Equity Commonwealth (EQC) 10-Q 25 Oct 18 2018 Q3 Quarterly report Financial data

Document and Entity Information

Document and Entity Information - shares	9 Months Ended
	Sep. 30, 2018	Oct. 19, 2018
Document and Entity Information
Entity Registrant Name	Equity Commonwealth
Entity Central Index Key	803,649
Document Type	10-Q
Document Period End Date	Sep. 30, 2018
Amendment Flag	false
Current Fiscal Year End Date	--12-31
Entity Current Reporting Status	Yes
Entity Filer Category	Large Accelerated Filer
Entity Emerging Growth Company	false
Entity Small Business	false
Entity Common Stock, Shares Outstanding (in shares)		121,482,673
Document Fiscal Year Focus	2,018
Document Fiscal Period Focus	Q3

CONDENSED CONSOLIDATED BALANCE

CONDENSED CONSOLIDATED BALANCE SHEETS - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Real estate properties:
Land	$ 137,329	$ 191,775
Buildings and improvements	1,000,822	1,555,836
Total real estate properties, at cost, gross	1,138,151	1,747,611
Accumulated depreciation	(370,854)	(450,718)
Total real estate properties, at cost, net	767,297	1,296,893
Assets held for sale	0	97,688
Acquired real estate leases, net	596	23,847
Cash and cash equivalents	2,673,328	2,351,693
Marketable securities	248,838	276,928
Restricted cash	9,708	8,897
Rents receivable, net of allowance for doubtful accounts of $5,816 and $4,771, respectively	50,103	93,436
Other assets, net	63,858	87,563
Total assets	3,813,728	4,236,945
LIABILITIES AND EQUITY
Senior unsecured debt, net	248,258	815,984
Mortgage notes payable, net	31,643	32,594
Liabilities related to properties held for sale	0	1,840
Accounts payable, accrued expenses and other	46,896	74,956
Rent collected in advance	8,182	11,076
Distributions payable	309,238	0
Total liabilities	644,217	936,450
Shareholders' equity:
Common shares of beneficial interest, $0.01 par value: 350,000,000 shares authorized; 121,482,673 and 124,217,616 shares issued and outstanding, respectively	1,215	1,242
Additional paid in capital	4,306,020	4,380,313
Cumulative net income	2,855,557	2,596,259
Cumulative other comprehensive loss	(1,006)	(95)
Cumulative common distributions	(3,418,995)	(3,111,868)
Cumulative preferred distributions	(693,736)	(685,748)
Total shareholders’ equity	3,168,318	3,299,366
Noncontrolling interest	1,193	1,129
Total equity	3,169,511	3,300,495
Total liabilities and equity	3,813,728	4,236,945
Series D
Shareholders' equity:
Series D preferred shares; 6 1/2% cumulative convertible; 4,915,196 shares issued and outstanding, aggregate liquidation preference of $122,880	119,263	119,263
Total equity	$ 119,263	$ 119,263

CONDENSED CONSOLIDATED BALANC_2

CONDENSED CONSOLIDATED BALANCE SHEETS (Parenthetical) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2018	Dec. 31, 2017
Statement of Financial Position [Abstract]
Rents receivable, allowance for doubtful accounts	$ 5,816	$ 4,771
Common shares of beneficial interest, par value (in dollars per share)	$ 0.01	$ 0.01
Common shares of beneficial interest, shares authorized (in shares)	350,000,000	350,000,000
Common shares of beneficial interest, shares issued (in shares)	121,482,673	124,217,616
Common shares of beneficial interest, shares outstanding (in shares)	121,482,673	124,217,616
Series D
Preferred shares of beneficial interest, par value (in dollars per share)	$ 0.01	$ 0.01
Preferred shares of beneficial interest, shares authorized (in shares)	50,000,000	50,000,000
Preferred shares, dividend yield	6.50%	6.50%
Preferred shares of beneficial interest, shares issued (in shares)	4,915,196	4,915,196
Preferred shares, of beneficial interest, shares outstanding (in shares)	4,915,196	4,915,196
Preferred shares, aggregate liquidation preference	$ 122,880	$ 122,880

CONDENSED CONSOLIDATED STATEMEN

CONDENSED CONSOLIDATED STATEMENTS OF OPERATIONS - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Revenues:
Total revenues	$ 46,873	$ 77,798	$ 154,097	$ 268,948
Expenses:
Operating expenses	20,257	32,380	64,377	110,751
Depreciation and amortization	11,287	21,133	38,211	71,970
General and administrative	10,905	11,689	35,466	35,727
Loss on asset impairment	0	0	12,087	19,714
Total expenses	42,449	65,202	150,141	238,162
Operating income	4,424	12,596	3,956	30,786
Interest and other income, net	12,626	7,596	31,074	17,987
Interest expense (including net amortization of debt discounts, premiums and deferred financing fees of $559, $784, $2,005 and $2,346, respectively)	(5,085)	(11,510)	(21,550)	(41,387)
Loss on early extinguishment of debt	0	(203)	(6,403)	(266)
Gain on sale of properties, net	20,877	25,080	253,025	44,670
Income before income taxes	32,842	33,559	260,102	51,790
Income tax expense	(65)	(335)	(2,616)	(555)
Net income	32,777	33,224	257,486	51,235
Net income attributable to noncontrolling interest	(13)	(12)	(90)	(18)
Net income attributable to Equity Commonwealth	32,764	33,212	257,396	51,217
Preferred distributions	(1,997)	(1,997)	(5,991)	(5,991)
Net income attributable to Equity Commonwealth common shareholders	$ 30,767	$ 31,215	$ 251,405	$ 45,226
Weighted average common shares outstanding — basic (in shares)	121,845	124,089	122,504	124,068
Weighted average common shares outstanding — diluted (in shares)	122,851	125,175	123,389	125,194
Earnings per common share attributable to Equity Commonwealth common shareholders:
Basic (in dollars per share)	$ 0.25	$ 0.25	$ 2.05	$ 0.36
Diluted (in dollars per share)	0.25	0.25	2.04	0.36
Distributions declared per common share (in dollars per share)	$ 2.50	$ 0	$ 2.50	$ 0
Rental income
Revenues:
Total revenues	$ 34,138	$ 61,091	$ 112,898	$ 215,648
Tenant reimbursements and other income
Revenues:
Total revenues	$ 12,735	$ 16,707	$ 41,199	$ 53,300

CONDENSED CONSOLIDATED STATEM_2

CONDENSED CONSOLIDATED STATEMENTS OF OPERATIONS (Parenthetical) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Income Statement [Abstract]
Amortization of debt discounts, premiums and deferred financing fees	$ 559	$ 784	$ 2,005	$ 2,346

CONDENSED CONSOLIDATED STATEM_3

CONDENSED CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Statement of Comprehensive Income [Abstract]
Net income	$ 32,777	$ 33,224	$ 257,486	$ 51,235
Other comprehensive income (loss), net of tax:
Unrealized (loss) gain on derivative instruments	0	(19)	456	(280)
Unrealized gain on marketable securities	463	1,455	535	3,159
Total comprehensive income	33,240	34,660	258,477	54,114
Comprehensive income attributable to the noncontrolling interest	(13)	(12)	(90)	(18)
Total comprehensive income attributable to Equity Commonwealth	$ 33,227	$ 34,648	$ 258,387	$ 54,096

