20, 2018USD ($)trustee$ / sharesshares	Jan. 29, 2018$ / sharesshares	Nov. 08, 2017shares	Jun. 20, 2017USD ($)trustee$ / sharesshares	Jan. 24, 2017$ / sharesshares	Jun. 15, 2016USD ($)trustee$ / sharesshares	Jan. 26, 2016$ / sharesshares	Jun. 16, 2015shares	Dec. 31, 2022shares	Dec. 31, 2021shares	Dec. 31, 2020shares	Dec. 31, 2019shares	Dec. 31, 2018USD ($)tranche$ / sharesshares	Dec. 31, 2017USD ($)$ / sharesshares	Dec. 31, 2016USD ($)$ / sharesshares	Dec. 31, 2015shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected term of incentive plan after effective date									10 years
Common shares available for issuance (in shares)														839,470
Common stock issued (in shares)	225,655			226,258										121,572,155	124,217,616
General and administrative expense
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Compensation expense | $														$ 19.7	$ 21.4	$ 18.5
Restricted shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period		1 year												4 years
Number of independent trustees | trustee		9			9		9
Share based compensation amount | $		$ 0.1					$ 0.1
Number of equity awards granted (in shares)		25,600	125,409		25,248	76,424	31,167	136,623
Price per share (in dollars per share) | $ / shares		$ 31.25			$ 31.69		$ 28.88
Granted (in dollars per share) | $ / shares			$ 29.78			$ 31.47		$ 26.93
Restricted shares | Independent Trustee
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of equity awards granted (in shares)		3,200					3,463
RSUs
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period														3 years
Number of tranches | tranche														2
Vested (in shares)	225,655			226,258
RSUs | Vesting in three years
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting percentage														50.00%
RSUs | Vesting in four years
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period														1 year
Vesting percentage														50.00%
RSUs at target
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of equity awards granted (in shares)			254,615			155,168		277,386
RSUs at maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of equity awards granted (in shares)			634,628			386,756		691,385
Granted (in dollars per share) | $ / shares														$ 14.90	$ 15.97	$ 15.57
LTIP units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Equity award, conversion basis (in shares)														1
Time-based LTIP Units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of equity awards granted (in shares)		3,200			3,156
Time-based LTIP Units | Independent Trustee
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of equity awards granted (in shares)					3,156
Time-based LTIP units at target
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of equity awards granted (in shares)						39,364
Market-based LTIP units at target
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of equity awards granted (in shares)						79,924
LTIP units at maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of equity awards granted (in shares)						199,211
Restricted Shares and Time-Based LTIP Units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vested (in shares)														407,273	217,449	191,498
Share based compensation amount | $					$ 0.1
Number of equity awards granted (in shares)														154,209	144,192	167,790
Granted (in dollars per share) | $ / shares														$ 30.05	$ 31.51	$ 27.29
Unvested (in shares)														425,888	678,952	752,209	776,456
Estimated future compensation expense for unvested shares | $														$ 6.1
Weighted average period over which compensation expense will be recorded														2 years 3 months 18 days
Restricted Shares and Time-Based LTIP Units | Scenario, Forecast
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vested (in shares)										61,885	87,772	124,959	151,272
OP Units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Conversions in period (in shares)														3,156
RSUs and market-based LTIP units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected term of incentive plan after effective date														4 years	4 years	4 years
Vested (in shares)														367,260	226,258	0
Number of equity awards granted (in shares)														634,628	585,967	691,385
Granted (in dollars per share) | $ / shares														$ 14.90	$ 15.97	$ 15.57
Unvested (in shares)														2,046,644	2,131,947	2,572,768	1,884,110
Estimated future compensation expense for unvested shares | $														$ 12.4
Weighted average period over which compensation expense will be recorded														2 years 1 month 6 days
Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common shares available for issuance (in shares)									3,250,000

