Equity Commonwealth (EQC) 10-K 15 Feb 18 2017 FY Annual report Financial data

Document and Entity Information

Document and Entity Information - USD ($) $ in Billions	12 Months Ended
	Dec. 31, 2017	Feb. 12, 2018	Jun. 30, 2017
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, 2017
Amendment Flag	false
Current Fiscal Year End Date	--12-31
Document Fiscal Year Focus	2,017
Document Fiscal Period Focus	FY
Entity Well-known Seasoned Issuer	Yes
Entity Current Reporting Status	Yes
Entity Voluntary Filers	No
Entity Filer Category	Large Accelerated Filer
Entity Common Stock, Shares Outstanding (in shares)		124,427,966
Entity Public Float			$ 3.9

CONSOLIDATED BALANCE SHEETS

CONSOLIDATED BALANCE SHEETS - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Real estate properties:
Land	$ 191,775	$ 286,186
Buildings and improvements	1,555,836	2,570,704
Total real estate properties, at cost, gross	1,747,611	2,856,890
Accumulated depreciation	(450,718)	(755,255)
Total real estate properties, at cost, net	1,296,893	2,101,635
Properties held for sale	97,688	0
Acquired real estate leases, net	23,847	48,281
Cash and cash equivalents	2,351,693	2,094,674
Marketable securities	276,928	0
Restricted cash	8,897	6,532
Rents receivable, net of allowance for doubtful accounts of $4,771 and $5,105, respectively	93,436	152,031
Other assets, net	87,563	122,922
Total assets	4,236,945	4,526,075
LIABILITIES AND EQUITY
Senior unsecured debt, net	815,984	1,063,950
Mortgage notes payable, net	32,594	77,717
Liabilities related to properties held for sale	1,840	0
Accounts payable and accrued expenses	69,220	95,395
Assumed real estate lease obligations, net	1,001	1,946
Rent collected in advance	11,076	18,460
Security deposits	4,735	8,160
Total liabilities	936,450	1,265,628
Commitments and contingencies
Shareholders’ equity:
Common shares of beneficial interest, $0.01 par value: 350,000,000 shares authorized; 124,217,616 and 123,994,465 shares issued and outstanding, respectively	1,242	1,240
Additional paid in capital	4,380,313	4,363,177
Cumulative net income	2,596,259	2,566,603
Cumulative other comprehensive loss	(95)	(208)
Cumulative common distributions	(3,111,868)	(3,111,868)
Cumulative preferred distributions	(685,748)	(677,760)
Total shareholders’ equity	3,299,366	3,260,447
Noncontrolling interest	1,129	0
Total equity	3,300,495	3,260,447
Total liabilities and equity	4,236,945	4,526,075
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, 2017	Dec. 31, 2016
Rents receivable, allowance for doubtful accounts	$ 4,771	$ 5,105
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)	124,217,616	123,994,465
Common shares of beneficial interest, shares outstanding (in shares)	124,217,616	123,994,465
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, 2017	Dec. 31, 2016	Dec. 31, 2015
Revenues:
Rental income	$ 270,320	$ 409,071	$ 570,382
Tenant reimbursements and other income	70,251	91,609	144,509
Total revenues	340,571	500,680	714,891
Expenses:
Operating expenses	141,425	200,706	324,948
Depreciation and amortization	90,708	131,806	194,001
General and administrative	47,760	50,256	57,457
Loss on asset impairment	19,714	58,476	17,162
Total expenses	299,607	441,244	593,568
Operating income	40,964	59,436	121,323
Interest and other income	26,380	10,331	5,989
Interest expense (including net amortization of debt discounts, premiums and deferred financing fees of $3,135, $3,725, and $1,028, respectively)	(52,183)	(84,329)	(107,316)
(Loss) gain on early extinguishment of debt	(493)	(2,680)	6,661
Foreign currency exchange loss	0	(5)	(8,857)
Gain on sale of properties, net	15,498	250,886	84,421
Income before income taxes	30,166	233,639	102,221
Income tax expense	(500)	(745)	(2,364)
Net income	29,666	232,894	99,857
Net income attributable to noncontrolling interest	(10)	0	0
Net income attributable to Equity Commonwealth	29,656	232,894	99,857
Preferred distributions	(7,988)	(17,956)	(27,924)
Excess fair value of consideration paid over carrying value of preferred shares	0	(9,609)	0
Net income attributable to Equity Commonwealth common shareholders	$ 21,668	$ 205,329	$ 71,933
Weighted average common shares outstanding — basic (in shares)	124,125	125,474	128,621
Weighted average common shares outstanding — diluted (in shares)	125,129	126,768	129,437
Earnings per common share attributable to Equity Commonwealth common shareholders:
Basic (in dollars per share)	$ 0.17	$ 1.64	$ 0.56
Diluted (in dollars per share)	$ 0.17	$ 1.62	$ 0.56

CONSOLIDATED STATEMENTS OF OPE5

CONSOLIDATED STATEMENTS OF OPERATIONS (Parenthetical) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Statement [Abstract]
Debt discounts, premiums and deferred financing fees	$ 3,135	$ 3,725	$ 1,028

CONSOLIDATED STATEMENTS OF COMP

CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Statement of Comprehensive Income [Abstract]
Net income	$ 29,666	$ 232,894	$ 99,857
Other comprehensive income (loss), net of tax:
Unrealized (loss) gain on derivative instruments and other assets	(248)	3,479	612
Foreign currency translation adjustments	0	0	48,917
Unrealized gain, net on marketable securities	361	0	0
Total comprehensive income	29,779	236,373	149,386
Comprehensive income attributable to the noncontrolling interest	(10)	0	0
Total comprehensive income attributable to Equity Commonwealth	$ 29,769	$ 236,373	$ 149,386

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, 2014		4,915,497	11,000,000		129,607,279
Balance at Dec. 31, 2014	$ 3,319,583	$ 119,266	$ 265,391	$ (622,271)	$ 1,296	$ (3,111,868)	$ 4,487,133	$ 2,233,852	$ (53,216)	$ 0
Comprehensive income:
Net income	99,857							99,857		0
Unrealized gain (loss) on derivative instrument	612								612
Unrealized gain on marketable securities	0
Foreign currency translation adjustments	48,917								48,917
Total comprehensive income	149,386
Purchase of shares	(87,983)				$ (35)		(87,948)			0
Purchase of shares (in shares)					(3,410,300)
Redemption of shares (in shares)		(301)			144
Redemption of shares	0	$ (3)		0	$ 0		3
Excess fair value of consideration paid over carrying value of preferred shares	0
Share-based compensation (in shares)					152,791
Share-based compensation	15,425				$ 2		15,423			0
Distributions	(27,924)			(27,924)		0
Distributions										0
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
Foreign currency translation adjustments	0
Total comprehensive income	236,373
Purchase of shares	(69,987)				$ (25)		(69,962)			0
Purchase of shares (in shares)					(2,491,675)
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
Distributions										0
Balance (in shares) at Dec. 31, 2016	123,994,465	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
Foreign currency translation adjustments	0
Total comprehensive income	29,779
Purchase of shares	(3,188)				$ 0		(3,188)
Purchase of shares (in shares)					(43,329)
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
Distributions										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

CONSOLIDATED STATEMENTS OF CASH

CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
CASH FLOWS FROM OPERATING ACTIVITIES:
Net income	$ 29,666	$ 232,894	$ 99,857
Adjustments to reconcile net income to cash provided by operating activities:
Depreciation	73,169	102,695	145,888
Net amortization of debt discounts, premiums and deferred financing fees	3,135	3,725	1,028
Straight line rental income	(14,425)	(14,083)	(5,255)
Amortization of acquired real estate leases	8,994	21,129	34,277
Other amortization	9,989	14,513	21,309
Share-based compensation	21,414	18,530	15,425
Loss on asset impairment	19,714	58,476	17,162
Loss (gain) on early extinguishment of debt	493	2,680	(6,661)
Foreign currency exchange loss	0	5	8,857
Net gain on sale of properties	(15,498)	(250,886)	(84,421)
Change in assets and liabilities:
Rents receivable and other assets	(23,708)	(23,921)	(47,472)
Accounts payable and accrued expenses	(9,047)	4,663	(4,643)
Rent collected in advance	(3,563)	(6,981)	(8,462)
Security deposits	(337)	264	1,225
Cash provided by operating activities	99,996	163,703	188,114
CASH FLOWS FROM INVESTING ACTIVITIES:
Real estate acquisitions	0	(2,802)	0
Real estate improvements	(64,813)	(121,450)	(70,633)
Insurance proceeds received	4,000	500	0
Principal payments received from direct financing lease	0	0	7,352
Principal payments received from real estate mortgages receivable	313	0	0
Proceeds from sale of properties, net	802,324	1,149,471	1,888,815
Purchase of marketable securities	(276,238)	0	0
Proceeds from sale of securities	0	0	27,068
Cash provided by investing activities	465,586	1,025,719	1,852,602
CASH FLOWS FROM FINANCING ACTIVITIES:
Purchase and retirement of common shares	(3,188)	(69,987)	(87,983)
Redemption of preferred shares	0	(275,000)	0
Payments on borrowings	(295,053)	(560,187)	(469,963)
Deferred financing fees	0	(52)	(7,143)
Contributions from holders of noncontrolling interest	31	0	0
Distributions to preferred shareholders	(7,988)	(17,956)	(27,924)
Cash used in financing activities	(306,198)	(923,182)	(593,013)
Effect of exchange rate changes on cash	0	(8)	(9,502)
Increase in cash, cash equivalents, and restricted cash	259,384	266,232	1,438,201
Cash, cash equivalents, and restricted cash at beginning of year	2,101,206	1,834,974	396,773
Cash, cash equivalents, and restricted cash at end of year	2,360,590	2,101,206	1,834,974
SUPPLEMENTAL CASH FLOW INFORMATION:
Interest paid	56,796	85,310	111,324
Taxes paid	910	327	6,028
NON-CASH INVESTING AND FINANCING ACTIVITIES:
Increase (decrease) in capital expenditures recorded as liabilities	3,970	10,331	(13,528)
Mortgage note payable and interest payable transferred in consensual foreclosure	0	0	43,255
Total cash, cash equivalents, and restricted cash shown in the statement of cash flows	$ 2,101,206	$ 1,834,974	$ 396,773

Organization

Organization	12 Months Ended
	Dec. 31, 2017
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 the ownership and operation of real estate, primarily office buildings, throughout 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, structure. 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, 99.97% of the outstanding shares of beneficial interest, designated as units, in the Operating Trust (OP Units) as of December 31, 2017 , and the Company is the sole trustee of the Operating Trust. As the sole trustee, the Company generally has the exclusive 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, 2017 , our portfolio, excluding properties held for sale, included 16 properties ( 26 buildings), with a combined 8.7 million square feet. As of December 31, 2017 , we had $2.6 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, 2017
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, 2017 , 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 (presented in our consolidated balance sheets as assumed real estate lease obligations) 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 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, cash flow or liquidity, and our decision to dispose of an asset before the end of its estimated useful life. 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, 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) recently quoted market prices, (ii) market prices for comparable properties, or (iii) the present value of future cash flows. During the years ended December 31, 2017 , 2016 , and 2015 we recorded a loss on asset impairment in continuing operations totaling $19.7 million , $58.5 million and $17.2 million respectively, to reduce the carrying value of properties to their estimated fair values (see Note 16). 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, and we have no current plans to remove any such asbestos. 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 at any of our properties that have a material adverse effect on us. However, no assurances can be given that such conditions are not present in our properties or that other costs we incur to remediate contamination will not have a material adverse effect on our business or financial condition. As of December 31, 2017 and 2016 , 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 consist 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 these securities are recognized in cumulative other comprehensive income (loss). Realized gains and losses are recognized in earnings only upon the sale of the securities. 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 some of our mortgage debts, 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. Accounting Policy for 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, 2017 , we have an interest rate cap agreement that qualifies as a cash flow hedge. 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 are entered into to manage interest rate risk associated with our 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. 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 real estate investment trust. However, we are subject to certain state and local taxes without regard to our REIT status. The Income Taxes Topic of the 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. Foreign Operations. During the year ended December 31, 2015, we sold our Australian properties and no longer have any foreign operations. The U.S. dollar was the functional currency for our consolidated subsidiaries operating in the United States prior to the sale of the Australian portfolio. The functional currency for our consolidated subsidiaries in countries other than the United States was the principal currency in which the entity's assets, liabilities, income and expenses were denominated. The functional currency of our consolidated subsidiary that operated in Australia was the Australian dollar. We translated our Australian subsidiary's financial statements into U.S. dollars when we consolidated that subsidiary's financial statements on a quarterly basis. Generally, we translated assets and liabilities at the exchange rate in effect as of the balance sheet date. The resulting translation adjustments were included in cumulative other comprehensive loss in our consolidated balance sheets. We translated income statement accounts using the average exchange rate for the period and income statement accounts that include significant non-recurring transactions at the rate in effect as of the date of the transaction. We were subject to foreign currency risk due to potential fluctuations in exchange rates between Australian and U.S. currencies. New Accounting Pronouncements. In August 2017, the FASB issued Accounting Standards Update (ASU) 2017-12, Derivatives and Hedging (Topic 815): Targeted Improvements to Accounting for Hedging Activities, to enable entities to better portray the economics of their risk management activities in the financial statements and enhance the transparency and understandability of hedge results. The amendments also simplify the application of hedge accounting in certain situations. This update is effective for fiscal years beginning after December 15, 2018, and for interim periods within those fiscal years. We are evaluating the impact ASU 2017-12 will have on our financial position and results of operations. 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 do not expect that the adoption of this ASU will 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. ASU 2017-05 is effective beginning January 1, 2018. Early adoption is permitted but the standard is required to be adopted concurrently with ASU 2014-09. We do not expect that the adoption of this ASU will have a material impact on our consolidated financial statements. In February 2017, the FASB issued ASU 2017-01, Clarifying the Definition of a Business, that changes the definition of a business to assist entities with evaluating when a set of transferred assets and activities is a business. This update is effective for fiscal years beginning after December 15, 2017, and for interim periods within those fiscal years. We early adopted this ASU effective January 1, 2017, and the adoption of this ASU did not have a material impact on our consolidated financial statements. In November 2016, the FASB issued ASU 2016-18, Statement of Cash Flows (Topic 230): Restricted Cash, which amends ASC Topic 230, Statements of Cash Flows, to add or clarify guidance on the classification and presentation of restricted cash in the statement of cash flows. This update is effective for fiscal years beginning after December 15, 2017, and for interim periods within those fiscal years. Early adoption is permitted, including adoption in an interim period. In August 2016, the FASB issued ASU 2016-15, Statement of Cash Flows (Topic 230): Classification of Certain Cash Receipts and Cash Payments, which addresses eight specific cash flow issues with the objective of reducing the existing diversity in practice. This update is effective for fiscal years beginning after December 15, 2017, and for interim periods within those fiscal years. Early adoption is permitted, including adoption in an interim period. We early adopted both ASU 2016-18 and ASU 2016-15 on December 31, 2017 and made the following reclassifications to the prior years' consolidated statements of cash flows to conform to the current year's presentation (in thousands): Statement of Cash Flow for the year ended December 31, 2016 Originally Reported Effect of Change As Adjusted Cash provided by operating activities $ 163,016 $ 687 $ 163,703 Cash provided by investing activities 1,052,119 (26,400 ) 1,025,719 Statement of Cash Flow for the year ended December 31, 2015 Originally Reported Effect of Change As Adjusted Cash provided by operating activities $ 181,544 $ 6,570 $ 188,114 Cash provided by investing activities 1,648,893 203,709 1,852,602 Cash used in financing activities (382,722 ) (210,291 ) (593,013 ) 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 in the process of evaluating the impact, if any, the adoption of this ASU will have on our consolidated financial statements. In March 2016, the FASB issued ASU 2016-09, Compensation-Stock Compensation (Topic 718): Improvements to Employee Share-Based Payment Accounting, which simplifies several aspects of the accounting for employee share-based payment transactions, including the income tax consequences, classification of awards as either equity or liabilities, and classification on the statement of cash flows. This update is effective for annual periods beginning after December 15, 2016, and interim periods within those annual periods, but early adoption is permitted. We adopted this ASU on January 1, 2017 and the adoption did not have a material impact 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. We are still assessing the impact of adopting ASU 2016-02. 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 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. The new standard was effective for us beginning on January 1, 2018 and it did not have a material effect on our consolidated financial statements. 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 expect to use the modified retrospective method of adoption that will result in a cumulative effect adjustment as of the date of adoption. We developed a project plan, which consisted of gathering and evaluating the inventory of our revenue streams. As our portfolio is currently comprised, the adoption of this ASU will not have a material impact on our consolidated financial statements. In April 2014, the FASB issued ASU No. 2014-08, Presentation of Financial Statements (Topic 205) and Property, Plant, and Equipment (Topic 360): Reporting Discontinued Operations and Disclosures of Disposals of Components of an Entity, or ASU 2014-08. ASU 2014-08 changes the criteria for reporting a discontinued operation. Under this pronouncement, a disposal of a part of an organization that has a major effect on its operations and financial results is a discontinued operation. We adopted ASU 2014-08 on January 1, 2015, and determined that our 2017 , 2016 and 2015 dispositions do not individually represent a strategic shift, as defined by the standard, that has or will have a major effect on our operations and financial results. As a result, the 2017 , 2016 and 2015 dispositions have not been presented as discontinued operations in the statements of operations.