CONDENSED CONSOLIDATED STATEM_4

CONDENSED CONSOLIDATED STATEMENTS OF EQUITY - USD ($) $ in Thousands	Total	Cumulative Preferred Distributions	Common Shares	Cumulative Common Distributions	Additional Paid in Capital	Cumulative Net Income	Cumulative Other Comprehensive Loss	Noncontrolling Interest	Series D
Increase (Decrease) in Stockholders' Equity
Reclassification pursuant to change in accounting principle | Accounting Standards Update 2016-01						$ 1,902	$ (1,902)
Balance as of beginning of period (in shares) at Dec. 31, 2017									4,915,196
Balance as of beginning of period at Dec. 31, 2017	$ 3,300,495	$ (685,748)	$ 1,242	$ (3,111,868)	$ 4,380,313	2,596,259	(95)	$ 1,129	$ 119,263
Balance as of beginning of period (in shares) at Dec. 31, 2017	124,217,616		124,217,616
Increase (Decrease) in Stockholders' Equity
Net income	$ 257,486					257,396		90
Unrealized gain on derivative instruments	456						456
Unrealized gain on marketable securities	535						535
Repurchase of shares (in shares)			(3,027,557)
Repurchase of shares	(89,910)		$ (30)		(89,880)
Share-based compensation (in shares)			292,614
Share-based compensation	15,677		$ 3		14,686			988
Contributions	1							1
Distributions	(315,229)	(7,988)		(307,127)				(114)
Adjustment for noncontrolling interest					901			(901)
Balance as of end of period (in shares) at Sep. 30, 2018									4,915,196
Balance as of end of period at Sep. 30, 2018	$ 3,169,511	(693,736)	$ 1,215	(3,418,995)	4,306,020	2,855,557	(1,006)	1,193	$ 119,263
Balance as of end of period (in shares) at Sep. 30, 2018	121,482,673		121,482,673
Increase (Decrease) in Stockholders' Equity
Net income	$ 32,777
Unrealized gain on derivative instruments	0
Unrealized gain on marketable securities	463
Balance as of end of period (in shares) at Sep. 30, 2018									4,915,196
Balance as of end of period at Sep. 30, 2018	$ 3,169,511	$ (693,736)	$ 1,215	$ (3,418,995)	$ 4,306,020	$ 2,855,557	$ (1,006)	$ 1,193	$ 119,263
Balance as of end of period (in shares) at Sep. 30, 2018	121,482,673		121,482,673

CONDENSED CONSOLIDATED STATEM_5

CONDENSED CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017
CASH FLOWS FROM OPERATING ACTIVITIES:
Net income	$ 257,486	$ 51,235
Adjustments to reconcile net income to cash provided by operating activities:
Depreciation	31,144	58,178
Net amortization of debt discounts, premiums and deferred financing fees	2,005	2,346
Straight line rental income	(3,985)	(12,487)
Amortization of acquired real estate leases	2,015	7,073
Other amortization	4,834	7,970
Share-based compensation	15,677	16,307
Loss on asset impairment	12,087	19,714
Loss on marketable securities	4,987	0
Loss on early extinguishment of debt	6,403	266
Net gain on sale of properties	(253,025)	(44,670)
Loss on sale of real estate mortgage receivable	2,117	0
Change in assets and liabilities:
Rents receivable and other assets	(19,749)	(20,762)
Accounts payable, accrued expenses and other	(6,523)	(18,742)
Rent collected in advance	(4,926)	(3,610)
Cash provided by operating activities	50,547	62,818
CASH FLOWS FROM INVESTING ACTIVITIES:
Real estate improvements	(41,718)	(51,700)
Insurance proceeds received	1,443	4,000
Proceeds from sale of properties, net	954,334	699,289
Proceeds from sale of real estate mortgage receivable	5,599	0
Purchase of marketable securities	0	(276,238)
Proceeds from sale of marketable securities	23,933	0
Cash provided by investing activities	943,591	375,351
CASH FLOWS FROM FINANCING ACTIVITIES:
Repurchase and retirement of common shares	(89,910)	(209)
Payments on borrowings	(575,792)	(292,472)
Contributions from holders of noncontrolling interest	1	31
Distributions to preferred shareholders	(5,991)	(5,991)
Cash used in financing activities	(671,692)	(298,641)
Increase in cash, cash equivalents, and restricted cash	322,446	139,528
Cash, cash equivalents, and restricted cash at beginning of period	2,360,590	2,101,206
Cash, cash equivalents, and restricted cash at end of period	2,683,036	2,240,734
SUPPLEMENTAL CASH FLOW INFORMATION:
Interest paid	25,899	50,408
Taxes paid, net	2,377	730
NON-CASH INVESTING ACTIVITIES:
Increase (decrease) in accrued capital expenditures	1,569	(3,876)
NON-CASH FINANCING ACTIVITIES:
Total cash, cash equivalents, and restricted cash shown in the statements of cash flows	$ 2,360,590	$ 2,101,206

Business

Business	9 Months Ended
	Sep. 30, 2018
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Business	Business 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, in the United States. On November 10, 2016, the Company converted to what is commonly referred to as an umbrella partnership real estate investment trust, or UPREIT, 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 99.96% of the outstanding shares of beneficial interest, designated as units, in the Operating Trust (OP Units) as of September 30, 2018 , 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 September 30, 2018 , our portfolio consisted of 11 properties ( 19 buildings), with a combined 5.4 million square feet. As of September 30, 2018 , we had $2.9 billion of cash and cash equivalents and marketable securities.