Share-Based Compensation - Summ

Share-Based Compensation - Summary of Restricted Share and Restricted Stock Unit Activity (Details) - $ / shares	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Restricted Shares and Time-Based LTIP Units
Number of Shares
Beginning balance (in shares)	678,952	752,209	776,456
Granted (in shares)	154,209	144,192	167,790
Vested (in shares)	(407,273)	(217,449)	(191,498)
Not earned (in shares)	0	0
Forfeited (in shares)			(539)
Ending balance (in shares)	425,888	678,952	752,209
Weighted Average Grant Date Fair Value
Beginning balance (in dollars per share)	$ 27.41	$ 26.79	$ 26.62
Granted (in dollars per share)	30.05	31.51	27.29
Vested (in dollars per share)	27.06	27.97	26.54
Not earned (in dollars per share)	0	0
Forfeited (in dollars per share)			26.82
Ending balance (in dollars per share)	$ 28.70	$ 27.41	$ 26.79
RSUs and market-based LTIP units
Number of Shares
Beginning balance (in shares)	2,131,947	2,572,768	1,884,110
Granted (in shares)	634,628	585,967	691,385
Vested (in shares)	(367,260)	(226,258)	0
Not earned (in shares)	(352,671)	(800,530)
Forfeited (in shares)			(2,727)
Ending balance (in shares)	2,046,644	2,131,947	2,572,768
Weighted Average Grant Date Fair Value
Beginning balance (in dollars per share)	$ 15.69	$ 15.75	$ 15.82
Granted (in dollars per share)	14.90	15.97	15.57
Vested (in dollars per share)	15.79	15.99	0
Not earned (in dollars per share)	15.47	15.99
Forfeited (in dollars per share)			15.54
Ending balance (in dollars per share)	$ 15.47	$ 15.69	$ 15.75

Share-Based Compensation - Su_2

Share-Based Compensation - Summary of Assumptions and Fair Values for Restricted Stock Units Granted in the Period (Details) - $ / shares	Jun. 16, 2015	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions and Methodology
Expected term	10 years
RSUs and market-based LTIP Units granted at the target amount
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions and Methodology
Fair value of RSUs and LTIP Units granted (in dollars per share)		$ 37.13	$ 39.81	$ 38.80
RSUs granted at the maximum amount
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions and Methodology
Fair value of RSUs and LTIP Units granted (in dollars per share)		14.90	15.97	15.57
RSUs and market-based LTIP units
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions and Methodology
Fair value of RSUs and LTIP Units granted (in dollars per share)		$ 14.90	$ 15.97	$ 15.57
Expected term		4 years	4 years	4 years
Expected volatility		0.00%	0.00%	0.00%
Expected dividend yield		1.68%	1.59%	1.86%
Risk-free rate		2.26%	1.49%	1.07%

Defined Contribution Plan (Deta

Defined Contribution Plan (Details) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Defined Contribution Plan Disclosure [Line Items]
Employer matching contribution, percent of match	100.00%
Employer matching contribution, percent of employees' gross pay	3.00%
Employer matching contribution in excess of three percent of employee contribution, percent of match	50.00%
Employer maximum contribution	$ 8,000
Employer contribution amount	$ 300,000	$ 300,000	$ 400,000
Minimum
Defined Contribution Plan Disclosure [Line Items]
Employer matching contribution, percent of employees' gross pay	3.00%
Maximum
Defined Contribution Plan Disclosure [Line Items]
Employer matching contribution, percent of employees' gross pay	5.00%

Fair Value of Assets and Liab_3

Fair Value of Assets and Liabilities - Schedule of Assets and Liabilities Measure at Fair Value (Details) - Recurring - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Interest rate cap
Recurring Fair Value Measurements:
Fair value of assets		$ 17
Quoted Prices in Active Markets for Identical Assets (Level 1) | Interest rate cap
Recurring Fair Value Measurements:
Fair value of assets		0
Significant Other Observable Inputs (Level 2) | Interest rate cap
Recurring Fair Value Measurements:
Fair value of assets		17
Significant Unobservable Inputs (Level 3) | Interest rate cap
Recurring Fair Value Measurements:
Fair value of assets		0
Marketable securities
Recurring Fair Value Measurements:
Fair value of assets	$ 249,602	276,928
Marketable securities | Quoted Prices in Active Markets for Identical Assets (Level 1)
Recurring Fair Value Measurements:
Fair value of assets	249,602	276,928
Marketable securities | Significant Other Observable Inputs (Level 2)
Recurring Fair Value Measurements:
Fair value of assets	0	0
Marketable securities | Significant Unobservable Inputs (Level 3)
Recurring Fair Value Measurements:
Fair value of assets	$ 0	$ 0