Real Estate Properties

Real Estate Properties	12 Months Ended
	Dec. 31, 2017
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 year ended December 31, 2017 . During the years ended December 31, 2017 , 2016 , and 2015 , we made improvements, excluding tenant-funded improvements, to our properties totaling $55.0 million , $ 110.7 million and $75.7 million , respectively. We committed $44.9 million for expenditures related to 1.3 million square feet of leases executed during 2017 . Committed but unspent tenant related obligations are leasing commissions and tenant improvements. Based on existing leases as of December 31, 2017 , committed but unspent tenant related obligations were $63.2 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, 2016 , we had no properties classified as held for sale. As of December 31, 2017 , we classified the following property as held for sale (dollars in thousands): Asset Date Sold Number of Number of Square Footage Gross Sales Price 1600 Market Street February 2018 1 1 825,968 $ 160,000 Summarized balance sheet information for the property classified as held for sale 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, 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 disposed of the following properties (dollars in thousands): Asset Date Sold Number of Number of Square Footage Gross Sales Price Gain (Loss) on Sale Properties Executive Park February 2016 1 9 427,443 $ 50,865 $ 16,531 3330 N Washington Boulevard March 2016 1 1 55,719 11,250 5,455 111 East Kilbourn Avenue March 2016 1 1 373,669 60,500 14,687 1525 Locust Street April 2016 1 1 98,009 17,700 8,956 633 Ahua Street April 2016 1 1 93,141 29,000 15,963 Asset Date Sold Number of Number of Square Footage Gross Sales Price Gain (Loss) on Sale Lakewood on the Park May 2016 1 2 180,558 37,100 13,616 Leased Land June 2016 1 7 — 48,450 15,914 9110 East Nichols Avenue June 2016 1 1 143,958 17,200 642 111 River Street (1) July 2016 1 1 566,215 235,000 78,207 Sky Park Centre July 2016 1 2 63,485 13,700 4,746 Raintree Industrial Park July 2016 1 12 563,182 11,500 (653 ) 8701 N Mopac August 2016 1 1 121,901 21,500 8,394 7800 Shoal Creek Boulevard October 2016 1 4 151,917 29,210 14,908 1200 Lakeside Drive November 2016 1 1 260,084 65,270 3,062 6200 Glenn Carlson Drive December 2016 1 1 338,000 23,050 7,706 Portfolios of properties 812 San Antonio Street May 2016 1 1 59,321 1601 Rio Grande Street May 2016 1 1 56,219 Downtown Austin portfolio 2 2 115,540 $ 32,600 $ 20,584 785 Schilinger Road South June 2016 1 1 72,000 401 Vine Street June 2016 1 1 53,980 633 Frazier Drive June 2016 1 1 150,000 9840 Gateway Boulevard North June 2016 1 1 72,000 3003 South Expressway 281 June 2016 1 1 150,000 1331 North Center Parkway June 2016 1 1 53,980 Movie theater portfolio 6 6 551,960 $ 109,100 $ 30,595 128 Crews Drive July 2016 1 1 185,600 111 Southchase Boulevard July 2016 1 1 168,087 1043 Global Avenue July 2016 1 1 450,000 South Carolina industrial portfolio 3 3 803,687 $ 30,000 $ 7,248 100 East Wisconsin Avenue August 2016 1 1 435,067 111 Monument Circle August 2016 1 2 1,121,764 101-115 W. Washington Street August 2016 1 1 634,058 North Point Office Complex August 2016 1 2 873,335 Midwest portfolio 4 6 3,064,224 $ 416,900 $ (15,800 ) Total 30 62 7,972,692 $ 1,259,895 $ 250,761 (1) Property sale represented a leasehold interest. During the year ended December 31, 2015, we sold 91 properties ( 135 buildings) with a combined 18,939,185 square feet for an aggregate gross sales price of $2.0 billion , excluding closing costs and mortgage debt repayments. Lease Payments Our real estate properties are generally leased on gross lease, modified gross lease or triple net lease bases pursuant to non-cancelable, fixed term operating leases expiring between 2018 and 2037. Our triple net leases generally require the lessee to pay all property operating costs. Our 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, 2017 are as follows (in thousands): 2018 $ 187,984 2019 177,325 2020 158,000 2021 141,937 2022 131,362 Thereafter 693,119 $ 1,489,727 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 years ended December 31, 2016 and 2015 , totaled $0.9 million and $1.8 million , respectively. Ground lease expense includes percentage rent.

Lease Intangibles

Lease Intangibles	12 Months Ended
	Dec. 31, 2017
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, 2017 , and 2016 , and (in thousands): December 31, 2017 2016 Acquired in-place leases $ 56,853 $ 102,102 Acquired above market leases 21,021 44,752 Acquired real estate leases 77,874 146,854 Accumulated amortization, acquired in-place leases (41,978 ) (71,320 ) Accumulated amortization, acquired above market leases (12,049 ) (27,253 ) Acquired real estate leases, net $ 23,847 $ 48,281 Acquired below market leases $ 6,093 $ 10,434 Accumulated amortization (5,092 ) (8,488 ) Assumed real estate lease obligations, net $ 1,001 $ 1,946 Amortization of the lease intangibles for the years ended December 31, 2017 , 2016 , and 2015 , is as follows (in thousands): December 31, Income Statement Location 2017 2016 2015 Amortization of acquired in-place leases Depreciation and amortization $ 7,220 $ 14,598 $ 26,787 Amortization of above and below market leases Increase (decrease) to rental income (1,774 ) (6,531 ) (7,515 ) 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, 2017 , are approximately (in thousands): 2018 $ 7,285 2019 3,702 2020 2,449 2021 2,236 2022 1,959 Thereafter 5,215 $ 22,846

Investment in Direct Financing

Investment in Direct Financing Lease	12 Months Ended
	Dec. 31, 2017
Capital Leases, Net Investment in Direct Financing Leases [Abstract]
Investment in Direct Financing Lease	Investment in Direct Financing Lease We had an investment in a direct financing lease that related to a lease with a term that exceeded 75% of the useful life of an office tower located within a mixed use property in Phoenix, AZ. In December 2015, we sold this property. Prior to the sale of the property, we recognized income using the effective interest method to produce a level yield on funds not yet recovered.

Marketable Securities

Marketable Securities	12 Months Ended
	Dec. 31, 2017
Investments, Debt and Equity Securities [Abstract]
Marketable Securities	Marketable Securities All of our marketable securities are classified as available-for-sale and consist of United States Treasury notes, which mature in 2019, and common stock. Below is a summary of our marketable securities as of December 31, 2017 (in thousands): December 31, 2017 Cost or Amortized Cost Unrealized Gain, Net Estimated Fair Value Marketable securities $ 276,567 $ 361 $ 276,928

Other Assets

Other Assets	12 Months Ended
	Dec. 31, 2017
Other Assets [Abstract]
Other Assets	Other Assets Real Estate Mortgages Receivable As of December 31, 2017 , and 2016 , we had total real estate mortgages receivable with an aggregate carrying value of $7.7 million and $8.1 million , respectively, included in other assets in our consolidated balance sheets. 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; this real estate mortgage requires monthly interest payments and matures on January 24, 2023 . 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. We monitor the payment history of the borrowers and have determined that no allowance for losses related to these real estate mortgages receivable was necessary at December 31, 2017 , and 2016 . 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, 2017 , and 2016 (in thousands): December 31, 2017 2016 Deferred financing fees $ 14,458 $ 14,458 Accumulated amortization (12,709 ) (11,093 ) Deferred financing fees, net $ 1,749 $ 3,365 Deferred leasing costs $ 85,606 $ 133,771 Accumulated amortization (22,067 ) (41,148 ) Deferred leasing costs, net $ 63,539 $ 92,623 Capitalized lease incentives $ 8,562 $ 10,344 Accumulated amortization (2,054 ) (2,680 ) Capitalized lease incentives, net $ 6,508 $ 7,664 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, 2017 are approximately (in thousands): Deferred Leasing Costs Capitalized Lease Incentives 2018 $ 7,707 $ 983 2019 7,694 798 2020 7,383 758 2021 6,595 592 2022 6,046 547 Thereafter 28,114 2,830 $ 63,539 $ 6,508

Indebtedness

Indebtedness	12 Months Ended
	Dec. 31, 2017
Debt Disclosure [Abstract]
Indebtedness	Indebtedness At December 31, 2017 and 2016 , our outstanding indebtedness included the following (in thousands): December 31, Interest Rate at December 31, 2017 Maturity Date 2017 2016 Unsecured revolving credit facility, at LIBOR plus a premium 2.61 % 1/28/2019 $ — $ — 5-year unsecured term loan, at LIBOR plus a premium 2.71 % 1/28/2020 200,000 200,000 7-year unsecured term loan, at LIBOR plus a premium 3.11 % 1/28/2022 200,000 200,000 Unsecured floating rate debt 2.91 % (1) $ 400,000 $ 400,000 6.65% Senior Unsecured Notes due 2018 — % — $ — $ 250,000 5.875% Senior Unsecured Notes due 2020 5.875 % 9/15/2020 250,000 250,000 5.75% Senior Unsecured Notes due 2042 (2) 5.75 % 8/1/2042 175,000 175,000 Unsecured fixed rate debt 5.82 % (1) $ 425,000 $ 675,000 Parkshore Plaza — % — $ — $ 41,275 206 East 9th Street 5.69 % 1/5/2021 26,536 27,041 33 Stiles Lane — % — — 2,415 97 Newberry Road 5.71 % 3/1/2026 5,404 5,903 Secured fixed rate debt 5.69 % (1) $ 31,940 $ 76,634 $ 856,940 $ 1,151,634 Unamortized net premiums, discounts and deferred financing fees (8,362 ) (9,967 ) $ 848,578 $ 1,141,667 (1) Represents weighted average interest rate at December 31, 2017 . (2) On January 30, 2018, we delivered notice of our intent to redeem at par our $175 million 5.75% senior unsecured notes due 2042 on March 7, 2018. The notes will be redeemed for cash at a price equal to 100% of the principal amount of the notes plus any accrued and unpaid interest up to, but excluding, the redemption date. 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. In connection with the termination of the prior agreements, we recognized a loss on early extinguishment of debt of $0.4 million from the write-off of unamortized deferred financing fees for the year ended December 31, 2015. 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. The revolving credit facility has a scheduled maturity date of January 28, 2019, which maturity date may be extended for up to two additional periods of six months at our option subject to satisfaction of certain conditions and the payment of an extension fee of 7.5 basis points of the aggregate amount available under the revolving credit facility. The 5 -year term loan and the 7 -year term loan have scheduled maturity dates of January 28, 2020 and January 28, 2022, respectively. We used the proceeds of borrowings under the credit agreement to repay all amounts outstanding and due under the previous term loan agreement. The credit agreement permits us to utilize up to $100.0 million of the revolving credit facility for the issuance of letters of credit. Amounts outstanding under the credit agreement generally may be prepaid at any time without premium or penalty, subject to certain exceptions. We have the right to request increases in the aggregate maximum amount of borrowings available under the revolving credit facility and term loans up to an additional $1.15 billion , subject to certain conditions. Borrowings under the 5 -year term loan and 7 -year term loan will, subject to certain exceptions, bear interest at a 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. Borrowings under the revolving credit facility will, subject to certain exceptions, bear interest at a rate equal to, at our option, either a LIBOR rate or a base rate plus a margin of 87.5 to 155 basis points for LIBOR rate advances and 0 to 55 basis points for base rate advances, in each case depending on our credit rating. In addition, we are 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. Borrowings under our revolving credit facility currently bear interest at LIBOR plus a spread, which was 105 basis points as of December 31, 2017 . As of December 31, 2017 , the interest rate payable on borrowings under our revolving credit facility was 2.61% . As of December 31, 2017 , we had no balance outstanding and $750.0 million available under our revolving credit facility and the facility fee as of December 31, 2017 was 20 basis points. Our term loans currently bear interest at a rate of LIBOR plus a spread, which was 115 and 155 basis points for the 5 -year and 7 -year term loan, respectively, as of December 31, 2017 . As of December 31, 2017 , the interest rates for the amounts outstanding under our 5 -year term loan and 7 -year term loan were 2.71% and 3.11% , respectively. As of December 31, 2017 , we had $200.0 million outstanding under each of our 5 -year and 7 -year term loans. Debt Covenants: Our public debt indenture and related supplements and our credit agreement contain a number of financial and other covenants, including covenants that restrict our ability to incur indebtedness or to make distributions under certain circumstances and require us to maintain financial ratios and a minimum net worth. At December 31, 2017 , we believe we were in compliance with all of our respective covenants under our public debt indenture and related supplements and our credit agreement. Senior Unsecured Notes: 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. On May 1, 2015, we redeemed at par $138.8 million of our 5.75% senior unsecured notes due 2015 and recognized a loss on early extinguishment of debt of $0.1 million for the year ended December 31, 2015 from the write-off of an unamortized discount and unamortized deferred financing fees. Mortgage Notes Payable: At December 31, 2017 , two of our properties with an aggregate net book value of $56.2 million had secured mortgage notes totaling $32.6 million (including net premiums and unamortized deferred financing fees) maturing in 2021 and 2026. 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. On December 1, 2015, we repaid at par $116.0 million of mortgage debt at 111 Monument Circle and recognized a gain on early extinguishment of debt of $0.6 million for the year ended December 31, 2015 from the write-off of an unamortized premium, net of the write-off of unamortized deferred financing fees. In accordance with the agreement to sell Illinois Center, we were required to deliver the property unencumbered. On August 3, 2015, prior to the sale, we defeased the outstanding $141.4 million balance of the mortgage loan secured by 111 East Wacker Drive, one of the buildings included in Illinois Center. The defeasance costs and write off of the unamortized deferred financing costs, net of the write off of the unamortized premium, resulted in a net loss on early extinguishment of debt of $3.9 million for the year ended December 31, 2015. In accordance with the agreement to sell 2501 20th Place South, we were required to deliver the property unencumbered. On June 5, 2015, we prepaid $10.0 million of mortgage debt at 2501 20th Place South and recognized a loss on early extinguishment of debt totaling $0.6 million for the year ended December 31, 2015, which consisted of a prepayment premium and the write off of unamortized deferred financing fees, net of the write off of an unamortized premium. In accordance with the agreement to sell 1320 Main Street, we were required to deliver the property unencumbered. On June 3, 2015, prior to the sale, we defeased the $38.7 million outstanding balance of the mortgage loan secured by 1320 Main Street. The defeasance costs and write off of the unamortized deferred financing costs, net of the write off of the unamortized premium, resulted in a net loss on early extinguishment of debt of $6.2 million for the year ended December 31, 2015. During the quarter ended June 30, 2014, we made the decision to cease making loan servicing payments on 225 Water Street in Jacksonville, Florida. The first payment we determined not to make for this property was due on June 11, 2014. The property was secured by a non-recourse mortgage loan. On October 10, 2014, we were notified by the lender that our decision to cease making loan servicing payments created an event of default effective July 11, 2014, and the lender exercised its option to accelerate the maturity of the unpaid balance of the loan. Beginning July 11, 2014, we accrued interest on this loan at 10.03% , to include the 4.0% of default interest. The lender filed a suit of foreclosure for this property and we cooperated with the lender to allow for a consensual foreclosure process. On May 22, 2015, title to 225 Water Street was transferred to the lender pursuant to the consensual foreclosure in full satisfaction of the mortgage debt, with a principal balance of $40.1 million . The transaction resulted in a gain on early extinguishment of debt of $17.3 million for the excess of the debt principal balance over the net book value of the property for the year ended December 31, 2015. Required Principal Payments: The required principal payments due during the next five years and thereafter under all of our outstanding debt at December 31, 2017 are as follows (in thousands): 2018 $ 1,063 2019 1,126 2020 451,189 2021 25,463 2022 200,663 Thereafter (1) 177,436 $ 856,940 (1) On January 30, 2018, we delivered notice of our intent to redeem at par our $175 million 5.75% senior unsecured notes due 2042 on March 7, 2018. The notes will be redeemed for cash at a price equal to 100% of the principal amount of the notes plus any accrued and unpaid interest up to, but excluding, the redemption date.

Shareholders' Equity

Shareholders' Equity	12 Months Ended
	Dec. 31, 2017
Stockholders' Equity Note [Abstract]
Shareholders' Equity	Shareholders’ Equity Common Share Issuances: During the year ended December 31, 2015, we issued 144 common shares to holders of 301 of our series D cumulative convertible preferred shares (series D preferred shares) who elected to convert their series D preferred shares into our common shares. See Note 14 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, which authorizes the repurchase of up to $100.0 million of our outstanding common shares over the twelve month period following the date of authorization. On September 14, 2015, our Board of Trustees authorized the repurchase of up to an additional $100.0 million of our outstanding common shares over the twelve month period following the date of authorization. In August and September 2016, the first two share repurchase authorizations, of which $86.6 million was not utilized, expired. On March 17, 2016, 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 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. During the year ended December 31, 2017, we did not purchase any common shares under our common share repurchase program. During the years ended December 31, 2016 and 2015, we purchased and retired 2,491,675 and 3,410,300 of our common shares, respectively, at a weighted average price of $27.68 and $25.76 per share, respectively, for a total investment of $69.0 million and $87.8 million , respectively. Since the inception of the common share repurchase program through December 31, 2017 , we have purchased and retired a total of 5,901,975 of our common shares at a weighted average price of $26.57 per share, for a total of $156.8 million . The $150.0 million of remaining authorization available under our share repurchase program as of December 31, 2017 is scheduled to expire in March 2018. During the years ended December 31, 2017 and 2016, certain of our employees surrendered 43,329 and 31,025 common shares, respectively, owned by them to satisfy their statutory tax withholding obligations in connection with the vesting of such common shares under the Equity Commonwealth 2015 Omnibus Incentive Plan (the 2015 Incentive Plan). Common Share Distributions: 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 8 for additional information regarding our credit agreement. We did not pay any cash distributions to our common shareholders in 2017 , 2016 and 2015 . 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 an initial conversion rate of 0.480775 common shares per series D preferred share, which is equivalent to an initial conversion price of $52.00 per common share, or 2,363,103 additional common shares at December 31, 2017 . 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, 2017 , we had 4,915,196 outstanding series D preferred shares that were convertible into 2,363,103 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 2017 , 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, 2017 January 30, 2017 February 15, 2017 $ 0.40625 April 10, 2017 April 28, 2017 May 15, 2017 $ 0.40625 June 30, 2017 July 28, 2017 August 15, 2017 $ 0.40625 October 12, 2017 October 30, 2017 November 15, 2017 $ 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% , 96.31% and 0% of each distribution to our shareholders for the tax years ended December 31, 2017 , 2016 , and 2015 , respectively, consists of an alternative minimum tax adjustment.

Noncontrolling Interest

Noncontrolling Interest	12 Months Ended
	Dec. 31, 2017
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 operating partnership unit (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, 2017 , the portion of the Operating Trust not beneficially owned by the Company is in the form of LTIP Units (see Note 14 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, 2017 : Common Shares LTIP Units Total Outstanding at January 1, 2017 123,994,465 — 123,994,465 Restricted share, time-based LTIP Unit grants and vested restricted stock units, net of forfeitures 223,151 42,520 265,671 Outstanding at December 31, 2017 124,217,616 42,520 124,260,136 Noncontrolling ownership interest in the Operating Trust 0.03 % The carrying value of the Noncontrolling interest is allocated based on the number of LTIP Units in proportion to the number of 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 99.97% for the year ended December 31, 2017 .