Summary of Significant Accounti

Summary of Significant Accounting Policies	9 Months Ended
	Sep. 30, 2018
Accounting Policies [Abstract]
Summary of Significant Accounting Policies	Summary of Significant Accounting Policies Basis of Presentation The accompanying condensed consolidated financial statements of EQC have been prepared without audit. Certain information and footnote disclosures required by U.S. generally accepted accounting principles, or GAAP, for complete financial statements have been condensed or omitted. We believe the disclosures made are appropriate. The accompanying condensed consolidated financial statements should be read in conjunction with the consolidated financial statements and notes contained in our Annual Report on Form 10-K (Annual Report) for the year ended December 31, 2017 . Capitalized terms used, but not defined in this Quarterly Report, have the same meanings as in our Annual Report. In the opinion of our management, all adjustments, which include only normal recurring adjustments considered necessary for a fair presentation, have been included. All intercompany transactions and balances with or among our subsidiaries have been eliminated. Operating results for interim periods are not necessarily indicative of the results that may be expected for the full year. Reclassifications have been made to the prior years’ financial statements to conform to the current year’s presentation. The preparation of financial statements in conformity with GAAP requires us to make estimates and assumptions that affect reported amounts. Actual results could differ from those estimates. Significant estimates in the condensed consolidated financial statements include the allowance for doubtful accounts, purchase price allocations, useful lives of fixed assets and impairment of real estate and intangible assets. Share amounts are presented in whole numbers, except where noted. Recent Accounting Pronouncements In August 2018, the Financial Accounting Standards Board (FASB) issued Accounting Standards Update (ASU) 2018-13, Fair Value Measurement (Topic 820): Disclosure Framework - Changes to the Disclosure Requirements for Fair Value Measurement, which changes the fair value measurement disclosure requirements of FASB Accounting Standards Codification (ASC) 820. This update is effective for fiscal years beginning after December 15, 2019, and for interim periods within those fiscal years. We do not expect the adoption of ASU 2018-13 to have a material impact on our consolidated financial statements. In June 2018, the FASB issued ASU 2018-07, Improvements to Nonemployee Share-Based Payment Accounting, which simplifies the accounting for share-based payments granted to nonemployees for goods and services. This update is effective for fiscal years beginning after December 15, 2018, and for interim periods within those fiscal years. We do not expect the adoption of ASU 2018-07 to have a material impact on our consolidated financial statements. In May 2017, the FASB issued ASU 2017-09 Compensation-Stock Compensation (Topic 718): Scope of Modification Accounting. ASU 2017-09 is designed to provide clarity and reduce both (1) diversity in practice and (2) cost and complexity when applying the guidance in Topic 718, to a change to the terms or conditions of a share-based payment award. ASU 2017-09 is effective for fiscal years beginning after December 15, 2017. We adopted ASU 2017-09 on January 1, 2018, and the adoption did not have a material impact on our consolidated financial statements. In February 2017, the FASB issued ASU 2017-05 Clarifying the Scope of Asset Derecognition Guidance and Accounting for Partial Sales of Nonfinancial Assets. ASU 2017-05 is designed to provide guidance on how to recognize gain and losses on sales, including partial sales, of nonfinancial assets to noncustomers. We adopted ASU 2017-05 on January 1, 2018 and the adoption did not have a material impact on our consolidated financial statements. In 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. 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. We early adopted both ASU 2016-18 and ASU 2016-15 for the period ended December 31, 2017 and made the following reclassifications to the prior year's condensed consolidated statement of cash flows to conform to the current year's presentation (in thousands): Statement of Cash Flows for the Nine Months Ended September 30, 2017 Originally Reported Effect of Change As Adjusted Cash provided by operating activities $ 64,326 $ (1,508 ) $ 62,818 Cash used in investing activities 372,718 2,633 375,351 In June 2016, the FASB issued ASU 2016-13, Financial Instruments-Credit Losses (Topic 326): Measurement of Credit Losses on Financial Instruments, which requires more timely recognition of credit losses associated with financial assets. This update is effective for fiscal years beginning after December 15, 2019, and for interim periods within those fiscal years. Early adoption is permitted for fiscal years, and interim periods within those years, beginning after December 15, 2018. We are currently evaluating the impact, if any, the adoption of ASU 2016-13 will have on our consolidated financial statements. In February 2016, the FASB issued ASU 2016-02, Leases, which sets out the principles for the recognition, measurement, presentation and disclosure of leases for both parties to a contract (i.e., lessees and lessors). ASU 2016-02 requires lessees to apply a dual approach, classifying leases as either finance or operating leases based on the principle of whether or not the lease is effectively a financed purchase of the leased asset by the lessee. This classification will determine whether the lease expense is recognized based on an effective interest method or on a straight-line basis over the term of the lease. A lessee is also required to record a right-of-use asset and a lease liability for all leases with a term of greater than 12 months regardless of their classification. Leases with a term of 12 months or less will be accounted for similar to existing guidance for operating leases today. ASU 2016-02 supersedes previous leasing standards. ASU 2016-02 is effective for us for reporting periods beginning after December 15, 2018, with early adoption permitted. 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 July 2018, the FASB issued ASU 2018-11 to provide entities with relief from the costs of implementing certain aspects of ASU 2016-02. The amendment to the new leases standard includes a practical expedient that provides lessors an option not to separate lease and non-lease components when certain criteria are met and instead account for those components as a single component under the new leases standard. The amendment also provides a transition option that permits the application of the new guidance as of the adoption date rather than to all periods presented. We anticipate electing the practical expedient to account for both our lease and non-lease components as a single component under the leases standard and electing the new transition option. In January 2016, the FASB issued ASU 2016-01, Financial Instruments-Overall: Recognition and Measurement of Financial Assets and Financial Liabilities, related to certain aspects of recognition, measurement, presentation, and disclosure of financial instruments. ASU 2016-01 will require entities to measure their equity investments at fair value and recognize any changes in fair value in net income, with certain exceptions, rather than other comprehensive income. This update is effective for fiscal years beginning after December 15, 2017, including interim periods within those fiscal years. We adopted ASU 2016-01 on January 1, 2018 and reclassified a $1.9 million unrealized gain from cumulative other comprehensive loss to cumulative net income on our condensed consolidated balance sheet (see Note 8). In May 2014, the FASB issued ASU 2014-09, Revenue from Contracts with Customers. The objective of ASU 2014-09, as amended, is to establish a single comprehensive model for entities to use in accounting for revenue arising from contracts with customers and will supersede most of the existing revenue recognition guidance, including industry-specific guidance. The core principle is that an entity should recognize revenue to depict the transfer of promised goods or services to customers in an amount that reflects the consideration to which the entity expects to be entitled in exchange for those goods or services. In applying ASU 2014-09, companies will perform a five-step analysis of transactions to determine when and how revenue is recognized. ASU 2014-09 applies to all contracts with customers except those that are within the scope of other topics in the FASB’s ASC, and more particularly lease contracts with customers, which are a scope exception. ASU 2014-09 is effective for the first interim period within annual reporting periods beginning after December 15, 2017, with early adoption permitted. We adopted ASU 2014-09 on January 1, 2018 and the adoption did not have a material impact on our consolidated financial statements.

Real Estate Properties

Real Estate Properties	9 Months Ended
	Sep. 30, 2018
Real Estate [Abstract]
Real Estate Properties	Real Estate Properties During each of the nine month periods ended September 30, 2018 and 2017 , we made improvements, excluding tenant-funded improvements, to our properties totaling $43.3 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 condensed consolidated balance sheets. As of December 31, 2017, we classified 1600 Market Street as held for sale. This property was sold in February 2018. As of September 30, 2018 , we did not have any properties classified as held for sale. 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 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 nine months ended September 30, 2018 , we sold the following properties (dollars in thousands): Asset Date Sold Number of Number of Square Footage Gross Sales Price Gain on Sale Properties 1600 Market Street February 2018 1 1 825,968 $ 160,000 $ 54,599 600 West Chicago Avenue (1) February 2018 1 2 1,561,477 510,000 107,790 5073, 5075, & 5085 S. Syracuse Street March 2018 1 1 248,493 115,186 42,762 1601 Dry Creek Drive May 2018 1 1 552,865 68,500 26,979 777 East Eisenhower Parkway August 2018 1 1 290,530 29,500 5,321 8750 Bryn Mawr Avenue September 2018 1 2 636,078 141,000 15,609 6 8 4,115,411 $ 1,024,186 $ 253,060 (1) The sale of this property did not represent a strategic shift under ASC Topic 205. However, the sale does represent an individually significant disposition. The operating results of this property are included in continued operations for all periods presented through the date of sale. Net income for this property was $0.1 million and $2.4 million for the three months ended September 30, 2018 and 2017, respectively, and $110.4 million and $7.4 million for the nine months ended September 30, 2018 and 2017, respectively.

Marketable Securities

Marketable Securities	9 Months Ended
	Sep. 30, 2018
Investments, Debt and Equity Securities [Abstract]
Marketable Securities	Marketable Securities During the nine months ended September 30, 2018 , our marketable securities consisted of United States Treasury notes and common stock. The United States Treasury notes are classified as available-for-sale and mature in 2019. Available-for-sale securities are presented on our condensed consolidated balance sheets at fair value. Changes in values of the United States Treasury notes are recognized in accumulated other comprehensive loss. On January 1, 2018 we adopted ASU 2016-01 (see Note 2) and reclassified a $1.9 million unrealized gain from cumulative other comprehensive loss to cumulative net income on our condensed consolidated balance sheet. Changes in values of common stock are recognized in interest and other income, net on the condensed consolidated statements of operations. In March 2018, we sold all common stock we held and recognized a loss of $5.0 million in interest and other income, net during the nine months ended September 30, 2018 . Below is a summary of our marketable securities as of September 30, 2018 and December 31, 2017 (in thousands): September 30, 2018 December 31, 2017 Amortized Cost Unrealized Loss Estimated Fair Value Cost or Amortized Cost Unrealized Gain, Net Estimated Fair Value Marketable securities $ 249,844 $ (1,006 ) $ 248,838 $ 276,567 $ 361 $ 276,928 The unrealized losses on our United States Treasury notes were caused by interest rate increases. The contractual terms of those investments do not permit the issuer to settle the securities at a price less than the amortized cost bases of the investments. Because we do not intend to sell the investments and it is not more likely than not that we will be required to sell the investments before recovery of their amortized cost bases, which may be maturity, we do not consider those investments to be other-than-temporarily impaired at September 30, 2018 .