Fair Value of Assets and Liab_4

Fair Value of Assets and Liabilities - Narrative (Details) $ in Thousands	Jun. 30, 2017USD ($)	Mar. 31, 2018USD ($)	Jun. 30, 2017USD ($)	Mar. 31, 2017USD ($)	Dec. 31, 2018USD ($)	Dec. 31, 2017USD ($)	Dec. 31, 2016USD ($)
Fair value of assets and liabilities
Asset impairment charges		$ 12,100	$ 18,400	$ 1,300	$ 12,087	$ 19,714	$ 58,476
Real estate properties, net					$ 763,674	$ 1,296,893
Customer concentration risk | Total annualized rents
Fair value of assets and liabilities
Concentration risk, benchmark percentage					0.075
Customer concentration risk | Total annualized rents | Tenant One
Fair value of assets and liabilities
Concentration risk					13.90%
Real estate properties held for sale
Fair value of assets and liabilities
Asset impairment charges	$ 18,400
Market Approach Valuation Technique | Real estate properties held for sale | Significant Unobservable Inputs (Level 3)
Fair value of assets and liabilities
Fair value of assets		29,700
Market Approach Valuation Technique | Real estate properties held for sale | Significant Other Observable Inputs (Level 2)
Fair value of assets and liabilities
Fair value of assets	80,600		80,600	23,300
Disposition Plan | Principal Balance | Nonrecurring
Fair value of assets and liabilities
Real estate properties, net	$ 99,000	$ 41,800	$ 99,000	$ 24,600

Fair Value of Assets and Liab_5

Fair Value of Assets and Liabilities - Schedule of Fair Value and Carrying Value of Financial Instruments (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Principal Balance
Fair value of financial instruments
Senior unsecured debt and mortgage notes payable	$ 276,000	$ 856,940
Fair Value
Fair value of financial instruments
Senior unsecured debt and mortgage notes payable	$ 283,214	$ 874,280

Earnings Per Common Share - Sch

Earnings Per Common Share - Schedule of Computation of Basic and Diluted Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Numerator for earnings per common share - basic:
Net income									$ 272,908	$ 29,666	$ 232,894
Net income attributable to noncontrolling interest									(95)	(10)	0
Preferred distributions									(7,988)	(7,988)	(17,956)
Excess fair value of consideration paid over carrying value of preferred shares									0	0	(9,609)
Net income attributable to Equity Commonwealth common shareholders	$ 13,420	$ 30,767	$ 35,036	$ 185,602	$ (23,558)	$ 31,215	$ (7,806)	$ 21,817	264,825	21,668	205,329
Net Income (Loss) Available to Common Stockholders, Diluted [Abstract]
Net income									272,908	29,666	232,894
Net income attributable to noncontrolling interest									(95)	0	0
Preferred distributions									(7,988)	(7,988)	(17,956)
Excess fair value of consideration paid over carrying value of preferred shares									0	0	(9,609)
Numerator for net income per share - diluted									$ 264,825	$ 21,678	$ 205,329
Denominator for earnings per common share - basic and diluted:
Weighted average common shares outstanding — basic (in shares)									122,314	124,125	125,474
Weighted average common shares outstanding — diluted (in shares)									123,385	125,129	126,768
Net income per common share attributable to Equity Commonwealth common shareholders:
Basic (in dollars per share)									$ 2.17	$ 0.17	$ 1.64
Diluted (in dollars per share)									$ 2.15	$ 0.17	$ 1.62
Series D
Anti-dilutive securities:
Effect of anti-dilutive securities (in shares)									2,563	2,363	2,363
Preferred shares, dividend yield									6.50%	6.50%	6.50%
LTIP units
Anti-dilutive securities:
Effect of anti-dilutive securities (in shares)									43	0	0
OP Units
Anti-dilutive securities:
Effect of anti-dilutive securities (in shares)									1	0	0
RSUs
Denominator for earnings per common share - basic and diluted:
Weighted average number of common shares outstanding, dilutive adjustment (in shares)									956	912	1,294
LTIP units
Denominator for earnings per common share - basic and diluted:
Weighted average number of common shares outstanding, dilutive adjustment (in shares)									115	92	0