Cumulative Other Comprehensive

Cumulative Other Comprehensive Loss	12 Months Ended
	Dec. 31, 2017
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, 2017 and 2016 (in thousands): Unrealized Loss on Derivative Instruments Unrealized Gain, Net on Marketable Securities Total Balances 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 Balances as of December 31, 2017 $ (456 ) $ 361 $ (95 ) Unrealized Loss Balance as of January 1, 2016 $ (3,687 ) Other comprehensive loss before reclassifications (554 ) Amounts reclassified from cumulative other comprehensive loss to net income 4,033 Net current period other comprehensive income 3,479 Balance as of December 31, 2016 $ (208 ) The following tables present reclassifications out of cumulative other comprehensive loss for the years ended December 31, 2017 and 2016 (in thousands): Amounts Reclassified from Cumulative Other Comprehensive Loss to Net Income Year Ended December 31, Details about Cumulative Other Comprehensive Loss Components 2017 2016 Affected Line Items in the Statement of Operations Interest rate swap contracts $ — $ 3,792 Interest expense Interest rate swap contracts — 241 Interest and other income Interest rate cap contract 49 — Interest expense $ 49 $ 4,033

Income Taxes

Income Taxes	12 Months Ended
	Dec. 31, 2017
Income Tax Disclosure [Abstract]
Income Taxes	Income Taxes Our provision for income taxes consists of the following (in thousands): Year Ended December 31, 2017 2016 2015 Current: State $ 500 $ 745 $ 270 Federal — — 513 Foreign — — 2,336 500 745 3,119 Deferred: Foreign — — (755 ) — — (755 ) Income tax expense $ 500 $ 745 $ 2,364 Federal income tax expense for the year ended December 31, 2015 relates to taxes incurred as a result of a taxable built-in gain triggered by the sale of a property that was previously owned by a C corporation. As a result of the disposition of the Australian properties, we incurred net income tax expense of $1.6 million for the year ended December 31, 2015. A reconciliation of our effective tax rate and the U.S. Federal statutory income tax rate is as follows: Year Ended December 31, 2017 2016 2015 Taxes at statutory U.S. federal income tax rate 35.00 % 35.00 % 35.00 % Dividends paid deduction and net operating loss utilization (35.00 )% (35.00 )% (35.00 )% Federal taxes on built-in gain — % — % 0.50 % State, local, and foreign income taxes 1.66 % 0.32 % 1.81 % Effective tax rate 1.66 % 0.32 % 2.31 % At December 31, 2017, we had net operating loss (NOL) carryforwards of approximately $120 million . 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. Our NOL carryforwards expire in years 2035 to 2037. We are still analyzing certain aspects of H.R. 1, the Tax Cuts and Jobs Act, and refining our calculations, which could potentially affect the measurement of our NOL carryforwards.

Derivative Instruments

Derivative Instruments	12 Months Ended
	Dec. 31, 2017
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. The only risk we currently manage by using derivative instruments is related to our interest rate risk. 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 the interest rate risk associated with our borrowings. The principal objective of such arrangements is to reduce the risks and/or costs associated with our operating and financial structure as well as to 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. We do not anticipate that any of the counterparties will fail to meet their obligations. Cash Flow Hedges of Interest Rate Risk We utilize interest rate derivatives to add stability to interest expense and to manage our exposure to interest rate movements. To accomplish this objective, we 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 2017 , 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. Amounts reported in cumulative other comprehensive loss related to derivatives will be reclassified to interest expense as interest payments are made on our variable-rate debt. During the next twelve months, we estimate that an additional $0.4 million will be reclassified from cumulative other comprehensive loss as an increase to interest expense. 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 4, 2016, we purchased an interest rate cap with a LIBOR strike price of 2.50% . The interest rate cap, effective April 1, 2016, has a notional amount of $400.0 million and a maturity date of March 1, 2019. As of December 31, 2017 , we had the following outstanding interest rate derivative that was designated as a cash flow hedge of interest rate risk: Interest Rate Derivative Number of Instruments Notional Amount (in thousands) Interest rate cap 1 $400,000 The table below presents the fair value of our derivative financial instrument as well as its classification on the consolidated balance sheets as of December 31, 2017 , and 2016 (amounts in thousands): Fair Value as of December 31, Interest Rate Derivative Designated as Hedging Instrument Balance Sheet Location 2017 2016 Interest rate cap Other assets $ 17 $ 314 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, 2017 , 2016 , and 2015 (amounts in thousands): Year Ended December 31, 2017 2016 2015 Amount of loss recognized in cumulative other comprehensive loss (effective portion) $ (297 ) $ (554 ) $ (1,149 ) Amount of loss reclassified from cumulative other comprehensive loss into interest expense (effective portion) 49 3,792 4,924 Amount of loss recognized in income (ineffective portion and amount excluded from effectiveness testing) — 241 — Credit-risk-related Contingent Features As of December 31, 2017 , we did not have any derivatives in a net liability.

Share-Based Compensation

Share-Based Compensation	12 Months Ended
	Dec. 31, 2017
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 the 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 performance period, the number of earned awards will be determined. The earned awards vest in two tranches with 50% of the earned award vesting at the end of the performance period and the remaining 50% of the earned award vesting one year after the end of the performance period, 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. 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. 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 9). On June 20, 2017, in accordance with the Company’s compensation plan for independent Trustees, the Compensation Committee of our Board of 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 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 24, 2017, the Committee approved a grant of 119,288 LTIP Units (which includes time-based LTIP Units and market-based LTIP Units), 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 LTIP Unit grant includes 39,364 time-based LTIP Units and 79,924 market-based LTIP Units at target ( 199,211 market-based LTIP Units at maximum). The restricted shares and time-based LTIP Units were granted on January 24, 2017 and were valued at $31.47 per share, the closing price of our common shares on the NYSE on that day. 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 vest one year after the date of the award. 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 that day. 2015 Equity Award Activity On June 16, 2015, 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 2015-2016 year of service on the Board of Trustees. These awards equated to 3,843 shares per Trustee, for a total of 34,587 shares, valued at $26.02 per share, the closing price of our common shares on the NYSE on that day. These shares vest one year after the date of the award. On January 28, 2015, the Committee approved a grant of 126,319 restricted common shares and 256,467 RSUs at target ( 639,250 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 2014. The restricted shares were granted on January 28, 2015 and were valued at $26.58 per share, the closing price of our common shares on the NYSE on that day. Outstanding Equity Awards The table below presents a summary of restricted share, RSU and LTIP Unit activity for the years ended December 31, 2017 , 2016 and 2015 : Number Weighted Number Weighted Outstanding at December 31, 2014 710,182 $ 26.66 1,266,237 $ 15.99 Granted 160,906 26.46 639,250 15.47 Vested (86,517 ) 26.66 — — Forfeited (8,115 ) 26.64 (21,377 ) 15.80 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 (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 678,952 unvested restricted shares and time-based LTIP Units as of December 31, 2017 are scheduled to vest as follows: 402,399 shares/units in 2018 , 123,764 shares/units in 2019 , 95,287 shares/units in 2020 and 57,502 shares/units in 2021 . As of December 31, 2017 , the estimated future compensation expense for all unvested restricted shares and time-based LTIP Units was $9.3 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 1.7 years. As of December 31, 2017 , the estimated future compensation expense for all unvested RSUs and market-based LTIP Units was $14.8 million . The weighted average period over which the future compensation expense will be recorded for the RSUs and market-based LTIP Units is approximately 1.9 years. The assumptions and fair values for the RSUs and market-based LTIP Units granted for the years ended December 31, 2017 , 2016 and 2015 are included in the following table on a per share and unit basis. 2017 2016 2015 Fair value of RSUs and market-based LTIP Units granted at the target amount $ 39.81 $ 38.80 $ 38.55 Fair value of RSUs granted at the maximum amount $ 15.97 $ 15.57 $ 15.47 Expected term (years) 4 4 4 Expected volatility — — — Expected dividend yield 1.59 % 1.86 % 1.88 % Risk-free rate 1.49 % 1.07 % 0.81 % During the years ended December 31, 2017 , 2016 and 2015 , we recorded $21.4 million , $18.5 million and $15.4 million , respectively, of compensation expense, net of forfeitures, in general and administrative expense for grants to our Trustees and employees related to our equity compensation plans. Forfeitures are recognized as they occur. At December 31, 2017 , 1,628,305 shares/units remain available for issuance under the 2015 Incentive Plan.

Defined Contribution Plan

Defined Contribution Plan	12 Months Ended
	Dec. 31, 2017
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 , $0.4 million and $0.3 million for the years ended December 31, 2017 , 2016 and 2015 , respectively.

Fair Value of Assets and Liabil

Fair Value of Assets and Liabilities	12 Months Ended
	Dec. 31, 2017
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 2017 and 2016 , categorized by the level of inputs used in the valuation of each asset and liability (dollars in thousands): 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 $ — $ — Fair Value at December 31, 2016 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 $ 314 $ — $ 314 $ — Effective Portion of Interest Rate Cap Contract The fair value of our interest rate cap contract is 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 are not significant to the overall valuation of our derivative. As a result, we have determined that our derivative valuation in its entirety is 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 in December 2016, 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 . As part of our disposition plan, and pursuant to our accounting policy, in 2016, 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 disposition plan and current estimates of market value, 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 second quarter of 2016, we recorded an impairment charge related to 111 Monument Circle, 101-115 W. Washington Street and 100 East Wisconsin Avenue of $43.7 million for the year ended December 31, 2016 in accordance with our impairment analysis procedures. We reduced the aggregate carrying value of these properties from $308.6 million to their estimated fair value of $264.9 million . These properties were sold in August 2016 (see Note 3 for additional information). In the fourth quarter of 2016, we recorded an impairment charge related to Parkshore Plaza, Cabot Business Park Land, 625 Crane Street and 111 Market Place of $14.7 million for the year ended December 31, 2016 in accordance with our impairment analysis procedures. We reduced the aggregate carrying value of these properties from $98.2 million to their estimated fair value of $83.5 million . These properties were sold in 2017 (see Note 3 for additional information). We determined these impairments based on third party offer prices and independent third party broker information, which are level 2 inputs according to the fair value hierarchy established in ASC 820. We evaluated each of our properties and determined there were no additional valuation adjustments necessary at December 31, 2016. Financial Instruments In addition to the assets and liabilities 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, 2017 and 2016 , the fair value of these additional financial instruments were not materially different from their carrying values, except as follows (in thousands): December 31, 2017 December 31, 2016 Principal Balance Fair Value Principal Balance Fair Value Senior unsecured debt and mortgage notes payable $ 856,940 $ 874,280 $ 1,151,634 $ 1,167,031 The fair values of our senior notes are based on quoted market prices (level 2 inputs) and 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, 2017 , no single tenant of ours is responsible for more than 8.9% of our total annualized rents. Our interest rate cap counterparty is a major financial institution and we monitor the amount of credit exposure to any one counterparty.

Earnings Per Common Share

Earnings Per Common Share	12 Months Ended
	Dec. 31, 2017
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, 2017 2016 2015 Numerator for earnings per common share - basic: Net income $ 29,666 $ 232,894 $ 99,857 Net income attributable to noncontrolling interest (10 ) — — Preferred distributions (7,988 ) (17,956 ) (27,924 ) Excess fair value of consideration paid over carrying value of preferred shares — (9,609 ) — Numerator for net income per share - basic $ 21,668 $ 205,329 $ 71,933 Numerator for earnings per common share - diluted: Net income $ 29,666 $ 232,894 $ 99,857 Preferred distributions (7,988 ) (17,956 ) (27,924 ) Excess fair value of consideration paid over carrying value of preferred shares — (9,609 ) — Numerator for net income per share - diluted $ 21,678 $ 205,329 $ 71,933 Denominator for earnings per common share - basic and diluted: Weighted average number of common shares outstanding - basic 124,125 125,474 128,621 RSUs 912 1,294 816 LTIP Units 92 — — Weighted average number of common shares outstanding - diluted (1) 125,129 126,768 129,437 Net income per common share attributable to Equity Commonwealth common shareholders: Basic $ 0.17 $ 1.64 $ 0.56 Diluted $ 0.17 $ 1.62 $ 0.56 Anti-dilutive securities: Effect of Series D preferred shares; 6 1/2% cumulative convertible (2) 2,363 2,363 2,363 (1) As of December 31, 2017 , we had granted RSUs and LTIP Units to certain employees, officers, and trustees. The RSUs and LTIP Units contain service and market-based vesting components. If the market-based vesting component of these awards was measured as of December 31, 2017 , 2016 and 2015 , 673 , 1,027 and 1,143 common shares would be issued, respectively. (2) 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 during the periods presented.

Segment Information

Segment Information	12 Months Ended
	Dec. 31, 2017
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 95% of our revenues for the year ended December 31, 2017 are from office properties.

Related Person Transactions

Related Person Transactions	12 Months Ended
	Dec. 31, 2017
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, 2017 , 2016 and 2015 . Two North Riverside Plaza Joint Venture Limited Partnership and Equity Group Investments and associated entities: 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 completed 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 lease with Two North Riverside Plaza Joint Venture Limited Partnership for storage space in the basement of Two North Riverside Plaza. The 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, 2017 and 2016, we recognized expense of $0.8 million and $0.8 million , respectively, pursuant to the 20th/21st Floor Office Lease and the related storage space. The future minimum lease payments scheduled to be paid by us during the current terms of this lease as of December 31, 2017 are as follows: $0.9 million in 2018 , $0.9 million in 2019 and $0.9 million in 2020 . As of December 31, 2017 and 2016 , we had $0.0 million and $0.1 million , respectively, due to Two North Riverside Plaza Joint Venture Limited Partnership pursuant to the 20th/21st Floor Office Lease and the related storage space. We had a license agreement with Equity Group Investments, a private investment firm (Equity Group), to use office space on the sixth floor at Two North Riverside Plaza. The license fee included the non-exclusive use of additional areas on the sixth floor (such as conference rooms and common areas), certain administrative services (such as mail room services and reception desk staffing), office equipment, office furniture, supplies, licensee’s share of building operating expenses and real estate taxes and access to one parking space. Mr. Zell, our Chairman, is the Chairman and Chief Executive Officer of Equity Group, and Mr. Helfand, our President and Chief Executive Officer, is the Co-President of Equity Group. The license agreement was terminated as of December 30, 2015. During the year ended December 31, 2015, we recognized expense of $0.2 million pursuant to the license agreement. We had a one -year lease with one 3 -month renewal option with Two North Riverside Plaza Joint Venture Limited Partnership to occupy office space on the fourteenth floor at Two North Riverside Plaza. This lease was terminated, effective January 31, 2015. During the year ended December 31, 2015, we recognized expense of $13,000 pursuant to the office space on the fourteenth floor. We had a sublease with Equity Residential Management, L.L.C., an entity associated with Mr Zell, our Chairman, to occupy office space on the tenth floor of Two North Riverside Plaza. Equity Residential Management, L.L.C. leases the space from Two North Riverside Plaza Joint Venture Limited Partnership. The sublease agreement was terminated as of December 30, 2015. During the year ended December 31, 2015, we recognized expense of $0.4 million pursuant to the tenth floor office sublease. 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 approximately $33.5 million 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. Approximately $16.7 million was paid during the year ended December 31, 2014. Up to $8.4 million was to be reimbursed only if the average closing price of our common shares was at least $26.00 (as adjusted for any share splits or share dividends) during the one year period after the date on which the reimbursement was approved by shareholders, and up to $8.4 million was to be reimbursed only if the average closing price of our common shares was at least $26.00 (as adjusted for any share splits or share dividends) during the one year period between the first and second anniversaries of the date on which the reimbursement was approved by shareholders. The average closing price of our common shares was at least $26.00 during both the first and second one year periods after the date on which the reimbursement was approved by shareholders, and as a result, in August 2016 and 2015, we paid $8.2 million and $8.4 million , respectively, to Related/Corvex. Termination and Cooperation Agreement: Pursuant to a termination and cooperation agreement entered into with Reit Management & Research LLC (RMR) and RMR Australia Asset Management Pty Ltd (together, Former Manager), Former Manager agreed to use best efforts to assist us in the transition of our management and operations. We paid Former Manager $1.2 million per month for transition services from October 1, 2014 to February 28, 2015, which included continued management and other services for the Australian assets pursuant to an Australia management agreement. Beginning March 1, 2015, we agreed to pay Former Manager $0.1 million per month until we no longer required such services or until the Australia management agreement was terminated, which was terminated in the third quarter of 2015, effective October 31, 2015. There is no future obligation to pay any fees to Former Manager.

Selected Quarterly Financial Da

Selected Quarterly Financial Data	12 Months Ended
	Dec. 31, 2017
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 2017 and 2016 (dollars in thousands): 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 . 2016 First Quarter Second Quarter Third Quarter Fourth Quarter Total revenues $ 137,135 $ 145,367 $ 114,632 $ 103,546 Net income attributable to Equity Commonwealth common shareholders 39,421 71,254 84,391 10,263 Net income attributable to Equity Commonwealth common shareholders per share—basic 0.31 0.57 0.67 0.08 Net income attributable to Equity Commonwealth common shareholders per share—diluted 0.31 0.56 0.67 0.08 The increase in the second quarter 2016 total revenues was primarily attributable to a $16.6 million lease termination fee received at 111 Monument Circle. The decrease in the third and fourth quarter 2016 total revenues is primarily attributable to properties sold in 2016. The increase in the second and third quarter 2016 net income attributable to Equity Commonwealth common shareholders was primarily attributable to the gain on sale of properties of $106.4 million and $82.2 million , respectively.

Subsequent Events

Subsequent Events	12 Months Ended
	Dec. 31, 2017
Subsequent Events [Abstract]
Subsequent Events	Subsequent Events On January 12, 2018, 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, 2018 to shareholders of record on January 30, 2018. On January 22, 2018, we entered into a contract to sell our 1,561,477 square foot property at 600 West Chicago Avenue, for a sales price of $510.0 million , excluding closing credits and closing costs. This transaction is subject to customary closing extensions and conditions, and there is no certainty that this transaction will close. On January 30, 2018, we delivered notice of our intent to redeem at par our $175 million 5.75% senior unsecured notes due 2042 on March 7, 2018. The notes will be redeemed for cash at a price equal to 100% of the principal amount of the notes plus any accrued and unpaid interest up to, but excluding, the redemption date (see Note 8). On February 14, 2018, we sold 1600 Market Street, a 825,968 square foot property, for a sales price of $160.0 million , excluding closing credits and closing costs. This property was classified as held for sale as of December 31, 2017 (see Note 3).