Indebtedness

Indebtedness	9 Months Ended
	Sep. 30, 2018
Debt Disclosure [Abstract]
Indebtedness	Indebtedness Unsecured Revolving Credit Facility and Term Loan: We are party to a credit agreement pursuant to which the lenders agreed to provide a $750.0 million unsecured revolving credit facility, a $200.0 million 5 -year term loan facility and a $200.0 million 7 -year term loan facility. 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. On May 4, 2018, we redeemed at par the total $400.0 million outstanding under our 5 -year and 7 -year term loans and recognized a loss on early extinguishment of debt of $1.5 million from the write off of unamortized deferred financing fees. 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. Prior to the redemption of the term loans, borrowings under the 5 -year term loan and 7 -year term loan, subject to certain exceptions, had interest rates of LIBOR rate plus a margin of 90 to 180 basis points for the 5 -year term loan and 140 to 235 basis points for the 7 -year term loan, in each case depending on our credit rating. 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 September 30, 2018 . As of September 30, 2018 , the interest rate payable under our revolving credit facility was 3.31% . As of September 30, 2018 , we had no balance outstanding and $750.0 million available under our revolving credit facility and the facility fee as of September 30, 2018 was 20 basis points. 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 September 30, 2018 , 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: At September 30, 2018 , we had senior unsecured notes of $250.0 million (excluding net discounts and unamortized deferred financing fees) maturing in 2020. On March 7, 2018, we redeemed at par all $175.0 million of our 5.75% senior unsecured notes due 2042 and recognized a loss on early extinguishment of debt of $4.9 million from the write off of unamortized deferred financing fees. Mortgage Notes Payable: At September 30, 2018 , two of our properties with an aggregate net book value of $51.7 million had secured mortgage notes totaling $31.6 million (including net premiums and unamortized deferred financing fees) maturing in 2021 and 2026.

Shareholders' Equity

Shareholders' Equity	9 Months Ended
	Sep. 30, 2018
Stockholders' Equity Note [Abstract]
Shareholders' Equity	Shareholders’ Equity Common Share Issuances: See Note 11 for information regarding equity issuances related to share-based compensation. Common Share Repurchases: On March 15, 2017, our Board of Trustees authorized the repurchase of up to $150.0 million of our outstanding common shares over the twelve month period following the date of authorization. In March 2018, this share repurchase authorization, of which $81.0 million was not utilized, expired. On March 14, 2018, our Board of Trustees authorized the repurchase of up to an additional $150.0 million of our outstanding common shares over the twelve month period following the date of authorization. During the nine months ended September 30, 2018 , we repurchased and retired 2,970,209 of our common shares, at a weighted average price of $29.67 per share, for a total investment of $88.1 million , of which $69.0 million was under the March 2017 authorization and $19.1 million was under the March 2018 authorization. The $130.9 million of remaining authorization available under our share repurchase program as of September 30, 2018 is scheduled to expire on March 14, 2019. During the nine months ended September 30, 2018 and 2017 , certain of our employees surrendered 57,348 and 6,694 common shares owned by them, respectively, to satisfy their statutory tax withholding obligations in connection with the vesting of such common shares. Common Share and Unit Distribution: On September 26, 2018 , our Board of Trustees declared a special, one-time cash distribution of $2.50 per common share/unit to shareholders/unitholders of record on October 9, 2018. On October 23, 2018, we paid this distribution to such shareholders/unitholders in the aggregate amount of $304.7 million . Preferred Share Distributions: In 2018 , our Board of Trustees declared distributions on our series D preferred shares to date as follows: Declaration Date Record Date Payment Date Series D Dividend Per Share January 12, 2018 January 30, 2018 February 15, 2018 $ 0.40625 April 11, 2018 April 27, 2018 May 15, 2018 $ 0.40625 July 12, 2018 July 30, 2018 August 15, 2018 $ 0.40625 September 26, 2018 October 29, 2018 November 15, 2018 $ 0.40625

Noncontrolling Interest

Noncontrolling Interest	9 Months Ended
	Sep. 30, 2018
Noncontrolling Interest [Abstract]
Noncontrolling Interest	Noncontrolling Interest Noncontrolling interest represents the portion of the units in the Operating Trust not beneficially owned by the Company. An OP Unit and a share of our common stock have essentially the same economic characteristics. Distributions with respect to OP Units will generally mirror distributions with respect to the Company’s common shares. Unitholders (other than the Company) generally have the right, commencing six months from the date of issuance of such OP Units, to cause the Operating Trust to redeem their OP Units in exchange for cash or, at the option of the Company, common shares of the Company on a one -for-one basis. As sole trustee, the Company will have the sole discretion to elect whether the redemption right will be satisfied by the Company in cash or the Company’s common shares. As a result, the Noncontrolling interest is classified as permanent equity. As of September 30, 2018 , the portion of the Operating Trust not beneficially owned by the Company is in the form of OP Units and LTIP Units (see Note 11 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 nine months ended September 30, 2018 : Common Shares OP Units and LTIP Units Total Outstanding at January 1, 2018 124,217,616 42,520 124,260,136 Repurchase of shares (2,970,209 ) — (2,970,209 ) Restricted share, time-based LTIP Unit grants and vested restricted stock units, net of forfeitures 235,266 3,200 238,466 Outstanding at September 30, 2018 121,482,673 45,720 121,528,393 Noncontrolling ownership interest in the Operating Trust 0.04 % The carrying value of the Noncontrolling interest is allocated based on the number of OP Units and LTIP Units in proportion to the number of OP Units and LTIP Units plus the number of common shares. We adjust the noncontrolling interest balance at the end of each period to reflect the noncontrolling partners’ interest in the net assets of the Operating Trust. Net income is allocated to the Noncontrolling interest in the Operating Trust based on the weighted average ownership percentage during the period. Equity Commonwealth’s weighted average ownership interest in the Operating Trust was 99.96% and 99.96% for the three and nine months ended September 30, 2018 , respectively.

Cumulative Other Comprehensive

Cumulative Other Comprehensive Loss	9 Months Ended
	Sep. 30, 2018
Accumulated Other Comprehensive Income (Loss), Net of Tax [Abstract]
Cumulative Other Comprehensive Loss	Cumulative Other Comprehensive Loss The following table presents the amounts recognized in cumulative other comprehensive loss for the three and nine months ended September 30, 2018 (in thousands): Unrealized Loss on Marketable Securities Balance as of July 1, 2018 $ (1,469 ) Other comprehensive income before reclassifications 463 Net current period other comprehensive income 463 Balance as of September 30, 2018 $ (1,006 ) Unrealized Loss on Derivative Instruments Unrealized Gain (Loss) on Marketable Securities Total Balance as of January 1, 2018 $ (456 ) $ 361 $ (95 ) Amounts reclassified from cumulative other comprehensive loss to cumulative net income pursuant to a change in accounting principle — (1,902 ) (1,902 ) Other comprehensive income before reclassifications 84 535 619 Amounts reclassified from cumulative other comprehensive loss to net income 372 — 372 Net current period other comprehensive income 456 535 991 Balance as of September 30, 2018 $ — $ (1,006 ) $ (1,006 ) The following table presents reclassifications out of cumulative other comprehensive loss for the three and nine months ended September 30, 2018 (in thousands): Amounts Reclassified from Cumulative Other Comprehensive Loss to Net Income Details about Cumulative Other Comprehensive Loss Components Three Months Ended September 30, 2018 Nine Months Ended September 30, 2018 Affected Line Items in the Statement of Operations Interest rate cap contract $ — $ 79 Interest expense Interest rate cap contract — 293 Interest and other income, net $ — $ 372

Income Taxes

Income Taxes	9 Months Ended
	Sep. 30, 2018
Income Tax Disclosure [Abstract]
Income Taxes	Income Taxes We have elected to be taxed as 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 a sufficient amount of our taxable income to our shareholders and meet other requirements for qualifying as a REIT. We are also subject to certain state and local taxes without regard to our REIT status. Our provision for income taxes consists of the following (in thousands): Three Months Ended September 30, Nine Months Ended September 30, 2018 2017 2018 2017 Current: State and local $ (65 ) $ (335 ) $ (2,616 ) $ (550 ) Federal — — — (5 ) Income tax expense (65 ) (335 ) $ (2,616 ) $ (555 ) The tax expense recorded in the current period is primarily the result of the taxable gains from sales of properties during the nine months ended September 30, 2018 .