Earnings Per Common Share - S_2

Earnings Per Common Share - Schedule of Computation of Basic and Diluted Earnings Per Share (Footnote) (Details) - shares shares in Thousands	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
RSUs and LTIP Units
Earnings Per Share, Diluted, by Common Class, Including Two Class Method [Line Items]
Common shares issued (in shares)	308	33	0

Segment Information - Narrative

Segment Information - Narrative (Details)	12 Months Ended
	Dec. 31, 2018segment
Segment Reporting [Abstract]
Number of reportable segments	1
Percentage of CBD and suburban properties that are office properties (more than)	90.00%
Expedia, Inc. | Customer concentration risk | Revenues
Concentration Risk [Line Items]
Concentration risk	10.30%

Related Person Transactions - T

Related Person Transactions - Two North Riverside Plaza Joint Venture Limited Partnership (Details) - Two North Riverside Plaza Joint Venture Limited Partnership	Jul. 20, 2015option	Dec. 31, 2018USD ($)	Dec. 31, 2017USD ($)	Dec. 31, 2016USD ($)
Related Party Transaction [Line Items]
Lease term	5 years
Number of renewal options | option	1
Renewal option term	5 years
Tenant improvements		$ 700,000
Operating lease, termination period duration	30 days
Expenses from transactions with related party		800,000	$ 800,000	$ 800,000
Operating Leases, Future Minimum Payments Due, Fiscal Year Maturity
2,019		900,000
2,020		900,000
Due to related party		$ 0	$ 0

Related Person Transactions - R

Related Person Transactions - Related/Corvex (Details) - Related/Corvex $ in Millions	1 Months Ended
	Aug. 31, 2016USD ($)
Related Party Transaction [Line Items]
Reimbursement amount paid	$ 8.2
Due to related parties	$ 33.3

Selected Quarterly Financial _3

Selected Quarterly Financial Data (Unaudited) - Summary of Unaudited Quarterly Results of Operations (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Quarterly Financial Information Disclosure [Abstract]
Total revenues	$ 42,925	$ 46,873	$ 48,636	$ 58,588	$ 71,623	$ 77,798	$ 91,599	$ 99,551	$ 197,022	$ 340,571	$ 500,680
Net income attributable to Equity Commonwealth common shareholders	$ 13,420	$ 30,767	$ 35,036	$ 185,602	$ (23,558)	$ 31,215	$ (7,806)	$ 21,817	$ 264,825	$ 21,668	$ 205,329
Net income (loss) attributable to Equity Commonwealth common shareholders per share—basic (in dollars per share)	$ 0.11	$ 0.25	$ 0.29	$ 1.50	$ (0.19)	$ 0.25	$ (0.06)	$ 0.18
Net income (loss) attributable to Equity Commonwealth common shareholders per share—diluted (in dollars per share)	$ 0.11	$ 0.25	$ 0.29	$ 1.48	$ (0.19)	$ 0.25	$ (0.06)	$ 0.17

Selected Quarterly Financial _4

Selected Quarterly Financial Data (Unaudited) - Narrative (Details) - USD ($) $ in Thousands	3 Months Ended				12 Months Ended
	Mar. 31, 2018	Dec. 31, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Quarterly Financial Information Disclosure [Abstract]
(Loss) gain on sale of properties	$ 205,200	$ (29,200)
Loss on asset impairment	$ 12,100		$ 18,400	$ 1,300	$ 12,087	$ 19,714	$ 58,476