SCHEDULE II VALUATION AND QUALI

SCHEDULE II VALUATION AND QUALIFYING ACCOUNTS	12 Months Ended
	Dec. 31, 2017
Valuation and Qualifying Accounts [Abstract]
Schedule II Valuation and Qualifying Accounts	Description Balance at Charged to Deductions Balance at Year Ended December 31, 2015: Allowance for doubtful accounts $ 6,565 $ 4,533 $ (3,383 ) $ 7,715 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

SCHEDULE III REAL ESTATE AND AC

SCHEDULE III REAL ESTATE AND ACCUMULATED DEPRECIATION	12 Months Ended
	Dec. 31, 2017
SEC Schedule III, 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 $ 28,273 $ (1,563 ) $ 22,400 $ 136,800 $ 159,200 $ 31,093 6/24/2009 1982 5073, 5075, & 5085 S. Syracuse Street Denver CO — 4,720 58,890 — — 4,720 58,890 63,610 11,287 4/16/2010 2007 1601 Dry Creek Drive Longmont CO — 3,714 24,397 7,498 (156 ) 3,714 31,739 35,453 10,751 10/26/2004 1982 97 Newberry Road East Windsor CT 5,404 2,960 12,360 30 — 2,943 12,407 15,350 3,485 10/24/2006 1989 1250 H Street, NW Washington DC — 5,975 53,778 14,906 (248 ) 5,975 68,436 74,411 30,990 6/23/1998 1992 Georgetown-Green and Harris Buildings Washington DC — 24,000 35,979 44 — 24,000 36,023 60,023 7,501 9/3/2009 1960;1975 600 West Chicago Avenue Chicago IL — 34,980 315,643 50,761 (322 ) 34,980 366,082 401,062 57,951 8/10/2011 1908 8750 Bryn Mawr Avenue Chicago IL — 6,600 77,764 14,234 (258 ) 6,600 91,740 98,340 17,342 10/28/2010 1985 109 Brookline Avenue Boston MA — 3,168 30,397 15,233 (1,397 ) 3,168 44,233 47,401 20,494 9/28/1995 1915 777 East Eisenhower Parkway Ann Arbor MI — 3,675 26,988 4,485 — 3,675 31,473 35,148 5,821 6/15/2010 1975;2006 1735 Market Street Philadelphia PA — 24,753 222,775 81,484 (14,927 ) 24,748 289,337 314,085 125,376 6/30/1998 1990 206 East 9th Street Austin TX 26,536 7,900 38,533 4,058 (12 ) 7,900 42,579 50,479 6,169 5/31/2012 1984 Bridgepoint Square Austin TX — 7,784 70,526 17,125 (770 ) 7,784 86,881 94,665 42,573 12/5/1997 1986;1996;1997 Research Park Austin TX — 15,859 60,175 17,439 — 21,213 72,260 93,473 34,941 10/7/1998; 10/11/2016 1968;1998; 2001 333 108th Avenue NE Bellevue WA — 14,400 136,412 2,781 (31 ) 14,400 139,162 153,562 29,467 11/12/2009 2008 600 108th Avenue NE Bellevue WA — 3,555 30,244 18,407 (857 ) 3,555 47,794 51,349 15,477 7/16/2004 1980 $ 31,940 $ 186,443 $ 1,304,951 $ 276,758 $ (20,541 ) $ 191,775 $ 1,555,836 $ 1,747,611 $ 450,718 Analysis of the carrying amount of real estate properties and accumulated depreciation: Real Estate Accumulated Balance at January 1, 2015 $ 5,728,443 $ 1,030,445 Additions(4) 68,118 144,844 Loss on asset impairment (17,162 ) — Disposals (1,892,047 ) (276,350 ) Balance at December 31, 2015 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 (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 $1,907,759 . (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. (4) Includes adjustments related to changes in foreign currency exchange rates for real estate properties additions of $0 , $0 and $(13,217) , and adjustments to accumulated depreciation additions of $0 , $0 and $(887) , during 2017 , 2016 and 2015 , respectively.

SCHEDULE IV MORTGAGE LOANS ON R

SCHEDULE IV MORTGAGE LOANS ON REAL ESTATE (Schedule)	12 Months Ended
	Dec. 31, 2017
Mortgage Loans on Real Estate [Abstract]
Schedule IV Mortgage Loans on Real Estate	Location Interest Rate Final Periodic Payment Terms Face Carrying Principal Amount of Dearborn, MI 6.00% 1/24/2023 Interest payable monthly in arrears. $7,688 due at maturity. $ 7,688 $ 7,688 $ — Reconciliation of the carrying amount of mortgage loans at the beginning of the period: Balance at December 31, 2015 $ 8,087 Collection of principal (7 ) Balance at December 31, 2016 $ 8,080 Collection of principal (320 ) Loss on repayment (72 ) Balance at December 31, 2017 $ 7,688 (1) Also represents cost for federal income tax purposes.

Summary of Significant Accoun33

Summary of Significant Accounting Policies (Policies)	12 Months Ended
	Dec. 31, 2017
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, 2017 , 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 (presented in our consolidated balance sheets as assumed real estate lease obligations) 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 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, cash flow or liquidity, and our decision to dispose of an asset before the end of its estimated useful life. 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, 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) recently quoted market prices, (ii) market prices for comparable properties, or (iii) the present value of future cash flows. During the years ended December 31, 2017 , 2016 , and 2015 we recorded a loss on asset impairment in continuing operations totaling $19.7 million , $58.5 million and $17.2 million respectively, to reduce the carrying value of properties to their estimated fair values (see Note 16). 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, and we have no current plans to remove any such asbestos. 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 at any of our properties that have a material adverse effect on us. However, no assurances can be given that such conditions are not present in our properties or that other costs we incur to remediate contamination will not have a material adverse effect on our business or financial condition. As of December 31, 2017 and 2016 , we did not have any accrued environmental remediation costs.
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 consist 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 these securities are recognized in cumulative other comprehensive income (loss). Realized gains and losses are recognized in earnings only upon the sale of the securities. 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 some of our mortgage debts, 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	Accounting Policy for 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, 2017 , we have an interest rate cap agreement that qualifies as a cash flow hedge. 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 are entered into to manage interest rate risk associated with our 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.
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 real estate investment trust. However, we are subject to certain state and local taxes without regard to our REIT status. The Income Taxes Topic of the 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.
Foreign Operations	Foreign Operations. During the year ended December 31, 2015, we sold our Australian properties and no longer have any foreign operations. The U.S. dollar was the functional currency for our consolidated subsidiaries operating in the United States prior to the sale of the Australian portfolio. The functional currency for our consolidated subsidiaries in countries other than the United States was the principal currency in which the entity's assets, liabilities, income and expenses were denominated. The functional currency of our consolidated subsidiary that operated in Australia was the Australian dollar. We translated our Australian subsidiary's financial statements into U.S. dollars when we consolidated that subsidiary's financial statements on a quarterly basis. Generally, we translated assets and liabilities at the exchange rate in effect as of the balance sheet date. The resulting translation adjustments were included in cumulative other comprehensive loss in our consolidated balance sheets. We translated income statement accounts using the average exchange rate for the period and income statement accounts that include significant non-recurring transactions at the rate in effect as of the date of the transaction. We were subject to foreign currency risk due to potential fluctuations in exchange rates between Australian and U.S. currencies.
New Accounting Pronouncements	New Accounting Pronouncements. In August 2017, the FASB issued Accounting Standards Update (ASU) 2017-12, Derivatives and Hedging (Topic 815): Targeted Improvements to Accounting for Hedging Activities, to enable entities to better portray the economics of their risk management activities in the financial statements and enhance the transparency and understandability of hedge results. The amendments also simplify the application of hedge accounting in certain situations. This update is effective for fiscal years beginning after December 15, 2018, and for interim periods within those fiscal years. We are evaluating the impact ASU 2017-12 will have on our financial position and results of operations. 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 do not expect that the adoption of this ASU will 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. ASU 2017-05 is effective beginning January 1, 2018. Early adoption is permitted but the standard is required to be adopted concurrently with ASU 2014-09. We do not expect that the adoption of this ASU will have a material impact on our consolidated financial statements. In February 2017, the FASB issued ASU 2017-01, Clarifying the Definition of a Business, that changes the definition of a business to assist entities with evaluating when a set of transferred assets and activities is a business. This update is effective for fiscal years beginning after December 15, 2017, and for interim periods within those fiscal years. We early adopted this ASU effective January 1, 2017, and the adoption of this ASU did not have a material impact on our consolidated financial statements. In November 2016, the FASB issued ASU 2016-18, Statement of Cash Flows (Topic 230): Restricted Cash, which amends ASC Topic 230, Statements of Cash Flows, to add or clarify guidance on the classification and presentation of restricted cash in the statement of cash flows. This update is effective for fiscal years beginning after December 15, 2017, and for interim periods within those fiscal years. Early adoption is permitted, including adoption in an interim period. In August 2016, the FASB issued ASU 2016-15, Statement of Cash Flows (Topic 230): Classification of Certain Cash Receipts and Cash Payments, which addresses eight specific cash flow issues with the objective of reducing the existing diversity in practice. This update is effective for fiscal years beginning after December 15, 2017, and for interim periods within those fiscal years. Early adoption is permitted, including adoption in an interim period. We early adopted both ASU 2016-18 and ASU 2016-15 on December 31, 2017 and made the following reclassifications to the prior years' consolidated statements of cash flows to conform to the current year's presentation (in thousands): Statement of Cash Flow for the year ended December 31, 2016 Originally Reported Effect of Change As Adjusted Cash provided by operating activities $ 163,016 $ 687 $ 163,703 Cash provided by investing activities 1,052,119 (26,400 ) 1,025,719 Statement of Cash Flow for the year ended December 31, 2015 Originally Reported Effect of Change As Adjusted Cash provided by operating activities $ 181,544 $ 6,570 $ 188,114 Cash provided by investing activities 1,648,893 203,709 1,852,602 Cash used in financing activities (382,722 ) (210,291 ) (593,013 ) 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 in the process of evaluating the impact, if any, the adoption of this ASU will have on our consolidated financial statements. In March 2016, the FASB issued ASU 2016-09, Compensation-Stock Compensation (Topic 718): Improvements to Employee Share-Based Payment Accounting, which simplifies several aspects of the accounting for employee share-based payment transactions, including the income tax consequences, classification of awards as either equity or liabilities, and classification on the statement of cash flows. This update is effective for annual periods beginning after December 15, 2016, and interim periods within those annual periods, but early adoption is permitted. We adopted this ASU on January 1, 2017 and the adoption did not have a material impact 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. We are still assessing the impact of adopting ASU 2016-02. 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 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. The new standard was effective for us beginning on January 1, 2018 and it did not have a material effect on our consolidated financial statements. 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 expect to use the modified retrospective method of adoption that will result in a cumulative effect adjustment as of the date of adoption. We developed a project plan, which consisted of gathering and evaluating the inventory of our revenue streams. As our portfolio is currently comprised, the adoption of this ASU will not have a material impact on our consolidated financial statements. In April 2014, the FASB issued ASU No. 2014-08, Presentation of Financial Statements (Topic 205) and Property, Plant, and Equipment (Topic 360): Reporting Discontinued Operations and Disclosures of Disposals of Components of an Entity, or ASU 2014-08. ASU 2014-08 changes the criteria for reporting a discontinued operation. Under this pronouncement, a disposal of a part of an organization that has a major effect on its operations and financial results is a discontinued operation. We adopted ASU 2014-08 on January 1, 2015, and determined that our 2017 , 2016 and 2015 dispositions do not individually represent a strategic shift, as defined by the standard, that has or will have a major effect on our operations and financial results. As a result, the 2017 , 2016 and 2015 dispositions have not been presented as discontinued operations in the statements of operations.
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.

Summary of Significant Accoun34

Summary of Significant Accounting Policies (Tables)	12 Months Ended
	Dec. 31, 2017
Accounting Policies [Abstract]
Summary of New Accounting Pronouncement, Early Adoption	We early adopted both ASU 2016-18 and ASU 2016-15 on December 31, 2017 and made the following reclassifications to the prior years' consolidated statements of cash flows to conform to the current year's presentation (in thousands): Statement of Cash Flow for the year ended December 31, 2016 Originally Reported Effect of Change As Adjusted Cash provided by operating activities $ 163,016 $ 687 $ 163,703 Cash provided by investing activities 1,052,119 (26,400 ) 1,025,719 Statement of Cash Flow for the year ended December 31, 2015 Originally Reported Effect of Change As Adjusted Cash provided by operating activities $ 181,544 $ 6,570 $ 188,114 Cash provided by investing activities 1,648,893 203,709 1,852,602 Cash used in financing activities (382,722 ) (210,291 ) (593,013 )

Real Estate Properties (Tables)

Real Estate Properties (Tables)	12 Months Ended
	Dec. 31, 2017
Real Estate [Abstract]
Disclosure of Properties Held-for-sale	As of December 31, 2017 , we classified the following property as held for sale (dollars in thousands): Asset Date Sold Number of Number of Square Footage Gross Sales Price 1600 Market Street February 2018 1 1 825,968 $ 160,000 Summarized balance sheet information for the property classified as held for sale 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, 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 disposed of the following properties (dollars in thousands): Asset Date Sold Number of Number of Square Footage Gross Sales Price Gain (Loss) on Sale Properties Executive Park February 2016 1 9 427,443 $ 50,865 $ 16,531 3330 N Washington Boulevard March 2016 1 1 55,719 11,250 5,455 111 East Kilbourn Avenue March 2016 1 1 373,669 60,500 14,687 1525 Locust Street April 2016 1 1 98,009 17,700 8,956 633 Ahua Street April 2016 1 1 93,141 29,000 15,963 Asset Date Sold Number of Number of Square Footage Gross Sales Price Gain (Loss) on Sale Lakewood on the Park May 2016 1 2 180,558 37,100 13,616 Leased Land June 2016 1 7 — 48,450 15,914 9110 East Nichols Avenue June 2016 1 1 143,958 17,200 642 111 River Street (1) July 2016 1 1 566,215 235,000 78,207 Sky Park Centre July 2016 1 2 63,485 13,700 4,746 Raintree Industrial Park July 2016 1 12 563,182 11,500 (653 ) 8701 N Mopac August 2016 1 1 121,901 21,500 8,394 7800 Shoal Creek Boulevard October 2016 1 4 151,917 29,210 14,908 1200 Lakeside Drive November 2016 1 1 260,084 65,270 3,062 6200 Glenn Carlson Drive December 2016 1 1 338,000 23,050 7,706 Portfolios of properties 812 San Antonio Street May 2016 1 1 59,321 1601 Rio Grande Street May 2016 1 1 56,219 Downtown Austin portfolio 2 2 115,540 $ 32,600 $ 20,584 785 Schilinger Road South June 2016 1 1 72,000 401 Vine Street June 2016 1 1 53,980 633 Frazier Drive June 2016 1 1 150,000 9840 Gateway Boulevard North June 2016 1 1 72,000 3003 South Expressway 281 June 2016 1 1 150,000 1331 North Center Parkway June 2016 1 1 53,980 Movie theater portfolio 6 6 551,960 $ 109,100 $ 30,595 128 Crews Drive July 2016 1 1 185,600 111 Southchase Boulevard July 2016 1 1 168,087 1043 Global Avenue July 2016 1 1 450,000 South Carolina industrial portfolio 3 3 803,687 $ 30,000 $ 7,248 100 East Wisconsin Avenue August 2016 1 1 435,067 111 Monument Circle August 2016 1 2 1,121,764 101-115 W. Washington Street August 2016 1 1 634,058 North Point Office Complex August 2016 1 2 873,335 Midwest portfolio 4 6 3,064,224 $ 416,900 $ (15,800 ) Total 30 62 7,972,692 $ 1,259,895 $ 250,761 (1) Property sale represented a leasehold interest.
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, 2017 are as follows (in thousands): 2018 $ 187,984 2019 177,325 2020 158,000 2021 141,937 2022 131,362 Thereafter 693,119 $ 1,489,727

Lease Intangibles (Tables)

Lease Intangibles (Tables)	12 Months Ended
	Dec. 31, 2017
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, 2017 , and 2016 , and (in thousands): December 31, 2017 2016 Acquired in-place leases $ 56,853 $ 102,102 Acquired above market leases 21,021 44,752 Acquired real estate leases 77,874 146,854 Accumulated amortization, acquired in-place leases (41,978 ) (71,320 ) Accumulated amortization, acquired above market leases (12,049 ) (27,253 ) Acquired real estate leases, net $ 23,847 $ 48,281 Acquired below market leases $ 6,093 $ 10,434 Accumulated amortization (5,092 ) (8,488 ) Assumed real estate lease obligations, net $ 1,001 $ 1,946
Schedule of Amortization of Lease Intangibles	Amortization of the lease intangibles for the years ended December 31, 2017 , 2016 , and 2015 , is as follows (in thousands): December 31, Income Statement Location 2017 2016 2015 Amortization of acquired in-place leases Depreciation and amortization $ 7,220 $ 14,598 $ 26,787 Amortization of above and below market leases Increase (decrease) to rental income (1,774 ) (6,531 ) (7,515 )
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, 2017 , are approximately (in thousands): 2018 $ 7,285 2019 3,702 2020 2,449 2021 2,236 2022 1,959 Thereafter 5,215 $ 22,846

Marketable Securities (Tables)

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

Other Assets (Tables)

Other Assets (Tables)	12 Months Ended
	Dec. 31, 2017
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, 2017 , and 2016 (in thousands): December 31, 2017 2016 Deferred financing fees $ 14,458 $ 14,458 Accumulated amortization (12,709 ) (11,093 ) Deferred financing fees, net $ 1,749 $ 3,365 Deferred leasing costs $ 85,606 $ 133,771 Accumulated amortization (22,067 ) (41,148 ) Deferred leasing costs, net $ 63,539 $ 92,623 Capitalized lease incentives $ 8,562 $ 10,344 Accumulated amortization (2,054 ) (2,680 ) Capitalized lease incentives, net $ 6,508 $ 7,664
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, 2017 are approximately (in thousands): Deferred Leasing Costs Capitalized Lease Incentives 2018 $ 7,707 $ 983 2019 7,694 798 2020 7,383 758 2021 6,595 592 2022 6,046 547 Thereafter 28,114 2,830 $ 63,539 $ 6,508

Indebtedness (Tables)

Indebtedness (Tables)	12 Months Ended
	Dec. 31, 2017
Debt Disclosure [Abstract]
Schedule of Outstanding Indebtedness	At December 31, 2017 and 2016 , our outstanding indebtedness included the following (in thousands): December 31, Interest Rate at December 31, 2017 Maturity Date 2017 2016 Unsecured revolving credit facility, at LIBOR plus a premium 2.61 % 1/28/2019 $ — $ — 5-year unsecured term loan, at LIBOR plus a premium 2.71 % 1/28/2020 200,000 200,000 7-year unsecured term loan, at LIBOR plus a premium 3.11 % 1/28/2022 200,000 200,000 Unsecured floating rate debt 2.91 % (1) $ 400,000 $ 400,000 6.65% Senior Unsecured Notes due 2018 — % — $ — $ 250,000 5.875% Senior Unsecured Notes due 2020 5.875 % 9/15/2020 250,000 250,000 5.75% Senior Unsecured Notes due 2042 (2) 5.75 % 8/1/2042 175,000 175,000 Unsecured fixed rate debt 5.82 % (1) $ 425,000 $ 675,000 Parkshore Plaza — % — $ — $ 41,275 206 East 9th Street 5.69 % 1/5/2021 26,536 27,041 33 Stiles Lane — % — — 2,415 97 Newberry Road 5.71 % 3/1/2026 5,404 5,903 Secured fixed rate debt 5.69 % (1) $ 31,940 $ 76,634 $ 856,940 $ 1,151,634 Unamortized net premiums, discounts and deferred financing fees (8,362 ) (9,967 ) $ 848,578 $ 1,141,667 (1) Represents weighted average interest rate at December 31, 2017 . (2) On January 30, 2018, we delivered notice of our intent to redeem at par our $175 million 5.75% senior unsecured notes due 2042 on March 7, 2018. The notes will be redeemed for cash at a price equal to 100% of the principal amount of the notes plus any accrued and unpaid interest up to, but excluding, the redemption date.
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, 2017 are as follows (in thousands): 2018 $ 1,063 2019 1,126 2020 451,189 2021 25,463 2022 200,663 Thereafter (1) 177,436 $ 856,940 (1) On January 30, 2018, we delivered notice of our intent to redeem at par our $175 million 5.75% senior unsecured notes due 2042 on March 7, 2018. The notes will be redeemed for cash at a price equal to 100% of the principal amount of the notes plus any accrued and unpaid interest up to, but excluding, the redemption date.