Derivative Instruments

Derivative Instruments	9 Months Ended
	Sep. 30, 2018
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Derivative Instruments	Derivative Instruments Risk Management Objective of Using Derivatives We are exposed to certain risks relating to our ongoing business operations, including the effect of changes in interest rates. We may use derivative financial instruments, including interest rate swaps, caps, options, floors and other interest rate derivative contracts, to hedge all or a portion of 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. Cash Flow Hedges of Interest Rate Risk Our objectives in using interest rate derivatives are to add stability to interest expense and to manage our exposure to interest rate movements. To accomplish this objective, we use interest rate caps as part of our interest rate risk management strategy. Interest rate caps designated as cash flow hedges involve the receipt of variable-rate amounts if interest rates rise above the cap strike rate. The effective portion of changes in the fair value of derivatives designated and that qualify as cash flow hedges is recorded in cumulative other comprehensive loss and is subsequently reclassified into earnings in the period that the hedged forecasted transaction affects earnings. During 2018, such derivatives were used to hedge the variable cash flows associated with variable-rate debt. The ineffective portion of the change in fair value of the derivatives is recognized directly in earnings. On May 8, 2018, we terminated an interest rate cap that had a LIBOR strike price of 2.50% , a notional amount of $400.0 million and a maturity date of March 1, 2019. We recognized $0.3 million of expense in interest and other income, net on the condensed consolidated statement of operations for the nine months ended September 30, 2018 related to the early termination of the interest rate cap agreement. As of September 30, 2018 , we do not have any outstanding interest rate derivatives designated as cash flow hedges of interest rate risk. The table below presents the fair value of derivative financial instruments as well as classification on the condensed consolidated balance sheets as of September 30, 2018 and December 31, 2017 (amounts in thousands): Fair Value as of Interest Rate Derivative Designated as Hedging Instrument Balance Sheet Location September 30, December 31, Interest rate cap Other assets $ — $ 17 The table below details the location in the financial statements of the gain or loss recognized on interest rate derivatives designated as cash flow hedges for the three and nine months ended September 30, 2018 and 2017 (amounts in thousands): Three Months Ended September 30, Nine Months Ended September 30, 2018 2017 2018 2017 Amount of (loss) gain recognized in cumulative other comprehensive loss (effective portion) $ — $ (32 ) $ 84 $ (300 ) Amount of loss reclassified from cumulative other comprehensive loss into interest expense (effective portion) — 13 79 20 Amount of loss recognized in income (ineffective portion and amount excluded from effectiveness testing) — — 293 —

Share-Based Compensation

Share-Based Compensation	9 Months Ended
	Sep. 30, 2018
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Share-Based Compensation	Share-Based Compensation Recipients of the Company’s restricted shares have the same voting rights as any other common shareholder. During the period of restriction, the Company’s unvested restricted shareholders are eligible to receive dividend payments on their shares at the same rate and on the same date as any other common shareholder. The restricted shares are service based awards and vest over a four -year period. Recipients of the Company’s restricted stock units (RSUs) are entitled to receive dividends with respect to the common shares underlying the RSUs if and when the RSUs are earned, at which time the recipient will be entitled to receive an amount in cash equal to the aggregate amount of cash dividends that would have been paid in respect of the common shares underlying the recipient’s earned RSUs had such common shares been issued to the recipient on the first day of the performance period. To the extent that an award does not vest, the dividends related to unvested RSUs will be forfeited. The RSUs are market-based awards with a service condition and recipients may earn RSUs based on the Company’s total shareholder return (TSR) relative to the TSRs of the companies that comprise the NAREIT Office Index over a three -year performance period. Following the end of the three -year performance period, the number of earned awards will be determined. The earned awards vest in two tranches with 50% of the earned award vesting following the end of the performance period on the date the Compensation Committee of our Board of Trustees (the Committee) determines the level of achievement of the performance metric and the remaining 50% of the earned award vesting approximately one year thereafter, subject to the grant recipient’s continued employment. Compensation expense for the RSUs is determined using a Monte Carlo simulation model and is recognized ratably from the grant date to the vesting date of each tranche. LTIP Units are a class of beneficial interests in the Operating Trust that may be issued to employees, officers or trustees of the Operating Trust, the Company or their subsidiaries (LTIP Units). Time-based LTIP Units have the same general characteristics as restricted shares and market-based LTIP Units have the same general characteristics as RSUs. Each LTIP Unit will convert automatically into an OP Unit on a one -for-one basis when the LTIP Unit becomes vested and its capital account is equalized with the per-unit capital account of the OP Units. Holders of LTIP Units generally will be entitled to receive the same per-unit distributions as the other outstanding OP Units in the Operating Trust, except that market-based LTIP Units will not participate in distributions until expiration of the applicable performance period, at which time any earned market-based LTIP Units generally will become entitled to receive a catch-up distribution for the periods prior to such time. 2018 Equity Award Activity On June 20, 2018, in accordance with the Company’s compensation plan for independent Trustees, the Committee awarded each of the nine independent Trustees $0.1 million in restricted shares or time-based LTIP Units as part of their compensation for the 2018-2019 year of service on the Board of Trustees. These awards equated to 3,200 shares or time-based LTIP Units per Trustee, for a total of 25,600 shares and 3,200 time-based LTIP Units, valued at $31.25 per share and unit, the closing price of our common shares on the New York Stock Exchange (NYSE) on that day. These shares and time-based LTIP Units vest one year after the date of the award. On January 29, 2018, the Committee approved a grant of 125,409 restricted shares and 254,615 RSUs at target ( 634,628 RSUs at maximum) to the Company’s officers, certain employees and to Mr. Zell, the Chairman of our Board of Trustees, as part of their compensation for fiscal year 2017. The restricted shares granted on January 29, 2018 were valued at $29.78 per share, the closing price of our common shares on the NYSE on that day. The assumptions and fair value for the RSUs granted during the nine months ended September 30, 2018 are included in the following table on a per share basis. 2018 Fair value of RSUs granted $ 37.13 Expected term (years) 4 Expected volatility — Expected dividend yield 1.68 % Risk-free rate 2.26 % 2017 Equity Award Activity On June 20, 2017, in accordance with the Company’s compensation plan for independent Trustees, the Committee awarded each of the nine independent Trustees $0.1 million in restricted shares or time-based LTIP Units as part of their compensation for the 2017-2018 year of service on the Board of Trustees. These awards equated to 3,156 shares or time-based LTIP Units per Trustee, for a total of 25,248 shares and 3,156 time-based LTIP Units, valued at $31.69 per share and unit, the closing price of our common shares on the NYSE on that day. These shares and time-based LTIP Units vested on June 20, 2018. On January 24, 2017, the Committee approved a grant of 39,364 time-based LTIP Units, 79,924 market-based LTIP Units at target ( 199,211 market-based LTIP Units at maximum), 76,424 restricted shares and 155,168 RSUs at target ( 386,756 RSUs at maximum) to the Company’s officers, certain employees and to Mr. Zell, the Chairman of our Board of Trustees, as part of their compensation for fiscal year 2016. The restricted shares and time-based LTIP Units were valued at $31.47 per share and unit, the closing price of our common shares on the NYSE on the grant date. The RSUs and market-based LTIP Units were valued at $39.81 per share and unit, their fair value on the grant date. Outstanding Equity Awards As of September 30, 2018 , the estimated future compensation expense for all unvested restricted shares and time-based LTIP Units was $7.5 million . Compensation expense for the restricted share and time-based LTIP Unit awards is being recognized on a straight-line basis over the requisite service period for each separately vesting portion of the award. The weighted average period over which the future compensation expense will be recorded for the restricted shares and time-based LTIP units is approximately 2.3 years. As of September 30, 2018 , the estimated future compensation expense for all unvested RSUs and market-based LTIP Units was $15.1 million . The weighted average period over which the future compensation expense will be recorded for the RSUs and market-based LTIP Units is approximately 2.2 years. During the three months ended September 30, 2018 and 2017 , we recorded $5.2 million and $5.6 million , respectively, and during the nine months ended September 30, 2018 and 2017 , we recorded $15.7 million and $16.3 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. The $15.7 million of compensation expense recorded during the nine months ended September 30, 2018 includes $0.4 million of accelerated vesting due to a staffing reduction. Forfeitures are recognized as they occur. At September 30, 2018 , 839,470 shares/units remain available for issuance under the Equity Commonwealth 2015 Omnibus Incentive Plan, as amended.