Subsequent Events - Narrative (

Subsequent Events - Narrative (Details) $ / shares in Units, $ in Thousands

Feb. 15, 2019$ / shares

Jan. 29, 2019USD ($)ft²

Jan. 11, 2019$ / shares

Nov. 15, 2018$ / shares

Oct. 29, 2018$ / shares

Aug. 15, 2018$ / shares

Jul. 30, 2018$ / shares

May 15, 2018$ / shares

Apr. 27, 2018$ / shares

Feb. 15, 2018$ / shares

Jan. 30, 2018$ / shares

Dec. 31, 2018USD ($)

Dec. 31, 2017USD ($)

Dec. 31, 2016USD ($)ft²

Disposed of by Sale

Subsequent Event [Line Items]

Property square feet (in sqft) | ft²

7,972,692

Gross sales price | $

$ 1,031,286

$ 862,624

$ 1,300,000

Series D

Subsequent Event [Line Items]

Dividend declared (in dollars per share)

$ 0.40625

$ 0.40625

$ 0.40625

$ 0.40625

Dividends paid (in dollars per share)

$ 0.40625

$ 0.40625

$ 0.40625

$ 0.40625

Subsequent event | 1735 Market Street | Disposed of by Sale

Subsequent Event [Line Items]

Percentage of equity interests in fee simple owner

100.00%

Property square feet (in sqft) | ft²

1,286,936

Gross sales price | $

$ 451,600

Subsequent event | Series D

Subsequent Event [Line Items]

Dividend declared (in dollars per share)

$ 0.40625

Dividends paid (in dollars per share)

$ 0.40625

SCHEDULE II VALUATION AND QUA_2

SCHEDULE II VALUATION AND QUALIFYING ACCOUNTS (Details) - Allowance for doubtful accounts - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Movement in valuation and qualifying accounts
Balance at Beginning of Period	$ 4,771	$ 5,105	$ 7,715
Charged to Costs and Expenses	1,273	935	1,056
Deductions	(1,070)	(1,269)	(3,666)
Balance at End of Period	$ 4,974	$ 4,771	$ 5,105