Shareholders' Equity (Tables)

Shareholders' Equity (Tables)	12 Months Ended
	Dec. 31, 2017
Stockholders' Equity Note [Abstract]
Schedule of Declared Distributions	In 2017 , 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, 2017 January 30, 2017 February 15, 2017 $ 0.40625 April 10, 2017 April 28, 2017 May 15, 2017 $ 0.40625 June 30, 2017 July 28, 2017 August 15, 2017 $ 0.40625 October 12, 2017 October 30, 2017 November 15, 2017 $ 0.40625

Noncontrolling Interest (Tables

Noncontrolling Interest (Tables)	12 Months Ended
	Dec. 31, 2017
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, 2017 : Common Shares LTIP Units Total Outstanding at January 1, 2017 123,994,465 — 123,994,465 Restricted share, time-based LTIP Unit grants and vested restricted stock units, net of forfeitures 223,151 42,520 265,671 Outstanding at December 31, 2017 124,217,616 42,520 124,260,136 Noncontrolling ownership interest in the Operating Trust 0.03 %
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, 2017 : Common Shares LTIP Units Total Outstanding at January 1, 2017 123,994,465 — 123,994,465 Restricted share, time-based LTIP Unit grants and vested restricted stock units, net of forfeitures 223,151 42,520 265,671 Outstanding at December 31, 2017 124,217,616 42,520 124,260,136 Noncontrolling ownership interest in the Operating Trust 0.03 % The table below presents a summary of restricted share, RSU and LTIP Unit activity for the years ended December 31, 2017 , 2016 and 2015 : Number Weighted Number Weighted Outstanding at December 31, 2014 710,182 $ 26.66 1,266,237 $ 15.99 Granted 160,906 26.46 639,250 15.47 Vested (86,517 ) 26.66 — — Forfeited (8,115 ) 26.64 (21,377 ) 15.80 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 (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 Comprehensiv42

Cumulative Other Comprehensive Loss (Tables)	12 Months Ended
	Dec. 31, 2017
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, 2017 and 2016 (in thousands): Unrealized Loss on Derivative Instruments Unrealized Gain, Net on Marketable Securities Total Balances 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 Balances as of December 31, 2017 $ (456 ) $ 361 $ (95 ) Unrealized Loss Balance as of January 1, 2016 $ (3,687 ) Other comprehensive loss before reclassifications (554 ) Amounts reclassified from cumulative other comprehensive loss to net income 4,033 Net current period other comprehensive income 3,479 Balance as of December 31, 2016 $ (208 )
Schedule of Reclassifications Out of Cumulative Other Comprehensive Loss	The following tables present reclassifications out of cumulative other comprehensive loss for the years ended December 31, 2017 and 2016 (in thousands): Amounts Reclassified from Cumulative Other Comprehensive Loss to Net Income Year Ended December 31, Details about Cumulative Other Comprehensive Loss Components 2017 2016 Affected Line Items in the Statement of Operations Interest rate swap contracts $ — $ 3,792 Interest expense Interest rate swap contracts — 241 Interest and other income Interest rate cap contract 49 — Interest expense $ 49 $ 4,033

Income Taxes (Tables)

Income Taxes (Tables)	12 Months Ended
	Dec. 31, 2017
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, 2017 2016 2015 Current: State $ 500 $ 745 $ 270 Federal — — 513 Foreign — — 2,336 500 745 3,119 Deferred: Foreign — — (755 ) — — (755 ) Income tax expense $ 500 $ 745 $ 2,364
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, 2017 2016 2015 Taxes at statutory U.S. federal income tax rate 35.00 % 35.00 % 35.00 % Dividends paid deduction and net operating loss utilization (35.00 )% (35.00 )% (35.00 )% Federal taxes on built-in gain — % — % 0.50 % State, local, and foreign income taxes 1.66 % 0.32 % 1.81 % Effective tax rate 1.66 % 0.32 % 2.31 %

Derivative Instruments (Tables)

Derivative Instruments (Tables)	12 Months Ended
	Dec. 31, 2017
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Schedule of Outstanding Interest Rate Derivatives Designated as Cash Flow Hedges of Interest Rate Risk	As of December 31, 2017 , we had the following outstanding interest rate derivative that was designated as a cash flow hedge of interest rate risk: Interest Rate Derivative Number of Instruments Notional Amount (in thousands) Interest rate cap 1 $400,000
Schedule of Fair Value of Derivative Financial Instruments	The table below presents the fair value of our derivative financial instrument as well as its classification on the consolidated balance sheets as of December 31, 2017 , and 2016 (amounts in thousands): Fair Value as of December 31, Interest Rate Derivative Designated as Hedging Instrument Balance Sheet Location 2017 2016 Interest rate cap Other assets $ 17 $ 314
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, 2017 , 2016 , and 2015 (amounts in thousands): Year Ended December 31, 2017 2016 2015 Amount of loss recognized in cumulative other comprehensive loss (effective portion) $ (297 ) $ (554 ) $ (1,149 ) Amount of loss reclassified from cumulative other comprehensive loss into interest expense (effective portion) 49 3,792 4,924 Amount of loss recognized in income (ineffective portion and amount excluded from effectiveness testing) — 241 —

Share-Based Compensation (Table

Share-Based Compensation (Tables)	12 Months Ended
	Dec. 31, 2017
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, 2017 : Common Shares LTIP Units Total Outstanding at January 1, 2017 123,994,465 — 123,994,465 Restricted share, time-based LTIP Unit grants and vested restricted stock units, net of forfeitures 223,151 42,520 265,671 Outstanding at December 31, 2017 124,217,616 42,520 124,260,136 Noncontrolling ownership interest in the Operating Trust 0.03 % The table below presents a summary of restricted share, RSU and LTIP Unit activity for the years ended December 31, 2017 , 2016 and 2015 : Number Weighted Number Weighted Outstanding at December 31, 2014 710,182 $ 26.66 1,266,237 $ 15.99 Granted 160,906 26.46 639,250 15.47 Vested (86,517 ) 26.66 — — Forfeited (8,115 ) 26.64 (21,377 ) 15.80 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 (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, 2017 , 2016 and 2015 are included in the following table on a per share and unit basis. 2017 2016 2015 Fair value of RSUs and market-based LTIP Units granted at the target amount $ 39.81 $ 38.80 $ 38.55 Fair value of RSUs granted at the maximum amount $ 15.97 $ 15.57 $ 15.47 Expected term (years) 4 4 4 Expected volatility — — — Expected dividend yield 1.59 % 1.86 % 1.88 % Risk-free rate 1.49 % 1.07 % 0.81 %

Fair Value of Assets and Liab46

Fair Value of Assets and Liabilities (Tables)	12 Months Ended
	Dec. 31, 2017
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 2017 and 2016 , categorized by the level of inputs used in the valuation of each asset and liability (dollars in thousands): 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 $ — $ — Fair Value at December 31, 2016 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 $ 314 $ — $ 314 $ —
Schedule of Fair Value and Carrying Value of Financial Instruments	At December 31, 2017 and 2016 , the fair value of these additional financial instruments were not materially different from their carrying values, except as follows (in thousands): December 31, 2017 December 31, 2016 Principal Balance Fair Value Principal Balance Fair Value Senior unsecured debt and mortgage notes payable $ 856,940 $ 874,280 $ 1,151,634 $ 1,167,031

Earnings Per Common Share (Tabl

Earnings Per Common Share (Tables)	12 Months Ended
	Dec. 31, 2017
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, 2017 2016 2015 Numerator for earnings per common share - basic: Net income $ 29,666 $ 232,894 $ 99,857 Net income attributable to noncontrolling interest (10 ) — — Preferred distributions (7,988 ) (17,956 ) (27,924 ) Excess fair value of consideration paid over carrying value of preferred shares — (9,609 ) — Numerator for net income per share - basic $ 21,668 $ 205,329 $ 71,933 Numerator for earnings per common share - diluted: Net income $ 29,666 $ 232,894 $ 99,857 Preferred distributions (7,988 ) (17,956 ) (27,924 ) Excess fair value of consideration paid over carrying value of preferred shares — (9,609 ) — Numerator for net income per share - diluted $ 21,678 $ 205,329 $ 71,933 Denominator for earnings per common share - basic and diluted: Weighted average number of common shares outstanding - basic 124,125 125,474 128,621 RSUs 912 1,294 816 LTIP Units 92 — — Weighted average number of common shares outstanding - diluted (1) 125,129 126,768 129,437 Net income per common share attributable to Equity Commonwealth common shareholders: Basic $ 0.17 $ 1.64 $ 0.56 Diluted $ 0.17 $ 1.62 $ 0.56 Anti-dilutive securities: Effect of Series D preferred shares; 6 1/2% cumulative convertible (2) 2,363 2,363 2,363 (1) As of December 31, 2017 , we had granted RSUs and LTIP Units to certain employees, officers, and trustees. The RSUs and LTIP Units contain service and market-based vesting components. If the market-based vesting component of these awards was measured as of December 31, 2017 , 2016 and 2015 , 673 , 1,027 and 1,143 common shares would be issued, respectively. (2) 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 during the periods presented.

Selected Quarterly Financial 48

Selected Quarterly Financial Data (Tables)	12 Months Ended
	Dec. 31, 2017
Quarterly Financial Information Disclosure [Abstract]
Summary of Unaudited Quarterly Results of Operations	2016 First Quarter Second Quarter Third Quarter Fourth Quarter Total revenues $ 137,135 $ 145,367 $ 114,632 $ 103,546 Net income attributable to Equity Commonwealth common shareholders 39,421 71,254 84,391 10,263 Net income attributable to Equity Commonwealth common shareholders per share—basic 0.31 0.57 0.67 0.08 Net income attributable to Equity Commonwealth common shareholders per share—diluted 0.31 0.56 0.67 0.08 The following is a summary of our unaudited quarterly results of operations for 2017 and 2016 (dollars in thousands): 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, 2017USD ($)ft²buildingproperty
Segment Reporting Information [Line Items]
Cash, cash equivalents, and marketable securities | $	$ 2.6
Consolidated portfolio
Segment Reporting Information [Line Items]
Number of properties | property	16
Number of buildings | building	26
Property square feet (in sqft) | ft²	8.7
EQC Operating Trust
Segment Reporting Information [Line Items]
Noncontrolling interest, ownership percentage by parent	99.97%

Summary of Significant Accoun50

Summary of Significant Accounting Policies - Narrative (Details) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Property, Plant and Equipment [Line Items]
Percentage of investments in subsidiaries			100.00%
Asset impairment charges	$ 18,400	$ 1,300	$ 19,714	$ 58,476	$ 17,162
Minimum percentage of likelihood of realization of tax benefits (greater than)			50.00%
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

Summary of Significant Accoun51

Summary of Significant Accounting Policies - Early Adoption (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
New Accounting Pronouncement, Early Adoption [Line Items]
Cash provided by operating activities	$ 99,996	$ 163,703	$ 188,114
Cash provided by investing activities	465,586	1,025,719	1,852,602
Cash used in financing activities	$ (306,198)	(923,182)	(593,013)
ASU 2016-18 and ASU 2016-15
New Accounting Pronouncement, Early Adoption [Line Items]
Cash provided by operating activities		163,703	188,114
Cash provided by investing activities		1,025,719	1,852,602
Cash used in financing activities			(593,013)
ASU 2016-18 and ASU 2016-15 | Originally Reported
New Accounting Pronouncement, Early Adoption [Line Items]
Cash provided by operating activities		163,016	181,544
Cash provided by investing activities		1,052,119	1,648,893
Cash used in financing activities			(382,722)
ASU 2016-18 and ASU 2016-15 | Effect of Change
New Accounting Pronouncement, Early Adoption [Line Items]
Cash provided by operating activities		687	6,570
Cash provided by investing activities		$ (26,400)	203,709
Cash used in financing activities			$ (210,291)

Real Estate Properties - Narrat

Real Estate Properties - Narrative (Details) $ in Thousands	1 Months Ended		12 Months Ended
	Oct. 31, 2016USD ($)	Jan. 31, 2013buildingproperty	Dec. 31, 2017USD ($)ft²building	Dec. 31, 2016USD ($)buildingproperty	Dec. 31, 2015USD ($)ft²buildingproperty
Real Estate Properties [Line Items]
Real estate property improvements			$ 55,000	$ 110,700	$ 75,700
Committed expenditures on leases executed during period			$ 44,900
Square feet of leases committed for expenditures during the period (in sqft) | ft²			1,300,000
Committed but unspent tenant related obligations based on existing leases			$ 63,200
Number of real estate properties held for sale | property				0
Number of properties sold | property		3
Number of buildings sold | building		18
Ground lease expenses				$ 900	$ 1,800
Disposed of by Sale
Real Estate Properties [Line Items]
Number of properties sold | property					91
Number of buildings sold | building			37	62	135
Property square feet (in sqft) | ft²					18,939,185
Gross sales price			$ 862,624	$ 1,259,895	$ 2,000,000
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	1 Months Ended
	Feb. 28, 2018USD ($)ft²buildingproperty	Jan. 31, 2013building	Dec. 31, 2017USD ($)	Dec. 31, 2016USD ($)
Long Lived Assets Held-for-sale [Line Items]
Number of Buildings | building		18
Liabilities related to properties held for sale			$ 1,840	$ 0
Disposal Group, Held-for-sale, Not Discontinued Operations
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
Rents receivable, allowance			$ 5
Subsequent event | 1600 Market Street | Disposal Group, Held-for-sale, Not Discontinued Operations
Long Lived Assets Held-for-sale [Line Items]
Number of Properties | property	1
Number of Buildings | building	1
Square Footage (in sqft) | ft²	825,968
Gross Sales Price	$ 160,000

Real Estate Properties - Summar

Real Estate Properties - Summary of Properties Disposed (Details) $ in Thousands

1 Months Ended

12 Months Ended

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, 2016USD ($)ft²buildingproperty

Nov. 30, 2016USD ($)ft²buildingproperty

Oct. 31, 2016USD ($)ft²buildingproperty

Aug. 31, 2016USD ($)ft²buildingproperty

Jul. 31, 2016USD ($)ft²buildingproperty

Jun. 30, 2016USD ($)ft²buildingproperty

May 31, 2016USD ($)ft²buildingproperty

Apr. 30, 2016USD ($)ft²buildingproperty

Mar. 31, 2016USD ($)ft²buildingproperty

Feb. 29, 2016USD ($)ft²buildingproperty

Jan. 31, 2013building

Dec. 31, 2017USD ($)ft²buildingproperty

Dec. 31, 2016USD ($)ft²buildingproperty

Dec. 31, 2015USD ($)building

Real Estate Properties [Line Items]

Number of Buildings | building

18

Gain (Loss) on Sale

$ 15,498

$ 250,886

$ 84,421

Disposed of by Sale

Real Estate Properties [Line Items]

Number of Properties | property

16

30

Number of Buildings | building

37

62

135

Square Footage (in sqft) | ft²

6,588,128

7,972,692

Gross Sales Price

$ 862,624

$ 1,259,895

$ 2,000,000

Gain (Loss) on Sale

$ 15,493

$ 250,761

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

Disposed of by Sale | Executive Park

Real Estate Properties [Line Items]

Number of Properties | property

1

Number of Buildings | building

9

Square Footage (in sqft) | ft²

427,443

Gross Sales Price

$ 50,865

Gain (Loss) on Sale

$ 16,531

Disposed of by Sale | 3330 N Washington Boulevard

Real Estate Properties [Line Items]

Number of Properties | property

1

Number of Buildings | building

1

Square Footage (in sqft) | ft²

55,719

Gross Sales Price

$ 11,250

Gain (Loss) on Sale

$ 5,455

Disposed of by Sale | 111 East Kilbourn Avenue

Real Estate Properties [Line Items]

Number of Properties | property

1

Number of Buildings | building

1

Square Footage (in sqft) | ft²

373,669

Gross Sales Price

$ 60,500

Gain (Loss) on Sale

$ 14,687

Disposed of by Sale | 1525 Locust Street

Real Estate Properties [Line Items]

Number of Properties | property

1

Number of Buildings | building

1

Square Footage (in sqft) | ft²

98,009

Gross Sales Price

$ 17,700

Gain (Loss) on Sale

$ 8,956

Disposed of by Sale | 633 Ahua Street

Real Estate Properties [Line Items]

Number of Properties | property

1

Number of Buildings | building

1

Square Footage (in sqft) | ft²

93,141

Gross Sales Price

$ 29,000

Gain (Loss) on Sale

$ 15,963

Disposed of by Sale | Lakewood on the Park

Real Estate Properties [Line Items]

Number of Properties | property

1

Number of Buildings | building

2

Square Footage (in sqft) | ft²

180,558

Gross Sales Price

$ 37,100

Gain (Loss) on Sale

$ 13,616

Disposed of by Sale | Leased Land

Real Estate Properties [Line Items]

Number of Properties | property

1

Number of Buildings | building

7

Square Footage (in sqft) | ft²

0

Gross Sales Price

$ 48,450

Gain (Loss) on Sale

$ 15,914

Disposed of by Sale | 9110 East Nichols Avenue

Real Estate Properties [Line Items]

Number of Properties | property

1

Number of Buildings | building

1

Square Footage (in sqft) | ft²

143,958

Gross Sales Price

$ 17,200

Gain (Loss) on Sale

$ 642

Disposed of by Sale | 111 River Street

Real Estate Properties [Line Items]