Fair Value of Assets and Liabil

Fair Value of Assets and Liabilities	9 Months Ended
	Sep. 30, 2018
Fair Value Disclosures [Abstract]
Fair Value of Assets and Liabilities	Fair Value of Assets and Liabilities The table below presents assets measured at fair value during 2018 , categorized by the level of inputs used in the valuation of the assets (dollars in thousands): Fair Value at September 30, 2018 Using Quoted Prices in Active Markets for Identical Assets Significant Other Observable Inputs Significant Unobservable Inputs Description Total (Level 1) (Level 2) (Level 3) Recurring Fair Value Measurements: Marketable securities $ 248,838 $ 248,838 $ — $ — Properties Held and Used As part of our office repositioning strategy adopted by our Board of Trustees, and pursuant to our accounting policy, in 2018, we evaluated the recoverability of the carrying values of each of the real estate assets that comprised our portfolio and determined that due to the shortening of the expected periods of ownership as a result of the office repositioning strategy and current estimates of market value less estimated costs to sell, it was necessary to reduce the net book value of a portion of the real estate assets in our portfolio to their estimated fair values. We anticipate the potential disposition of certain properties prior to the end of their remaining useful lives. As a result, in the first quarter of 2018, we recorded an impairment charge related to 777 East Eisenhower Parkway and 97 Newberry Road of $12.1 million in accordance with our impairment analysis procedures. We determined this impairment based on independent third party broker information, which are level 3 inputs according to the fair value hierarchy established in ASC 820. We reduced the aggregate carrying value of these properties from $41.8 million to their estimated fair value less estimated costs to sell of $29.7 million . We evaluated each of our properties and determined there were no additional valuation adjustments necessary at September 30, 2018 . Financial Instruments In addition to the assets described in the above table, our financial instruments include our cash and cash equivalents, real estate mortgage receivable, restricted cash, marketable securities, senior unsecured debt and mortgage notes payable. At September 30, 2018 and December 31, 2017 , the fair value of these additional financial instruments were not materially different from their carrying values, except as follows (in thousands): September 30, 2018 December 31, 2017 Carrying Amount Fair Value Carrying Amount Fair Value Senior unsecured debt and mortgage notes payable $ 281,149 $ 289,047 $ 856,940 $ 874,280 The fair values of our senior notes are based on quoted market prices (level 2 inputs) and the fair values of our mortgage notes payable are based on estimates using discounted cash flow analyses and currently prevailing interest rates adjusted by credit risk spreads (level 3 inputs). Other financial instruments that potentially subject us to concentrations of credit risk consist principally of rents receivable; however, as of September 30, 2018 , no single tenant of ours is responsible for more than 7.5% of our total annualized rents, other than one tenant that is responsible for 13.9% of our total annualized rents.

Earnings Per Common Share

Earnings Per Common Share	9 Months Ended
	Sep. 30, 2018
Earnings Per Share [Abstract]
Earnings Per Common Share	Earnings Per Common Share The following table sets forth the computation of basic and diluted earnings per share (amounts in thousands except per share amounts): Three Months Ended September 30, Nine Months Ended September 30, 2018 2017 2018 2017 Numerator for earnings per common share - basic: Net income $ 32,777 $ 33,224 $ 257,486 $ 51,235 Net income attributable to noncontrolling interest (13 ) (12 ) (90 ) (18 ) Preferred distributions (1,997 ) (1,997 ) (5,991 ) (5,991 ) Numerator for net income per share - basic $ 30,767 $ 31,215 $ 251,405 $ 45,226 Numerator for earnings per common share - diluted: Net income $ 32,777 $ 33,224 $ 257,486 $ 51,235 Net income attributable to noncontrolling interests (13 ) — (90 ) — Preferred distributions (1,997 ) (1,997 ) (5,991 ) (5,991 ) Numerator for net income per share - diluted $ 30,767 $ 31,227 $ 251,405 $ 45,244 Denominator for earnings per common share - basic and diluted: Weighted average number of common shares outstanding - basic (1) 121,845 124,089 122,504 124,068 RSUs (2) 894 983 795 1,023 LTIP Units (3) 112 103 90 103 Weighted average number of common shares outstanding - diluted 122,851 125,175 123,389 125,194 Net income per common share attributable to Equity Commonwealth common shareholders: Basic $ 0.25 $ 0.25 $ 2.05 $ 0.36 Diluted $ 0.25 $ 0.25 $ 2.04 $ 0.36 Anti-dilutive securities: Effect of Series D preferred shares; 6 1/2% cumulative convertible (4) 2,363 2,363 2,363 2,363 Effect of LTIP Units 45 — 43 — Effect of OP Units (5) 1 — 1 — (1) The three months ended September 30, 2018 and 2017 , includes 362 and 0 weighted-average, unvested, earned RSUs, respectively, and the nine months ended September 30, 2018 and 2017 , includes 344 and 0 weighted-average, unvested, earned RSUs, respectively. (2) Represents weighted-average number of common shares that would have been issued if the quarter-end was the measurement date for RSUs. (3) Represents the weighted-average dilutive shares issuable from LTIP Units if the quarter-end was the measurement date for the periods shown. (4) The Series D preferred shares are excluded from the diluted earnings per share calculation because including the Series D preferred shares would also require that the preferred distributions be added back to net income, resulting in anti-dilution. (5) Beneficial interests in the Operating Trust.

Segment Information

Segment Information	9 Months Ended
	Sep. 30, 2018
Segment Reporting [Abstract]
Segment Information	Segment Information Our primary business is the ownership and operation of office properties, and we currently have one reportable segment. More than 90% of our revenues for the nine months ended September 30, 2018 were from office properties.

Related Person Transactions

Related Person Transactions	9 Months Ended
	Sep. 30, 2018
Related Party Transactions [Abstract]
Related Person Transactions	Related Person Transactions The following discussion includes a description of our related person transactions for the nine months ended September 30, 2018 and 2017 . Two North Riverside Plaza Joint Venture Limited Partnership: We have 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 three months ended September 30, 2018 and 2017 , we recognized expense of $0.2 million and $0.2 million , respectively, and during the nine months ended September 30, 2018 and 2017 , we recognized expense of $0.6 million and $0.6 million , respectively, pursuant to the 20 th /21 st Floor Office Lease and the related storage space.

Subsequent Events

Subsequent Events	9 Months Ended
	Sep. 30, 2018
Subsequent Events [Abstract]
Subsequent Events	Subsequent Events On September 26, 2018 , we announced that our Board of Trustees declared a special, one-time cash distribution of $2.50 per common share/unit to shareholders/unitholders of record on October 9, 2018. On October 23, 2018, we paid this distribution to such shareholders/unitholders in the aggregate amount of $304.7 million (see Note 6).