SCHEDULE III REAL ESTATE AND _2

SCHEDULE III REAL ESTATE AND ACCUMULATED DEPRECIATION (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	$ 26,000
Initial Cost to Company
Land	129,794
Buildings and Improvements	788,909
Costs Capitalized Subsequent to Acquisition, Net	243,438
Impairment/Write Downs	(22,499)
Cost Amount Carried at Close of Period
Land	135,142
Buildings and Improvements	1,004,500
Total	1,139,642
Accumulated Depreciation	375,968	$ 450,718	$ 755,255	$ 898,939
1225 Seventeenth Street
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	0
Initial Cost to Company
Land	22,400
Buildings and Improvements	110,090
Costs Capitalized Subsequent to Acquisition, Net	33,108
Impairment/Write Downs	(2,750)
Cost Amount Carried at Close of Period
Land	22,400
Buildings and Improvements	140,448
Total	162,848
Accumulated Depreciation	35,066
1250 H Street, NW
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	0
Initial Cost to Company
Land	5,975
Buildings and Improvements	53,778
Costs Capitalized Subsequent to Acquisition, Net	15,895
Impairment/Write Downs	(579)
Cost Amount Carried at Close of Period
Land	5,975
Buildings and Improvements	69,094
Total	75,069
Accumulated Depreciation	33,383
Georgetown-Green and Harris Buildings
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	0
Initial Cost to Company
Land	24,000
Buildings and Improvements	35,979
Costs Capitalized Subsequent to Acquisition, Net	1,076
Impairment/Write Downs	0
Cost Amount Carried at Close of Period
Land	24,000
Buildings and Improvements	37,055
Total	61,055
Accumulated Depreciation	8,402
109 Brookline Avenue
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	0
Initial Cost to Company
Land	3,168
Buildings and Improvements	30,397
Costs Capitalized Subsequent to Acquisition, Net	15,636
Impairment/Write Downs	(1,397)
Cost Amount Carried at Close of Period
Land	3,168
Buildings and Improvements	44,636
Total	47,804
Accumulated Depreciation	21,853
1735 Market Street
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	0
Initial Cost to Company
Land	24,753
Buildings and Improvements	222,775
Costs Capitalized Subsequent to Acquisition, Net	95,918
Impairment/Write Downs	(14,927)
Cost Amount Carried at Close of Period
Land	24,748
Buildings and Improvements	303,771
Total	328,519
Accumulated Depreciation	136,425
206 East 9th Street
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	26,000
Initial Cost to Company
Land	7,900
Buildings and Improvements	38,533
Costs Capitalized Subsequent to Acquisition, Net	5,362
Impairment/Write Downs	(444)
Cost Amount Carried at Close of Period
Land	7,900
Buildings and Improvements	43,451
Total	51,351
Accumulated Depreciation	7,073
Bridgepoint Square
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	0
Initial Cost to Company
Land	7,784
Buildings and Improvements	70,526
Costs Capitalized Subsequent to Acquisition, Net	20,637
Impairment/Write Downs	(1,172)
Cost Amount Carried at Close of Period
Land	7,784
Buildings and Improvements	89,991
Total	97,775
Accumulated Depreciation	45,146
Research Park
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	0
Initial Cost to Company
Land	15,859
Buildings and Improvements	60,175
Costs Capitalized Subsequent to Acquisition, Net	33,383
Impairment/Write Downs	0
Cost Amount Carried at Close of Period
Land	21,212
Buildings and Improvements	88,205
Total	109,417
Accumulated Depreciation	38,117
333 108th Avenue NE
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	0
Initial Cost to Company
Land	14,400
Buildings and Improvements	136,412
Costs Capitalized Subsequent to Acquisition, Net	2,746
Impairment/Write Downs	(31)
Cost Amount Carried at Close of Period
Land	14,400
Buildings and Improvements	139,127
Total	153,527
Accumulated Depreciation	33,117
600 108th Avenue NE
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	0
Initial Cost to Company
Land	3,555
Buildings and Improvements	30,244
Costs Capitalized Subsequent to Acquisition, Net	19,677
Impairment/Write Downs	(1,199)
Cost Amount Carried at Close of Period
Land	3,555
Buildings and Improvements	48,722
Total	52,277
Accumulated Depreciation	$ 17,386

SCHEDULE III REAL ESTATE AND _3

SCHEDULE III REAL ESTATE AND ACCUMULATED DEPRECIATION - Analysis of Carrying Amount of Real Estate Properties and Accumulated Depreciation (Details) - USD ($) $ in Thousands	3 Months Ended			12 Months Ended
	Mar. 31, 2018	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Real Estate Properties
Additions				$ 56,300	$ 55,000	$ 110,700
Loss on asset impairment	$ (12,100)	$ (18,400)	$ (1,300)	(12,087)	(19,714)	(58,476)
Properties classified to held for sale					(138,130)
Balance at the end of the year				1,139,642
Accumulated Depreciation
Balance at the beginning of the year	450,718		755,255	450,718	755,255	898,939
Additions				39,161	71,979	101,647
Loss on asset impairment				0	0	(119)
Properties classified to held for sale					(62,064)
Disposals				(113,911)	(314,452)	(245,212)
Balance at the end of the year				375,968	450,718	755,255
Real Estate Properties
Real Estate Properties
Balance at the beginning of the year	$ 1,747,611		$ 2,856,890	1,747,611	2,856,890	3,887,352
Additions				58,618	55,041	113,258
Loss on asset impairment				(12,032)	(6,711)	(58,595)
Disposals				(654,555)	(1,019,479)	(1,085,125)
Balance at the end of the year				$ 1,139,642	$ 1,747,611	$ 2,856,890

SCHEDULE III REAL ESTATE AND _4

SCHEDULE III REAL ESTATE AND ACCUMULATED DEPRECIATION - Analysis of Carrying Amount of Real Estate Properties and Accumulated Depreciation (Footnote) (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2018USD ($)
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation [Line Items]
Aggregate cost of properties for federal income tax purposes	$ 1,129,227
Maximum | Buildings and improvements
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation [Line Items]
Estimated useful lives	40 years
Maximum | Personal property
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation [Line Items]
Estimated useful lives	12 years