Number of Properties | property

1

Number of Buildings | building

1

Square Footage (in sqft) | ft²

566,215

Gross Sales Price

$ 235,000

Gain (Loss) on Sale

$ 78,207

Disposed of by Sale | Sky Park Centre

Real Estate Properties [Line Items]

Number of Properties | property

1

Number of Buildings | building

2

Square Footage (in sqft) | ft²

63,485

Gross Sales Price

$ 13,700

Gain (Loss) on Sale

$ 4,746

Disposed of by Sale | Raintree Industrial Park

Real Estate Properties [Line Items]

Number of Properties | property

1

Number of Buildings | building

12

Square Footage (in sqft) | ft²

563,182

Gross Sales Price

$ 11,500

Gain (Loss) on Sale

$ (653)

Disposed of by Sale | 8701 N Mopac

Real Estate Properties [Line Items]

Number of Properties | property

1

Number of Buildings | building

1

Square Footage (in sqft) | ft²

121,901

Gross Sales Price

$ 21,500

Gain (Loss) on Sale

$ 8,394

Disposed of by Sale | 7800 Shoal Creek Boulevard

Real Estate Properties [Line Items]

Number of Properties | property

1

Number of Buildings | building

4

Square Footage (in sqft) | ft²

151,917

Gross Sales Price

$ 29,210

Gain (Loss) on Sale

$ 14,908

Disposed of by Sale | 1200 Lakeside Drive

Real Estate Properties [Line Items]

Number of Properties | property

1

Number of Buildings | building

1

Square Footage (in sqft) | ft²

260,084

Gross Sales Price

$ 65,270

Gain (Loss) on Sale

$ 3,062

Disposed of by Sale | 6200 Glenn Carlson Drive

Real Estate Properties [Line Items]

Number of Properties | property

1

Number of Buildings | building

1

Square Footage (in sqft) | ft²

338,000

Gross Sales Price

$ 23,050

Gain (Loss) on Sale

$ 7,706

Disposed of by Sale | 812 San Antonio Street

Real Estate Properties [Line Items]

Number of Properties | property

1

Number of Buildings | building

1

Square Footage (in sqft) | ft²

59,321

Disposed of by Sale | 1601 Rio Grande Street

Real Estate Properties [Line Items]

Number of Properties | property

1

Number of Buildings | building

1

Square Footage (in sqft) | ft²

56,219

Disposed of by Sale | Downtown Austin portfolio

Real Estate Properties [Line Items]

Number of Properties | property

2

Number of Buildings | building

2

Square Footage (in sqft) | ft²

115,540

Gross Sales Price

$ 32,600

Gain (Loss) on Sale

$ 20,584

Disposed of by Sale | 785 Schilinger Road South

Real Estate Properties [Line Items]

Number of Properties | property

1

Number of Buildings | building

1

Square Footage (in sqft) | ft²

72,000

Disposed of by Sale | 401 Vine Street

Real Estate Properties [Line Items]

Number of Properties | property

1

Number of Buildings | building

1

Square Footage (in sqft) | ft²

53,980

Disposed of by Sale | 633 Frazier Drive

Real Estate Properties [Line Items]

Number of Properties | property

1

Number of Buildings | building

1

Square Footage (in sqft) | ft²

150,000

Disposed of by Sale | 9840 Gateway Boulevard North

Real Estate Properties [Line Items]

Number of Properties | property

1

Number of Buildings | building

1

Square Footage (in sqft) | ft²

72,000

Disposed of by Sale | 3003 South Expressway 281

Real Estate Properties [Line Items]

Number of Properties | property

1

Number of Buildings | building

1

Square Footage (in sqft) | ft²

150,000

Disposed of by Sale | 1331 North Center Parkway

Real Estate Properties [Line Items]

Number of Properties | property

1

Number of Buildings | building

1

Square Footage (in sqft) | ft²

53,980

Disposed of by Sale | Movie theater portfolio

Real Estate Properties [Line Items]

Number of Properties | property

6

Number of Buildings | building

6

Square Footage (in sqft) | ft²

551,960

Gross Sales Price

$ 109,100

Gain (Loss) on Sale

$ 30,595

Disposed of by Sale | 128 Crews Drive

Real Estate Properties [Line Items]

Number of Properties | property

1

Number of Buildings | building

1

Square Footage (in sqft) | ft²

185,600

Disposed of by Sale | 111 Southchase Boulevard

Real Estate Properties [Line Items]

Number of Properties | property

1

Number of Buildings | building

1

Square Footage (in sqft) | ft²

168,087

Disposed of by Sale | 1043 Global Avenue

Real Estate Properties [Line Items]

Number of Properties | property

1

Number of Buildings | building

1

Square Footage (in sqft) | ft²

450,000

Disposed of by Sale | South Carolina industrial portfolio

Real Estate Properties [Line Items]

Number of Properties | property

3

Number of Buildings | building

3

Square Footage (in sqft) | ft²

803,687

Gross Sales Price

$ 30,000

Gain (Loss) on Sale

$ 7,248

Disposed of by Sale | 100 East Wisconsin Avenue

Real Estate Properties [Line Items]

Number of Properties | property

1

Number of Buildings | building

1

Square Footage (in sqft) | ft²

435,067

Disposed of by Sale | 111 Monument Circle

Real Estate Properties [Line Items]

Number of Properties | property

1

Number of Buildings | building

2

Square Footage (in sqft) | ft²

1,121,764

Disposed of by Sale | 101-115 W. Washington Street

Real Estate Properties [Line Items]

Number of Properties | property

1

Number of Buildings | building

1

Square Footage (in sqft) | ft²

634,058

Disposed of by Sale | North Point Office Complex

Real Estate Properties [Line Items]

Number of Properties | property

1

Number of Buildings | building

2

Square Footage (in sqft) | ft²

873,335

Disposed of by Sale | Midwest portfolio

Real Estate Properties [Line Items]

Number of Properties | property

4

Number of Buildings | building

6

Square Footage (in sqft) | ft²

3,064,224

Gross Sales Price

$ 416,900

Gain (Loss) on Sale

$ (15,800)

Real Estate Properties - Schedu

Real Estate Properties - Schedule of Future Minimum Lease Payment Receivables and Payments (Details) $ in Thousands	Dec. 31, 2017USD ($)
Future minimum lease payments receivable:
2,018	$ 187,984
2,019	177,325
2,020	158,000
2,021	141,937
2,022	131,362
Thereafter	693,119
Total	$ 1,489,727

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, 2017	Dec. 31, 2016
Finite-Lived Intangible Assets [Line Items]
Acquired real estate leases	$ 77,874	$ 146,854
Acquired real estate leases, net	23,847	48,281
Leases, Acquired-in-Place
Finite-Lived Intangible Assets [Line Items]
Acquired in-place leases	56,853	102,102
Accumulated amortization	(41,978)	(71,320)
Leases Acquired-in-Place, Above Market Lease
Finite-Lived Intangible Assets [Line Items]
Acquired above market leases	21,021	44,752
Accumulated amortization	(12,049)	(27,253)
Leases Acquired-in-Place, Below Market Lease
Finite-Lived Intangible Assets [Line Items]
Acquired below market leases	6,093	10,434
Accumulated amortization	(5,092)	(8,488)
Assumed real estate lease obligations, net	$ 1,001	$ 1,946

Lease Intangibles - Schedule of

Lease Intangibles - Schedule of Amortization of Lease Intangibles (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Leases [Abstract]
Amortization of acquired in-place leases, Depreciation and amortization	$ 7,220	$ 14,598	$ 26,787
Amortization of above and below market leases, Increase (decrease) to rental income	$ (1,774)	$ (6,531)	$ (7,515)

Lease Intangibles - Schedule 58

Lease Intangibles - Schedule of Future Amortization of Net Intangible Lease Assets and Liabilities (Details) $ in Thousands	Dec. 31, 2017USD ($)
Leases [Abstract]
2,018	$ 7,285
2,019	3,702
2,020	2,449
2,021	2,236
2,022	1,959
Thereafter	5,215
Acquired real estate leases, net	$ 22,846

Investment in Direct Financin59

Investment in Direct Financing Lease - Narrative (Details)	Dec. 31, 2017
Capital Leases, Net Investment in Direct Financing Leases [Abstract]
Percentage of term exceeding useful life of one office tower	75.00%

Marketable Securities (Details)

Marketable Securities (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Marketable securities	$ 276,928	$ 0
Marketable securities
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	276,567
Unrealized Gain, Net	361
Marketable securities	$ 276,928

Other Assets - Narrative (Detai

Other Assets - Narrative (Details)	1 Months Ended			12 Months Ended
	Sep. 30, 2017USD ($)	Jan. 31, 2013USD ($)buildingproperty	Apr. 30, 2012USD ($)	Dec. 31, 2017USD ($)building	Dec. 31, 2016USD ($)building	Dec. 31, 2015USD ($)buildingproperty
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total real estate mortgage receivable included in other assets, carrying value				$ 7,700,000	$ 8,100,000
Number of properties sold or agreed to be sold | property		3
Number of buildings sold | building		18
Principal payments received from real estate mortgages receivable				313,000	0	$ 0
Real estate mortgages receivable
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for losses related to real estate mortgages receivable				$ 0	$ 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						91
Number of buildings sold | building				37	62	135
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 notes 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%

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, 2017	Dec. 31, 2016
Other Assets [Abstract]
Deferred financing fees	$ 14,458	$ 14,458
Accumulated amortization	(12,709)	(11,093)
Deferred financing fees, net	1,749	3,365
Deferred leasing costs	85,606	133,771
Accumulated amortization	(22,067)	(41,148)
Deferred leasing costs, net	63,539	92,623
Capitalized lease incentives	8,562	10,344
Accumulated amortization	(2,054)	(2,680)
Capitalized lease incentives, net	$ 6,508	$ 7,664

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, 2017	Dec. 31, 2016
Deferred Leasing Costs
2,018	$ 7,707
2,019	7,694
2,020	7,383
2,021	6,595
2,022	6,046
Thereafter	28,114
Deferred leasing costs, net	63,539	$ 92,623
Capitalized Lease Incentives
2,018	983
2,019	798
2,020	758
2,021	592
2,022	547
Thereafter	2,830
Capitalized lease incentives, net	$ 6,508	$ 7,664

Indebtedness - Schedule of Outs

Indebtedness - Schedule of Outstanding Indebtedness (Details) - USD ($)	Jan. 30, 2018	Jul. 15, 2017	Dec. 31, 2017	Dec. 31, 2016
Debt Instrument [Line Items]
Outstanding indebtedness			$ 856,940,000	$ 1,151,634,000
Unamortized net premiums, discounts and deferred financing fees			(8,362,000)	(9,967,000)
Total			$ 848,578,000	1,141,667,000
Unsecured revolving credit facility, at LIBOR plus a premium
Debt Instrument [Line Items]
Interest rate stated percentage			2.61%
Outstanding indebtedness			$ 0	0
5-year unsecured term loan, at LIBOR plus a premium
Debt Instrument [Line Items]
Interest rate stated percentage			2.71%
Outstanding indebtedness			$ 200,000,000	200,000,000
Total			$ 200,000,000
7-year unsecured term loan, at LIBOR plus a premium
Debt Instrument [Line Items]
Interest rate stated percentage			3.11%
Outstanding indebtedness			$ 200,000,000	200,000,000
Unsecured floating rate debt
Debt Instrument [Line Items]
Interest rate stated percentage			2.91%
Outstanding indebtedness			$ 400,000,000	400,000,000
6.65% Senior Unsecured Notes due 2018
Debt Instrument [Line Items]
Interest rate stated percentage		6.65%	0.00%
Outstanding indebtedness			$ 0	250,000,000
Amount of debt intended to be redeemed		$ 250,000,000
5.875% Senior Unsecured Notes due 2020
Debt Instrument [Line Items]
Interest rate stated percentage			5.875%
Outstanding indebtedness			$ 250,000,000	250,000,000
5.75% Senior Unsecured Notes due 2042
Debt Instrument [Line Items]
Interest rate stated percentage			5.75%
Outstanding indebtedness			$ 175,000,000	175,000,000
5.75% Senior Unsecured Notes due 2042 | Subsequent event
Debt Instrument [Line Items]
Interest rate stated percentage	5.75%
Amount of debt intended to be redeemed	$ 175,000,000
Percentage of principal amount intended to be redeemed	100.00%
Unsecured fixed rate debt
Debt Instrument [Line Items]
Interest rate stated percentage			5.82%
Outstanding indebtedness			$ 425,000,000	675,000,000
Secured Fixed Rate Debt
Debt Instrument [Line Items]
Interest rate stated percentage			5.69%
Outstanding indebtedness			$ 31,940,000	76,634,000
Secured Fixed Rate Debt | Parkshore Plaza
Debt Instrument [Line Items]
Interest rate stated percentage			0.00%
Outstanding indebtedness			$ 0	41,275,000
Secured Fixed Rate Debt | 206 East 9th Street
Debt Instrument [Line Items]
Interest rate stated percentage			5.69%
Outstanding indebtedness			$ 26,536,000	27,041,000
Secured Fixed Rate Debt | 33 Stiles Lane
Debt Instrument [Line Items]
Interest rate stated percentage			0.00%
Outstanding indebtedness			$ 0	2,415,000
Secured Fixed Rate Debt | 97 Newberry Road
Debt Instrument [Line Items]
Interest rate stated percentage			5.71%
Outstanding indebtedness			$ 5,404,000	$ 5,903,000

Indebtedness - Narrative (Detai

Indebtedness - Narrative (Details)

Jul. 15, 2017USD ($)

Dec. 15, 2016USD ($)

Nov. 10, 2016USD ($)

Feb. 16, 2016USD ($)

Dec. 01, 2015USD ($)

Aug. 03, 2015USD ($)

Jun. 05, 2015USD ($)

Jun. 03, 2015USD ($)

May 01, 2015USD ($)

Jan. 29, 2015USD ($)option

Dec. 31, 2017USD ($)property

Apr. 30, 2017USD ($)

May 22, 2015USD ($)

Dec. 31, 2015

Dec. 31, 2017USD ($)property

Dec. 31, 2016USD ($)

Dec. 31, 2015USD ($)

Debt Instrument [Line Items]

(Loss) gain on early extinguishment of debt

$ (493,000)

$ (2,680,000)

$ 6,661,000

Term loan amount outstanding

$ 848,578,000

848,578,000

1,141,667,000

Real estate properties, net

1,296,893,000

1,296,893,000

2,101,635,000

Mortgage notes payable, net

32,594,000

32,594,000

77,717,000

Outstanding indebtedness

856,940,000

856,940,000

1,151,634,000

Designated as hedging instrument | Cash flow hedging

Debt Instrument [Line Items]

Expense included in interest and other income

0

241,000

0

Mortgage notes | 33 Stiles Lane

Debt Instrument [Line Items]

(Loss) gain on early extinguishment of debt

(200,000)

Amount of debt, redeemed, defeased, or prepaid

2,000,000

Mortgage notes | Parkshore Plaza

Debt Instrument [Line Items]

(Loss) gain on early extinguishment of debt

$ (100,000)

Amount of debt, redeemed, defeased, or prepaid

$ 41,300,000

Mortgage notes | 1735 Market Street

Debt Instrument [Line Items]

(Loss) gain on early extinguishment of debt

(2,400,000)

Amount of debt, redeemed, defeased, or prepaid

$ 167,800,000

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

Mortgage notes | 111 Monument Circle

Debt Instrument [Line Items]

(Loss) gain on early extinguishment of debt

$ 600,000

Amount of debt, redeemed, defeased, or prepaid

$ 116,000,000

Mortgage notes | 111 East Wacker Drive

Debt Instrument [Line Items]

(Loss) gain on early extinguishment of debt

(3,900,000)

Amount of debt, redeemed, defeased, or prepaid

$ 141,400,000

Mortgage notes | 2501 20th Place South

Debt Instrument [Line Items]

(Loss) gain on early extinguishment of debt

(600,000)

Amount of debt, redeemed, defeased, or prepaid

$ 10,000,000

Mortgage notes | 1320 Main Street

Debt Instrument [Line Items]

(Loss) gain on early extinguishment of debt

(6,200,000)

Amount of debt, redeemed, defeased, or prepaid

$ 38,700,000

Mortgage notes | 225 Water Street in Jacksonville, Florida

Debt Instrument [Line Items]

(Loss) gain on early extinguishment of debt

17,300,000

Interest accrual rate

10.03%

Outstanding indebtedness

$ 40,100,000

Default interest

4.00%

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 | LIBOR

Debt Instrument [Line Items]

Basis points on variable rate

1.05%

Revolving credit facility | LIBOR | Minimum

Debt Instrument [Line Items]

Basis points on variable rate

0.875%

Revolving credit facility | LIBOR | Maximum

Debt Instrument [Line Items]

Basis points on variable rate

1.55%

Revolving credit facility | Base rate | Minimum

Debt Instrument [Line Items]

Basis points on variable rate

0.00%

Revolving credit facility | Base rate | Maximum

Debt Instrument [Line Items]

Basis points on variable rate

0.55%

Unsecured revolving credit facility | Revolving credit facility

Debt Instrument [Line Items]

Maximum borrowing capacity

$ 750,000,000

Number of extension options | option

2

Extension option term

6 months

Extension option fee, percent

0.075%

Commitment fee percentage

0.20%

Interest rate payable percentage

2.61%

Revolving credit facility

0

$ 0

Amount available for borrowing

750,000,000

750,000,000

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

Debt Instrument [Line Items]

(Loss) gain on early extinguishment of debt

$ (400,000)

Unsecured revolving credit facility | Letter of credit

Debt Instrument [Line Items]

Maximum borrowing capacity

$ 100,000,000

Loan facility, 5-year term

Debt Instrument [Line Items]

Maximum borrowing capacity

$ 200,000,000

Term loan amount outstanding

$ 200,000,000

$ 200,000,000

Interest rate stated percentage

2.71%

2.71%

Outstanding indebtedness

$ 200,000,000

$ 200,000,000

200,000,000

Loan facility, 5-year term | LIBOR

Debt Instrument [Line Items]

Basis points on variable rate

1.15%

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, 5-year term | Revolving credit facility

Debt Instrument [Line Items]

Debt instrument, term

5 years

Interest accrual rate

2.71%

Loan facility, 7-year term

Debt Instrument [Line Items]

Maximum borrowing capacity

$ 200,000,000

Interest rate stated percentage

3.11%

3.11%

Outstanding indebtedness

$ 200,000,000

$ 200,000,000

200,000,000

Loan facility, 7-year term | LIBOR

Debt Instrument [Line Items]

Basis points on variable rate

1.55%

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%

Loan facility, 7-year term | Revolving credit facility

Debt Instrument [Line Items]