Summary of Significant Accoun_2

Summary of Significant Accounting Policies (Policies)	9 Months Ended
	Sep. 30, 2018
Accounting Policies [Abstract]
Basis of Presentation	Basis of Presentation The accompanying condensed consolidated financial statements of EQC have been prepared without audit. Certain information and footnote disclosures required by U.S. generally accepted accounting principles, or GAAP, for complete financial statements have been condensed or omitted. We believe the disclosures made are appropriate. The accompanying condensed consolidated financial statements should be read in conjunction with the consolidated financial statements and notes contained in our Annual Report on Form 10-K (Annual Report) for the year ended December 31, 2017 . Capitalized terms used, but not defined in this Quarterly Report, have the same meanings as in our Annual Report. In the opinion of our management, all adjustments, which include only normal recurring adjustments considered necessary for a fair presentation, have been included. All intercompany transactions and balances with or among our subsidiaries have been eliminated. Operating results for interim periods are not necessarily indicative of the results that may be expected for the full year. Reclassifications have been made to the prior years’ financial statements to conform to the current year’s presentation. The preparation of financial statements in conformity with GAAP requires us to make estimates and assumptions that affect reported amounts. Actual results could differ from those estimates. Significant estimates in the condensed consolidated financial statements include the allowance for doubtful accounts, purchase price allocations, useful lives of fixed assets and impairment of real estate and intangible assets. Share amounts are presented in whole numbers, except where noted.
Recent Accounting Pronouncements	Recent Accounting Pronouncements In August 2018, the Financial Accounting Standards Board (FASB) issued Accounting Standards Update (ASU) 2018-13, Fair Value Measurement (Topic 820): Disclosure Framework - Changes to the Disclosure Requirements for Fair Value Measurement, which changes the fair value measurement disclosure requirements of FASB Accounting Standards Codification (ASC) 820. This update is effective for fiscal years beginning after December 15, 2019, and for interim periods within those fiscal years. We do not expect the adoption of ASU 2018-13 to have a material impact on our consolidated financial statements. In June 2018, the FASB issued ASU 2018-07, Improvements to Nonemployee Share-Based Payment Accounting, which simplifies the accounting for share-based payments granted to nonemployees for goods and services. This update is effective for fiscal years beginning after December 15, 2018, and for interim periods within those fiscal years. We do not expect the adoption of ASU 2018-07 to have a material impact on our consolidated financial statements. In May 2017, the FASB issued ASU 2017-09 Compensation-Stock Compensation (Topic 718): Scope of Modification Accounting. ASU 2017-09 is designed to provide clarity and reduce both (1) diversity in practice and (2) cost and complexity when applying the guidance in Topic 718, to a change to the terms or conditions of a share-based payment award. ASU 2017-09 is effective for fiscal years beginning after December 15, 2017. We adopted ASU 2017-09 on January 1, 2018, and the adoption did not have a material impact on our consolidated financial statements. In February 2017, the FASB issued ASU 2017-05 Clarifying the Scope of Asset Derecognition Guidance and Accounting for Partial Sales of Nonfinancial Assets. ASU 2017-05 is designed to provide guidance on how to recognize gain and losses on sales, including partial sales, of nonfinancial assets to noncustomers. We adopted ASU 2017-05 on January 1, 2018 and the adoption did not have a material impact on our consolidated financial statements. In 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. 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. We early adopted both ASU 2016-18 and ASU 2016-15 for the period ended December 31, 2017 and made the following reclassifications to the prior year's condensed consolidated statement of cash flows to conform to the current year's presentation (in thousands): Statement of Cash Flows for the Nine Months Ended September 30, 2017 Originally Reported Effect of Change As Adjusted Cash provided by operating activities $ 64,326 $ (1,508 ) $ 62,818 Cash used in investing activities 372,718 2,633 375,351 In June 2016, the FASB issued ASU 2016-13, Financial Instruments-Credit Losses (Topic 326): Measurement of Credit Losses on Financial Instruments, which requires more timely recognition of credit losses associated with financial assets. This update is effective for fiscal years beginning after December 15, 2019, and for interim periods within those fiscal years. Early adoption is permitted for fiscal years, and interim periods within those years, beginning after December 15, 2018. We are currently evaluating the impact, if any, the adoption of ASU 2016-13 will have on our consolidated financial statements. In February 2016, the FASB issued ASU 2016-02, Leases, which sets out the principles for the recognition, measurement, presentation and disclosure of leases for both parties to a contract (i.e., lessees and lessors). ASU 2016-02 requires lessees to apply a dual approach, classifying leases as either finance or operating leases based on the principle of whether or not the lease is effectively a financed purchase of the leased asset by the lessee. This classification will determine whether the lease expense is recognized based on an effective interest method or on a straight-line basis over the term of the lease. A lessee is also required to record a right-of-use asset and a lease liability for all leases with a term of greater than 12 months regardless of their classification. Leases with a term of 12 months or less will be accounted for similar to existing guidance for operating leases today. ASU 2016-02 supersedes previous leasing standards. ASU 2016-02 is effective for us for reporting periods beginning after December 15, 2018, with early adoption permitted. 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 July 2018, the FASB issued ASU 2018-11 to provide entities with relief from the costs of implementing certain aspects of ASU 2016-02. The amendment to the new leases standard includes a practical expedient that provides lessors an option not to separate lease and non-lease components when certain criteria are met and instead account for those components as a single component under the new leases standard. The amendment also provides a transition option that permits the application of the new guidance as of the adoption date rather than to all periods presented. We anticipate electing the practical expedient to account for both our lease and non-lease components as a single component under the leases standard and electing the new transition option. In January 2016, the FASB issued ASU 2016-01, Financial Instruments-Overall: Recognition and Measurement of Financial Assets and Financial Liabilities, related to certain aspects of recognition, measurement, presentation, and disclosure of financial instruments. ASU 2016-01 will require entities to measure their equity investments at fair value and recognize any changes in fair value in net income, with certain exceptions, rather than other comprehensive income. This update is effective for fiscal years beginning after December 15, 2017, including interim periods within those fiscal years. We adopted ASU 2016-01 on January 1, 2018 and reclassified a $1.9 million unrealized gain from cumulative other comprehensive loss to cumulative net income on our condensed consolidated balance sheet (see Note 8). In May 2014, the FASB issued ASU 2014-09, Revenue from Contracts with Customers. The objective of ASU 2014-09, as amended, is to establish a single comprehensive model for entities to use in accounting for revenue arising from contracts with customers and will supersede most of the existing revenue recognition guidance, including industry-specific guidance. The core principle is that an entity should recognize revenue to depict the transfer of promised goods or services to customers in an amount that reflects the consideration to which the entity expects to be entitled in exchange for those goods or services. In applying ASU 2014-09, companies will perform a five-step analysis of transactions to determine when and how revenue is recognized. ASU 2014-09 applies to all contracts with customers except those that are within the scope of other topics in the FASB’s ASC, and more particularly lease contracts with customers, which are a scope exception. ASU 2014-09 is effective for the first interim period within annual reporting periods beginning after December 15, 2017, with early adoption permitted. We adopted ASU 2014-09 on January 1, 2018 and the adoption did not have a material impact on our consolidated financial statements.
Properties Held For Sale	We classify all properties that meet the criteria outlined in the Property, Plant and Equipment Topic of the FASB ASC as held for sale on our condensed consolidated balance sheets.
Marketable Securities	our marketable securities consisted of United States Treasury notes and common stock. The United States Treasury notes are classified as available-for-sale and mature in 2019. Available-for-sale securities are presented on our condensed consolidated balance sheets at fair value. Changes in values of the United States Treasury notes are recognized in accumulated other comprehensive loss. On January 1, 2018 we adopted ASU 2016-01 (see Note 2) and reclassified a $1.9 million unrealized gain from cumulative other comprehensive loss to cumulative net income on our condensed consolidated balance sheet. Changes in values of common stock are recognized in interest and other income, net on the condensed consolidated statements of operations.