Debt instrument, term

7 years

Interest accrual rate

3.11%

Term loan amount outstanding

$ 200,000,000

$ 200,000,000

Revolving credit facility and term loans

Debt Instrument [Line Items]

Additional increases in maximum amount of borrowings available

$ 1,150,000,000

6.65% Senior Unsecured Notes due 2018

Debt Instrument [Line Items]

(Loss) gain on early extinguishment of debt

$ (200,000)

Amount of debt, redeemed, defeased, or prepaid

$ 250,000,000

Interest rate stated percentage

6.65%

0.00%

0.00%

Outstanding indebtedness

$ 0

$ 0

$ 250,000,000

6.25% Senior Unsecured Notes due 2017

Debt Instrument [Line Items]

(Loss) gain on early extinguishment of debt

$ (100,000)

Amount of debt, redeemed, defeased, or prepaid

$ 250,000,000

Interest rate stated percentage

6.25%

6.25% Senior Unsecured Notes due 2016

Debt Instrument [Line Items]

(Loss) gain on early extinguishment of debt

$ (100,000)

Amount of debt, redeemed, defeased, or prepaid

$ 139,100,000

Interest rate stated percentage

6.25%

5.75% Senior Unsecured Notes due 2015

Debt Instrument [Line Items]

(Loss) gain on early extinguishment of debt

$ (100,000)

Amount of debt, redeemed, defeased, or prepaid

$ 138,800,000

Interest rate stated percentage

5.75%

Mortgage notes

Debt Instrument [Line Items]

Number of real estate properties secured by mortgage | property

2

2

Real estate properties, net

$ 56,200,000

$ 56,200,000

Mortgage notes payable, net

$ 32,600,000

$ 32,600,000

Indebtedness - Schedule of Requ

Indebtedness - Schedule of Required Principal Payments on Outstanding Debt (Details) - USD ($)	Jan. 30, 2018	Dec. 31, 2017	Dec. 31, 2016
Debt Disclosure [Abstract]
2,018		$ 1,063,000
2,019		1,126,000
2,020		451,189,000
2,021		25,463,000
2,022		200,663,000
Thereafter		177,436,000
Total		856,940,000	$ 1,151,634,000
5.75% Senior Unsecured Notes due 2042
Debt Disclosure [Abstract]
Total		$ 175,000,000	$ 175,000,000
Debt Instrument [Line Items]
Interest rate stated percentage		5.75%
Subsequent event | 5.75% Senior Unsecured Notes due 2042
Debt Instrument [Line Items]
Amount of debt intended to be redeemed	$ 175,000,000
Interest rate stated percentage	5.75%
Percentage of principal amount intended to be redeemed	100.00%

Shareholders' Equity - Narrativ

Shareholders' Equity - Narrative (Details)	May 15, 2016USD ($)$ / sharesshares	Mar. 17, 2016USD ($)	Aug. 24, 2015USD ($)	Mar. 31, 2017USD ($)	Sep. 30, 2016USD ($)authorization	Dec. 31, 2017USD ($)$ / sharesshares	Dec. 31, 2016USD ($)$ / sharesshares	Dec. 31, 2015USD ($)$ / sharesshares	Dec. 31, 2017USD ($)$ / sharesshares	Mar. 15, 2017USD ($)	Sep. 14, 2015USD ($)
Class of Stock [Line Items]
Stock repurchase program, period in force		12 months	12 months
Stock repurchase program, number of authorizations expired | authorization					2
Stock repurchase program, amount not utilized				$ 106,600,000	$ 86,600,000
Redemption of shares							$ 275,000,000	$ 0
Preferred stock redemption premium						$ 0	$ 9,609,000	$ 0
Distribution percentage to shareholders consisting of an alternative minimum tax adjustment						0.00%	96.31%	0.00%
Common Shares
Class of Stock [Line Items]
Common shares issued to holders of preferred shares (in shares) | shares								144
Number of shares repurchased (in shares) | shares							2,491,675	3,410,300	5,901,975
Stock repurchase program, authorized amount			$ 100,000,000
Stock repurchase program, additional authorized amount		$ 150,000,000								$ 150,000,000	$ 100,000,000
Stock repurchased and retired during period, price per share (in dollars per share) | $ / shares							$ 27.68	$ 25.76	$ 26.57
Redemption of shares							$ 69,000,000	$ 87,800,000	$ 156,800,000
Stock repurchase program, remaining authorized repurchase amount						$ 150,000,000			$ 150,000,000
Stock repurchase program, number of shares surrendered (in shares) | shares						43,329	31,025
Common shares from conversion of preferred shares (in shares) | shares						2,363,103			2,363,103
Series D
Class of Stock [Line Items]
Number of shares repurchased (in shares) | shares							0	301
Redemption of shares							$ 0	$ 3,000
Preferred shares, of beneficial interest, shares outstanding (in shares) | shares						4,915,196	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			0.480775
Initial conversion price per share (in dollars per share) | $ / shares						$ 52			$ 52
Additional common shares (in shares) | shares						2,363,103
Liquidation preference (in dollars per share) | $ / shares						$ 25			$ 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			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 Declared Distributions (Details) - $ / shares	Oct. 30, 2017	Jul. 28, 2017	Apr. 28, 2017	Jan. 30, 2017
Series D
Class of Stock [Line Items]
Dividend Per Share (in dollars per share)	$ 0.40625	$ 0.40625	$ 0.40625	$ 0.40625

Noncontrolling Interest (Detail

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

Noncontrolling Interest - Commo

Noncontrolling Interest - Common Shares and Units Activity (Details)	12 Months Ended
	Dec. 31, 2017shares
Increase (Decrease) in Stockholders' Equity
Ownership percentage by noncontrolling owners	0.03%
Common Shares
Increase (Decrease) in Stockholders' Equity
Outstanding, beginning balance (in shares)	123,994,465
Restricted share and time-based LTIP Unit grants, net of forfeitures (in shares)	223,151
Outstanding, ending balance (in shares)	124,217,616
LTIP units | Noncontrolling Interest
Increase (Decrease) in Stockholders' Equity
Outstanding, beginning balance (in shares)	0
Restricted share and time-based LTIP Unit grants, net of forfeitures (in shares)	42,520
Outstanding, ending balance (in shares)	42,520
Common Shares and LTIP Units
Increase (Decrease) in Stockholders' Equity
Outstanding, beginning balance (in shares)	123,994,465
Restricted share and time-based LTIP Unit grants, net of forfeitures (in shares)	265,671
Outstanding, ending balance (in shares)	124,260,136

Cumulative Other Comprehensiv71

Cumulative Other Comprehensive Loss - Schedule of Amounts Recognized in Cumulative Other Comprehensive Loss (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Amounts recognized in cumulative other comprehensive income (Loss) by component
Balance as of beginning of period	$ (208)
Other comprehensive (loss) income before reclassifications	64
Amounts reclassified from cumulative other comprehensive loss to net income	49
Net current period other comprehensive (loss) income	113
Balance as of end of period	(95)	$ (208)
Unrealized Loss on Derivative Instruments
Amounts recognized in cumulative other comprehensive income (Loss) by component
Balance as of beginning of period	(208)	(3,687)
Other comprehensive (loss) income before reclassifications	(297)	(554)
Amounts reclassified from cumulative other comprehensive loss to net income	49	4,033
Net current period other comprehensive (loss) income	(248)	3,479
Balance as of end of period	(456)	(208)
Unrealized Gain, Net on Marketable Securities
Amounts recognized in cumulative other comprehensive income (Loss) by component
Balance as of beginning of period	0
Other comprehensive (loss) income before reclassifications	361
Amounts reclassified from cumulative other comprehensive loss to net income	0
Net current period other comprehensive (loss) income	361
Balance as of end of period	$ 361	$ 0

Cumulative Other Comprehensiv72

Cumulative Other Comprehensive Loss - Schedule of Reclassifications Out of Cumulative Other Comprehensive Loss (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Reclassifications out of cumulative other comprehensive income (Loss)
Interest expense	$ 52,183	$ 84,329	$ 107,316
Interest and other income	26,380	10,331	$ 5,989
Amounts reclassified from cumulative other comprehensive loss to net income	(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	49	4,033
Amounts Reclassified from Cumulative Other Comprehensive Loss to Net Income | Interest rate swap contracts
Reclassifications out of cumulative other comprehensive income (Loss)
Interest expense	0	3,792
Interest and other income	0	241
Amounts Reclassified from Cumulative Other Comprehensive Loss to Net Income | Interest rate cap contract
Reclassifications out of cumulative other comprehensive income (Loss)
Interest expense	$ 49	$ 0

Income Taxes - Schedule of Prov

Income Taxes - Schedule of Provision for Income Taxes (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Current:
State	$ 500	$ 745	$ 270
Federal	0	0	513
Foreign	0	0	2,336
Current income tax expense (benefit)	500	745	3,119
Deferred:
Foreign	0	0	(755)
Deferred income tax expense (benefit)	0	0	(755)
Income tax expense	$ 500	$ 745	$ 2,364

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, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Disclosure [Abstract]
Taxes at statutory U.S. federal income tax rate	35.00%	35.00%	35.00%
Dividends paid deduction and net operating loss utilization	(35.00%)	(35.00%)	(35.00%)
Federal taxes on built-in gain	0.00%	0.00%	0.50%
State, local, and foreign income taxes	1.66%	0.32%	1.81%
Effective tax rate	1.66%	0.32%	2.31%

Income Taxes - Narrative (Detai

Income Taxes - Narrative (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2017
Real Estate Properties [Line Items]
Net operating loss carryforwards		$ 120
Australian Properties
Real Estate Properties [Line Items]
Income tax expense (benefit)	$ 1.6

Derivative Instruments - Narrat

Derivative Instruments - Narrative (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Mar. 04, 2016
Derivative [Line Items]
Interest and other income	$ 26,380	$ 10,331	$ 5,989
Interest rate cap | LIBOR
Derivative [Line Items]
Derivative, cap interest rate				2.50%
Cash flow hedging | Designated as hedging instrument
Derivative [Line Items]
Estimated loss reclassification from other comprehensive loss to income	400
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		$ 200
Cash flow hedging | Designated as hedging instrument | Interest rate cap
Derivative [Line Items]
Derivative, notional amount	$ 400,000			$ 400,000

Derivative Instruments - Schedu

Derivative Instruments - Schedule of Outstanding Interest Rate Derivatives Designated as Cash Flow Hedges of Interest Rate Risk (Details) - Interest rate cap - Cash flow hedging - Designated as hedging instrument $ in Thousands	Dec. 31, 2017USD ($)financial_instrument	Mar. 04, 2016USD ($)
Derivative [Line Items]
Number of Instruments | financial_instrument	1
Notional Amount | $	$ 400,000	$ 400,000

Derivative Instruments - Sche78

Derivative Instruments - Schedule of Fair Value of Derivative Financial Instruments (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Interest rate cap | Cash flow hedging | Designated as hedging instrument | Other assets
Derivatives, Fair Value [Line Items]
Interest Rate Derivative Designated as Hedging Instrument	$ 17	$ 314

Derivative Instruments - Sche79

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, 2017	Dec. 31, 2016	Dec. 31, 2015
Derivative Instruments, Gain (Loss) [Line Items]
Amount of loss recognized in cumulative other comprehensive loss (effective portion)	$ (297)	$ (554)	$ (1,149)
Amount of loss reclassified from cumulative other comprehensive loss into interest expense (effective portion)	49	3,792	4,924
Amount of loss recognized in income (ineffective portion and amount excluded from effectiveness testing)	$ 0	$ 241	$ 0

Share-Based Compensation - Narr

Share-Based Compensation - Narrative (Details) $ / shares in Units, $ in Millions	Nov. 08, 2017shares	Jun. 20, 2017USD ($)trustee$ / sharesshares	Jan. 24, 2017$ / sharesshares	Jun. 15, 2016USD ($)trustee$ / sharesshares	Jan. 26, 2016$ / sharesshares	Jun. 16, 2015USD ($)trustee$ / sharesshares	Jan. 28, 2015$ / sharesshares	Dec. 31, 2021shares	Dec. 31, 2020shares	Dec. 31, 2019shares	Dec. 31, 2018shares	Dec. 31, 2017USD ($)tranche$ / sharesshares	Dec. 31, 2016USD ($)$ / sharesshares	Dec. 31, 2015USD ($)$ / sharesshares	Dec. 31, 2014shares
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)												1,628,305
Common stock issued (in shares)	226,258											124,217,616	123,994,465
General and administrative expense
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Compensation expense | $												$ 21.4	$ 18.5	$ 15.4
Restricted shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period				1 year		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)		25,248	76,424	31,167	136,623	34,587	126,319
Price per share (in dollars per share) | $ / shares		$ 31.69		$ 28.88		$ 26.02
Granted (in dollars per share) | $ / shares			$ 31.47		$ 26.93		$ 26.58
Restricted shares | Independent Trustee
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of equity awards granted (in shares)				3,463		3,843
RSUs
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of tranches | tranche												2
Vested (in shares)	226,258
RSUs | Vesting in three years
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting percent												50.00%
RSUs | Vesting in four years
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting percent												50.00%
RSUs at target
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of equity awards granted (in shares)			155,168		277,386		256,467
RSUs at maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of equity awards granted (in shares)			386,756		691,385		639,250
Granted (in dollars per share) | $ / shares												$ 15.97	$ 15.57	$ 15.47
LTIP units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Equity award, conversion basis (in shares)												1
Number of equity awards granted (in shares)			119,288
Time-based LTIP Units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of equity awards granted (in shares)		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
Number of Restricted Shares and Time-Based LTIP Units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period		1 year
Vested (in shares)												217,449	191,498	86,517
Share based compensation amount | $		$ 0.1
Number of equity awards granted (in shares)												144,192	167,790	160,906
Granted (in dollars per share) | $ / shares												$ 31.51	$ 27.29	$ 26.46
Unvested (in shares)												678,952	752,209	776,456	710,182
Estimated future compensation expense for unvested shares | $												$ 9.3
Weighted average period over which compensation expense will be recorded												1 year 8 months 12 days
Number of Restricted Shares and Time-Based LTIP Units | Scenario, Forecast
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vested (in shares)								57,502	95,287	123,764	402,399
RSUs and market-based LTIP units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vested (in shares)												226,258	0	0
Number of equity awards granted (in shares)												585,967	691,385	639,250
Granted (in dollars per share) | $ / shares												$ 15.97	$ 15.57	$ 15.47
Unvested (in shares)												2,131,947	2,572,768	1,884,110	1,266,237
Estimated future compensation expense for unvested shares | $												$ 14.8
Weighted average period over which compensation expense will be recorded												1 year 10 months 24 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, 2017	Dec. 31, 2016	Dec. 31, 2015
Number of Restricted Shares and Time-Based LTIP Units
Number of Shares
Beginning balance (in shares)	752,209	776,456	710,182
Granted (in shares)	144,192	167,790	160,906
Vested (in shares)	(217,449)	(191,498)	(86,517)
Not earned (in shares)	0
Forfeited (in shares)		(539)	(8,115)
Ending balance (in shares)	678,952	752,209	776,456
Weighted Average Grant Date Fair Value
Beginning balance (in dollars per share)	$ 26.79	$ 26.62	$ 26.66
Granted (in dollars per share)	31.51	27.29	26.46
Vested (in dollars per share)	27.97	26.54	26.66
Not earned (in dollars per share)	0
Forfeited (in dollars per share)		26.82	26.64
Ending balance (in dollars per share)	$ 27.41	$ 26.79	$ 26.62
Number of RSUs and Market-Based LTIP Units
Number of Shares
Beginning balance (in shares)	2,572,768	1,884,110	1,266,237
Granted (in shares)	585,967	691,385	639,250
Vested (in shares)	(226,258)	0	0
Not earned (in shares)	(800,530)
Forfeited (in shares)		(2,727)	(21,377)
Ending balance (in shares)	2,131,947	2,572,768	1,884,110
Weighted Average Grant Date Fair Value
Beginning balance (in dollars per share)	$ 15.75	$ 15.82	$ 15.99
Granted (in dollars per share)	15.97	15.57	15.47
Vested (in dollars per share)	15.99	0	0
Not earned (in dollars per share)	15.99
Forfeited (in dollars per share)		15.54	15.80
Ending balance (in dollars per share)	$ 15.69	$ 15.75	$ 15.82

Share-Based Compensation - Su82

Share-Based Compensation - Summary of Assumptions and Fair Values for Restricted Stock Units Granted in the Period (Details) - $ / shares	Jun. 16, 2015	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
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)		$ 39.81	$ 38.80	$ 38.55
RSUs at maximum
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)		$ 15.97	$ 15.57	$ 15.47
Number of Restricted Shares and Time-Based LTIP Units
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions and Methodology
Expected term		4 years	4 years	4 years
Expected volatility		0.00%	0.00%	0.00%
Expected dividend yield		1.59%	1.86%	1.88%
Risk-free rate		1.49%	1.07%	0.81%

Defined Contribution Plan (Deta

Defined Contribution Plan (Details) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
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	$ 400,000	$ 300,000
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 Liab84

Fair Value of Assets and Liabilities - Schedule of Assets and Liabilities Measure at Fair Value (Details) - Recurring - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Marketable securities
Recurring Fair Value Measurements:
Fair value of assets	$ 276,928
Marketable securities | Level 1
Recurring Fair Value Measurements:
Fair value of assets	276,928
Marketable securities | Level 2
Recurring Fair Value Measurements:
Fair value of assets	0
Marketable securities | Level 3
Recurring Fair Value Measurements:
Fair value of assets	0
Interest rate cap
Recurring Fair Value Measurements:
Fair value of assets	17	$ 314
Interest rate cap | Level 1
Recurring Fair Value Measurements:
Fair value of assets	0	0
Interest rate cap | Level 2
Recurring Fair Value Measurements:
Fair value of assets	17	314
Interest rate cap | Level 3
Recurring Fair Value Measurements:
Fair value of assets	$ 0	$ 0

Fair Value of Assets and Liab85

Fair Value of Assets and Liabilities - Narrative (Details) - USD ($) $ in Thousands	Jun. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Jun. 30, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Fair value of assets and liabilities
Asset impairment charges		$ 18,400	$ 1,300		$ 19,714	$ 58,476	$ 17,162
Real estate properties, net					$ 1,296,893	2,101,635
Credit concentration | Total rents
Fair value of assets and liabilities
Concentration risk					8.90%
111 Monument Circle, 101-115 W. Washington Street and 100 East Wisconsin Avenue
Fair value of assets and liabilities
Asset impairment charges				$ 43,700
Parkshore Plaza, Cabot Business Park Land, 625 Crane Street and 111 Market Place
Fair value of assets and liabilities
Asset impairment charges						14,700
Real estate properties held for sale
Fair value of assets and liabilities
Asset impairment charges	$ 18,400
Level 2 | Market Approach Valuation Technique | Real estate properties held for sale
Fair value of assets and liabilities
Fair value of assets	80,600	80,600	23,300
Level 2 | Market Approach Valuation Technique | Real estate properties held for sale | 111 Monument Circle, 101-115 W. Washington Street and 100 East Wisconsin Avenue
Fair value of assets and liabilities
Fair value of assets				264,900
Level 2 | Market Approach Valuation Technique | Real estate properties held for sale | Parkshore Plaza, Cabot Business Park Land, 625 Crane Street and 111 Market Place
Fair value of assets and liabilities
Fair value of assets						83,500
Disposition Plan | Principal Balance | Nonrecurring
Fair value of assets and liabilities
Real estate properties, net	$ 99,000	$ 99,000	$ 24,600
Disposition Plan | Principal Balance | Nonrecurring | 111 Monument Circle, 101-115 W. Washington Street and 100 East Wisconsin Avenue
Fair value of assets and liabilities
Real estate properties, net				$ 308,600
Disposition Plan | Principal Balance | Nonrecurring | Parkshore Plaza, Cabot Business Park Land, 625 Crane Street and 111 Market Place
Fair value of assets and liabilities
Real estate properties, net						$ 98,200

Fair Value of Assets and Liab86

Fair Value of Assets and Liabilities - Schedule of Fair Value and Carrying Value of Financial Instruments (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Principal Balance
Fair value of financial instruments
Senior unsecured debt and mortgage notes payable	$ 856,940	$ 1,151,634
Fair Value
Fair value of financial instruments
Senior unsecured debt and mortgage notes payable	$ 874,280	$ 1,167,031

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, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Numerator for earnings per common share - basic:
Net income									$ 29,666	$ 232,894	$ 99,857
Net income attributable to noncontrolling interest									(10)	0	0
Preferred distributions									(7,988)	(17,956)	(27,924)
Excess fair value of consideration paid over carrying value of preferred shares									0	(9,609)	0
Net income attributable to Equity Commonwealth common shareholders	$ (23,558)	$ 31,215	$ (7,806)	$ 21,817	$ 10,263	$ 84,391	$ 71,254	$ 39,421	21,668	205,329	71,933
Net Income (Loss) Available to Common Stockholders, Diluted [Abstract]
Net income									29,666	232,894	99,857
Preferred distributions									(7,988)	(17,956)	(27,924)
Excess fair value of consideration paid over carrying value of preferred shares									0	(9,609)	0
Numerator for net income per share - diluted									$ 21,678	$ 205,329	$ 71,933
Denominator for earnings per common share - basic and diluted:
Weighted average common shares outstanding — basic (in shares)									124,125	125,474	128,621
Weighted average common shares outstanding — diluted (in shares)									125,129	126,768	129,437
Net income per common share attributable to Equity Commonwealth common shareholders:
Basic (in dollars per share)									$ 0.17	$ 1.64	$ 0.56
Diluted (in dollars per share)									$ 0.17	$ 1.62	$ 0.56
Series D
Anti-dilutive securities:
Effect of Series D preferred shares; 6 1/2% cumulative convertible (in shares)									2,363	2,363	2,363
Preferred shares, dividend yield									6.50%	6.50%	6.50%
RSUs
Denominator for earnings per common share - basic and diluted:
Weighted average number of common shares outstanding, dilutive adjustment (in shares)									912	1,294	816
LTIP units
Denominator for earnings per common share - basic and diluted:
Weighted average number of common shares outstanding, dilutive adjustment (in shares)									92	0	0

Earnings Per Common Share - S88

Earnings Per Common Share - Schedule of Computation of Basic and Diluted Earnings Per Share (Footnote) (Details) - shares shares in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
RSUs and LTIP Units
Earnings Per Share, Diluted, by Common Class, Including Two Class Method [Line Items]
Common shares issued (in shares)	673	1,027	1,143

Segment Information - Narrative

Segment Information - Narrative (Details)	12 Months Ended
	Dec. 31, 2017segment
Segment Reporting [Abstract]
Number of reportable segments	1
Percentage of CBD and suburban properties that are office properties (more than)	95.00%

Related Person Transactions - T

Related Person Transactions - Two North Riverside Plaza Joint Venture Limited Partnership and Equity Group Investments and Associated Entities (Details) $ in Thousands	Dec. 29, 2015parking_space	Jul. 20, 2015option	Jan. 30, 2015option	Dec. 31, 2017USD ($)	Dec. 31, 2016USD ($)	Dec. 31, 2015USD ($)
Two North Riverside Plaza Joint Venture Limited Partnership
Related Party Transaction [Line Items]
Lease term		5 years
Number of renewal options | option		1
Renewal option term		5 years
Tenant improvements				$ 700
Operating lease, termination period duration				30 days
Expenses from transactions with related party				$ 800	$ 800
Operating Leases, Future Minimum Payments Due, Fiscal Year Maturity
2,018				900
2,019				900
2,020				900
Due to related party				$ 0	$ 100
Equity Group
Related Party Transaction [Line Items]
Lease term			1 year
Number of renewal options | option			1
Renewal option term			3 months
Expenses from transactions with related party						$ 13
Equity Group | Terminated Agreement
Related Party Transaction [Line Items]
Expenses from transactions with related party						200
Operating Leases, Future Minimum Payments Due, Fiscal Year Maturity
Number of parking spaces | parking_space	1
Equity Residential Management, L.L.C.
Related Party Transaction [Line Items]
Expenses from transactions with related party						$ 400

Related Person Transactions - R

Related Person Transactions - Related/Corvex (Details) - Related/Corvex - USD ($) $ / shares in Units, $ in Millions	1 Months Ended		12 Months Ended
	Aug. 31, 2016	Aug. 31, 2015	Dec. 31, 2014	Jul. 31, 2014
Related Party Transaction [Line Items]
Amount of reimbursements payable to related party				$ 33.5
Reimbursement amount paid	$ 8.2	$ 8.4	$ 16.7
Amount of reimbursements payable to related party if specified minimum common share price maintain during the one year period after the date on which the reimbursement is approved by shareholders				8.4
Amount of reimbursements payable to related party if specified minimum common share price maintain during the one year period between the first and second anniversaries of the date on which the reimbursement is approved by shareholders				$ 8.4
Minimum
Related Party Transaction [Line Items]
Price per common share during one year period after date (in dollars per share)				$ 26
Price per common share during first and second year after date (in dollars per share)				$ 26

Related Person Transactions -92

Related Person Transactions - Termination and Cooperation Agreement (Details) - USD ($) $ in Millions	Mar. 01, 2015	Feb. 28, 2015
RMR and RMR Australia
Related Party Transaction [Line Items]
Monthly management fees	$ 0.1	$ 1.2

Selected Quarterly Financial 93

Selected Quarterly Financial Data - Summary of Unaudited Quarterly Results of Operations (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Quarterly Financial Information Disclosure [Abstract]
Total revenues	$ 71,623	$ 77,798	$ 91,599	$ 99,551	$ 103,546	$ 114,632	$ 145,367	$ 137,135	$ 340,571	$ 500,680	$ 714,891
Net income (loss) attributable to Equity Commonwealth common shareholders	$ (23,558)	$ 31,215	$ (7,806)	$ 21,817	$ 10,263	$ 84,391	$ 71,254	$ 39,421	$ 21,668	$ 205,329	$ 71,933
Net income (loss) attributable to Equity Commonwealth common shareholders per share—basic (in dollars per share)	$ (0.19)	$ 0.25	$ (0.06)	$ 0.18	$ 0.08	$ 0.67	$ 0.57	$ 0.31
Net income (loss) attributable to Equity Commonwealth common shareholders per share—diluted (in dollars per share)	$ (0.19)	$ 0.25	$ (0.06)	$ 0.17	$ 0.08	$ 0.67	$ 0.56	$ 0.31

Selected Quarterly Financial 94

Selected Quarterly Financial Data - Narrative (Details) - USD ($) $ in Thousands	3 Months Ended					12 Months Ended
	Dec. 31, 2017	Jun. 30, 2017	Mar. 31, 2017	Sep. 30, 2016	Jun. 30, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Quarterly Financial Information Disclosure [Abstract]
Loss on asset impairment		$ 18,400	$ 1,300			$ 19,714	$ 58,476	$ 17,162
(Loss) gain on sale of properties	$ (29,200)			$ 82,200	$ 106,400
111 Monument Circle
Schedule of Equity Method Investments [Line Items]
Lease termination fee					$ 16,600

Subsequent Events - Narrative (

Subsequent Events - Narrative (Details)

Feb. 15, 2018$ / shares

Feb. 14, 2018USD ($)ft²

Jan. 30, 2018USD ($)

Jan. 22, 2018USD ($)ft²

Jan. 12, 2018$ / shares

Oct. 30, 2017$ / shares

Jul. 28, 2017$ / shares

Apr. 28, 2017$ / shares

Jan. 30, 2017$ / shares

Dec. 31, 2017USD ($)

Dec. 31, 2016USD ($)

Dec. 31, 2015USD ($)ft²

5.75% Senior Unsecured Notes due 2042

Subsequent Event [Line Items]

Interest rate stated percentage

5.75%

Disposed of by Sale

Subsequent Event [Line Items]

Property square feet (in sqft) | ft²

18,939,185

Gross sales price

$ 862,624,000

$ 1,259,895,000

$ 2,000,000,000

Series D

Subsequent Event [Line Items]

Dividend declared (in dollars per share) | $ / shares

$ 0.40625

$ 0.40625

$ 0.40625

$ 0.40625

Subsequent event | 5.75% Senior Unsecured Notes due 2042

Subsequent Event [Line Items]

Amount of debt intended to be redeemed

$ 175,000,000

Interest rate stated percentage

5.75%

Percentage of principal amount intended to be redeemed

100.00%

Subsequent event | 600 West Chicago Avenue | Disposed of by Sale

Subsequent Event [Line Items]

Property square feet (in sqft) | ft²

1,561,477

Gross sales price

$ 510,000,000

Subsequent event | 1600 Market Street | Disposed of by Sale

Subsequent Event [Line Items]

Property square feet (in sqft) | ft²

825,968

Gross sales price

$ 160,000,000

Subsequent event | Series D

Subsequent Event [Line Items]

Dividend declared (in dollars per share) | $ / shares

$ 0.40625

Dividends paid (in dollars per share) | $ / shares

$ 0.40625

SCHEDULE II VALUATION AND QUA96

SCHEDULE II VALUATION AND QUALIFYING ACCOUNTS (Details) - Allowance for doubtful accounts - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Movement in valuation and qualifying accounts
Balance at Beginning of Period	$ 5,105	$ 7,715	$ 6,565
Charged to Costs and Expenses	935	1,056	4,533
Deductions	(1,269)	(3,666)	(3,383)
Balance at End of Period	$ 4,771	$ 5,105	$ 7,715

SCHEDULE III REAL ESTATE AND 97

SCHEDULE III REAL ESTATE AND ACCUMULATED DEPRECIATION (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	$ 31,940
Initial Cost to Company
Land	186,443
Buildings and Improvements	1,304,951
Costs Capitalized Subsequent to Acquisition, Net	276,758
Impairment/Write Downs	(20,541)
Cost Amount Carried at Close of Period
Land	191,775
Buildings and Improvements	1,555,836
Total	1,747,611
Accumulated Depreciation	450,718	$ 755,255	$ 898,939	$ 1,030,445
1225 Seventeenth Street
SEC Schedule III, 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	28,273
Impairment/Write Downs	(1,563)
Cost Amount Carried at Close of Period
Land	22,400
Buildings and Improvements	136,800
Total	159,200
Accumulated Depreciation	31,093
5073, 5075, & 5085 S. Syracuse Street
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	0
Initial Cost to Company
Land	4,720
Buildings and Improvements	58,890
Costs Capitalized Subsequent to Acquisition, Net	0
Impairment/Write Downs	0
Cost Amount Carried at Close of Period
Land	4,720
Buildings and Improvements	58,890
Total	63,610
Accumulated Depreciation	11,287
1601 Dry Creek Drive
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	0
Initial Cost to Company
Land	3,714
Buildings and Improvements	24,397
Costs Capitalized Subsequent to Acquisition, Net	7,498
Impairment/Write Downs	(156)
Cost Amount Carried at Close of Period
Land	3,714
Buildings and Improvements	31,739
Total	35,453
Accumulated Depreciation	10,751
97 Newberry Road
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	5,404
Initial Cost to Company
Land	2,960
Buildings and Improvements	12,360
Costs Capitalized Subsequent to Acquisition, Net	30
Impairment/Write Downs	0
Cost Amount Carried at Close of Period
Land	2,943
Buildings and Improvements	12,407
Total	15,350
Accumulated Depreciation	3,485
1250 H Street, NW
SEC Schedule III, 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	14,906
Impairment/Write Downs	(248)
Cost Amount Carried at Close of Period
Land	5,975
Buildings and Improvements	68,436
Total	74,411
Accumulated Depreciation	30,990
Georgetown-Green and Harris Buildings
SEC Schedule III, 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	44
Impairment/Write Downs	0
Cost Amount Carried at Close of Period
Land	24,000
Buildings and Improvements	36,023
Total	60,023
Accumulated Depreciation	7,501
600 West Chicago Avenue
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	0
Initial Cost to Company
Land	34,980
Buildings and Improvements	315,643
Costs Capitalized Subsequent to Acquisition, Net	50,761
Impairment/Write Downs	(322)
Cost Amount Carried at Close of Period
Land	34,980
Buildings and Improvements	366,082
Total	401,062
Accumulated Depreciation	57,951
8750 Bryn Mawr Avenue
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	0
Initial Cost to Company
Land	6,600
Buildings and Improvements	77,764
Costs Capitalized Subsequent to Acquisition, Net	14,234
Impairment/Write Downs	(258)
Cost Amount Carried at Close of Period
Land	6,600
Buildings and Improvements	91,740
Total	98,340
Accumulated Depreciation	17,342
109 Brookline Avenue
SEC Schedule III, 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	15,233
Impairment/Write Downs	(1,397)
Cost Amount Carried at Close of Period
Land	3,168
Buildings and Improvements	44,233
Total	47,401
Accumulated Depreciation	20,494
777 East Eisenhower Parkway
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	0
Initial Cost to Company
Land	3,675
Buildings and Improvements	26,988
Costs Capitalized Subsequent to Acquisition, Net	4,485
Impairment/Write Downs	0
Cost Amount Carried at Close of Period
Land	3,675
Buildings and Improvements	31,473
Total	35,148
Accumulated Depreciation	5,821
1735 Market Street
SEC Schedule III, 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	81,484
Impairment/Write Downs	(14,927)
Cost Amount Carried at Close of Period
Land	24,748
Buildings and Improvements	289,337
Total	314,085
Accumulated Depreciation	125,376
206 East 9th Street
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	26,536
Initial Cost to Company
Land	7,900
Buildings and Improvements	38,533
Costs Capitalized Subsequent to Acquisition, Net	4,058
Impairment/Write Downs	(12)
Cost Amount Carried at Close of Period
Land	7,900
Buildings and Improvements	42,579
Total	50,479
Accumulated Depreciation	6,169
Bridgepoint Square
SEC Schedule III, 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	17,125
Impairment/Write Downs	(770)
Cost Amount Carried at Close of Period
Land	7,784
Buildings and Improvements	86,881
Total	94,665
Accumulated Depreciation	42,573
Research Park
SEC Schedule III, 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	17,439
Impairment/Write Downs	0
Cost Amount Carried at Close of Period
Land	21,213
Buildings and Improvements	72,260
Total	93,473
Accumulated Depreciation	34,941
333 108th Avenue NE
SEC Schedule III, 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	2,781
Impairment/Write Downs	(31)
Cost Amount Carried at Close of Period
Land	14,400
Buildings and Improvements	139,162
Total	153,562
Accumulated Depreciation	29,467
600 108th Avenue NE
SEC Schedule III, 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	18,407
Impairment/Write Downs	(857)
Cost Amount Carried at Close of Period
Land	3,555
Buildings and Improvements	47,794
Total	51,349
Accumulated Depreciation	$ 15,477

SCHEDULE III REAL ESTATE AND 98

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
	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Real Estate Properties
Additions			$ 55,000	$ 110,700	$ 75,700
Loss on asset impairment	$ (18,400)	$ (1,300)	(19,714)	(58,476)	(17,162)
Properties classified to held for sale			(138,130)
Balance at the end of the year			1,747,611
Accumulated Depreciation
Balance at the beginning of the year		755,255	755,255	898,939	1,030,445
Additions			71,979	101,647	144,844
Loss on asset impairment			0	(119)	0
Properties classified to held for sale			(62,064)
Disposals			(314,452)	(245,212)	(276,350)
Balance at the end of the year			450,718	755,255	898,939
Real Estate
Real Estate Properties
Balance at the beginning of the year		$ 2,856,890	2,856,890	3,887,352	5,728,443
Additions			55,041	113,258	68,118
Loss on asset impairment			(6,711)	(58,595)	(17,162)
Disposals			(1,019,479)	(1,085,125)	(1,892,047)
Balance at the end of the year			$ 1,747,611	$ 2,856,890	$ 3,887,352

SCHEDULE III REAL ESTATE AND 99

SCHEDULE III REAL ESTATE AND ACCUMULATED DEPRECIATION - Analysis of Carrying Amount of Real Estate Properties and Accumulated Depreciation (Footnote) (Details) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Aggregate cost of properties for federal income tax purposes	$ 1,907,759,000
Real estate properties additions due to changes in foreign currency exchange	0	$ 0	$ (13,217,000)
Accumulated depreciation additions due to changes in foreign currency exchange	$ 0	$ 0	$ (887,000)
Maximum | Buildings and improvements
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Estimated useful lives	40 years
Maximum | Personal property
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Estimated useful lives	12 years

SCHEDULE IV MORTGAGE LOANS O100

SCHEDULE IV MORTGAGE LOANS ON REAL ESTATE (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
SCHEDULE IV MORTGAGE LOANS ON REAL ESTATE
Carrying Amount of Mortgage	$ 7,700	$ 8,100
Reconciliation of the carrying amount of mortgage loans at the beginning of the period:
Balance at the beginning of period	8,080	8,087
Collection of principal	(320)	(7)
Loss on repayment	(72)
Balance at the end of period	$ 7,688	$ 8,080
Dearborn, MI
SCHEDULE IV MORTGAGE LOANS ON REAL ESTATE
Interest Rate	6.00%
Periodic Payment Terms	$ 7,688
Face Amount of Mortgage	7,688
Carrying Amount of Mortgage	7,688
Principal Amount of Loans Subject to Delinquent Principal or Interest	$ 0