Summary of Significant Accoun_3

Summary of Significant Accounting Policies (Tables)	9 Months Ended
	Sep. 30, 2018
Accounting Policies [Abstract]
New Accounting Pronouncement, Early Adoption	We early adopted both ASU 2016-18 and ASU 2016-15 for the period ended December 31, 2017 and made the following reclassifications to the prior year's condensed consolidated statement of cash flows to conform to the current year's presentation (in thousands): Statement of Cash Flows for the Nine Months Ended September 30, 2017 Originally Reported Effect of Change As Adjusted Cash provided by operating activities $ 64,326 $ (1,508 ) $ 62,818 Cash used in investing activities 372,718 2,633 375,351

Real Estate Properties (Tables)

Real Estate Properties (Tables)	9 Months Ended
	Sep. 30, 2018
Real Estate [Abstract]
Disclosure of Long Lived Assets Held-for-sale	As of September 30, 2018 , we did not have any properties classified as held for sale. 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 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 Sold and Income Statement Information for Properties Disposed of	During the nine months ended September 30, 2018 , we sold the following properties (dollars in thousands): Asset Date Sold Number of Number of Square Footage Gross Sales Price Gain on Sale Properties 1600 Market Street February 2018 1 1 825,968 $ 160,000 $ 54,599 600 West Chicago Avenue (1) February 2018 1 2 1,561,477 510,000 107,790 5073, 5075, & 5085 S. Syracuse Street March 2018 1 1 248,493 115,186 42,762 1601 Dry Creek Drive May 2018 1 1 552,865 68,500 26,979 777 East Eisenhower Parkway August 2018 1 1 290,530 29,500 5,321 8750 Bryn Mawr Avenue September 2018 1 2 636,078 141,000 15,609 6 8 4,115,411 $ 1,024,186 $ 253,060 (1) The sale of this property did not represent a strategic shift under ASC Topic 205. However, the sale does represent an individually significant disposition. The operating results of this property are included in continued operations for all periods presented through the date of sale. Net income for this property was $0.1 million and $2.4 million for the three months ended September 30, 2018 and 2017, respectively, and $110.4 million and $7.4 million for the nine months ended September 30, 2018 and 2017, respectively.

Marketable Securities (Tables)

Marketable Securities (Tables)	9 Months Ended
	Sep. 30, 2018
Investments, Debt and Equity Securities [Abstract]
Summary of Marketable Securities	Below is a summary of our marketable securities as of September 30, 2018 and December 31, 2017 (in thousands): September 30, 2018 December 31, 2017 Amortized Cost Unrealized Loss Estimated Fair Value Cost or Amortized Cost Unrealized Gain, Net Estimated Fair Value Marketable securities $ 249,844 $ (1,006 ) $ 248,838 $ 276,567 $ 361 $ 276,928

Shareholders' Equity (Tables)

Shareholders' Equity (Tables)	9 Months Ended
	Sep. 30, 2018
Stockholders' Equity Note [Abstract]
Schedule of Declared Distributions	In 2018 , our Board of Trustees declared distributions on our series D preferred shares to date as follows: Declaration Date Record Date Payment Date Series D Dividend Per Share January 12, 2018 January 30, 2018 February 15, 2018 $ 0.40625 April 11, 2018 April 27, 2018 May 15, 2018 $ 0.40625 July 12, 2018 July 30, 2018 August 15, 2018 $ 0.40625 September 26, 2018 October 29, 2018 November 15, 2018 $ 0.40625

Noncontrolling Interest (Tables

Noncontrolling Interest (Tables)	9 Months Ended
	Sep. 30, 2018
Noncontrolling Interest [Abstract]
Schedule of Issued and Outstanding Common Shares	The following table presents the changes in Equity Commonwealth’s issued and outstanding common shares and units for the nine months ended September 30, 2018 : Common Shares OP Units and LTIP Units Total Outstanding at January 1, 2018 124,217,616 42,520 124,260,136 Repurchase of shares (2,970,209 ) — (2,970,209 ) Restricted share, time-based LTIP Unit grants and vested restricted stock units, net of forfeitures 235,266 3,200 238,466 Outstanding at September 30, 2018 121,482,673 45,720 121,528,393 Noncontrolling ownership interest in the Operating Trust 0.04 %
Schedule of Issued and Outstanding Units	The following table presents the changes in Equity Commonwealth’s issued and outstanding common shares and units for the nine months ended September 30, 2018 : Common Shares OP Units and LTIP Units Total Outstanding at January 1, 2018 124,217,616 42,520 124,260,136 Repurchase of shares (2,970,209 ) — (2,970,209 ) Restricted share, time-based LTIP Unit grants and vested restricted stock units, net of forfeitures 235,266 3,200 238,466 Outstanding at September 30, 2018 121,482,673 45,720 121,528,393 Noncontrolling ownership interest in the Operating Trust 0.04 %

Cumulative Other Comprehensiv_2

Cumulative Other Comprehensive Loss (Tables)	9 Months Ended
	Sep. 30, 2018
Accumulated Other Comprehensive Income (Loss), Net of Tax [Abstract]
Schedule of Amounts Recognized in Cumulative Other Comprehensive Loss	The following table presents the amounts recognized in cumulative other comprehensive loss for the three and nine months ended September 30, 2018 (in thousands): Unrealized Loss on Marketable Securities Balance as of July 1, 2018 $ (1,469 ) Other comprehensive income before reclassifications 463 Net current period other comprehensive income 463 Balance as of September 30, 2018 $ (1,006 ) Unrealized Loss on Derivative Instruments Unrealized Gain (Loss) on Marketable Securities Total Balance as of January 1, 2018 $ (456 ) $ 361 $ (95 ) Amounts reclassified from cumulative other comprehensive loss to cumulative net income pursuant to a change in accounting principle — (1,902 ) (1,902 ) Other comprehensive income before reclassifications 84 535 619 Amounts reclassified from cumulative other comprehensive loss to net income 372 — 372 Net current period other comprehensive income 456 535 991 Balance as of September 30, 2018 $ — $ (1,006 ) $ (1,006 )
Schedule of Reclassifications Out of Cumulative Other Comprehensive Loss	The following table presents reclassifications out of cumulative other comprehensive loss for the three and nine months ended September 30, 2018 (in thousands): Amounts Reclassified from Cumulative Other Comprehensive Loss to Net Income Details about Cumulative Other Comprehensive Loss Components Three Months Ended September 30, 2018 Nine Months Ended September 30, 2018 Affected Line Items in the Statement of Operations Interest rate cap contract $ — $ 79 Interest expense Interest rate cap contract — 293 Interest and other income, net $ — $ 372

Income Taxes (Tables)

Income Taxes (Tables)	9 Months Ended
	Sep. 30, 2018
Income Tax Disclosure [Abstract]
Schedule of Provision for Income Taxes	Our provision for income taxes consists of the following (in thousands): Three Months Ended September 30, Nine Months Ended September 30, 2018 2017 2018 2017 Current: State and local $ (65 ) $ (335 ) $ (2,616 ) $ (550 ) Federal — — — (5 ) Income tax expense (65 ) (335 ) $ (2,616 ) $ (555 )

Derivative Instruments (Tables)

Derivative Instruments (Tables)	9 Months Ended
	Sep. 30, 2018
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Schedule of Fair Value of Derivative Financial Instruments	The table below presents the fair value of derivative financial instruments as well as classification on the condensed consolidated balance sheets as of September 30, 2018 and December 31, 2017 (amounts in thousands): Fair Value as of Interest Rate Derivative Designated as Hedging Instrument Balance Sheet Location September 30, December 31, Interest rate cap Other assets $ — $ 17
Schedule of Gain or Loss Recognized on Interest Rate Derivatives Designated as Cash Flow Hedges	The table below details the location in the financial statements of the gain or loss recognized on interest rate derivatives designated as cash flow hedges for the three and nine months ended September 30, 2018 and 2017 (amounts in thousands): Three Months Ended September 30, Nine Months Ended September 30, 2018 2017 2018 2017 Amount of (loss) gain recognized in cumulative other comprehensive loss (effective portion) $ — $ (32 ) $ 84 $ (300 ) Amount of loss reclassified from cumulative other comprehensive loss into interest expense (effective portion) — 13 79 20 Amount of loss recognized in income (ineffective portion and amount excluded from effectiveness testing) — — 293 —

Share-Based Compensation (Table

Share-Based Compensation (Tables)	9 Months Ended
	Sep. 30, 2018
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Summary of Assumptions and Fair Values for RSUs Granted in the Period	The assumptions and fair value for the RSUs granted during the nine months ended September 30, 2018 are included in the following table on a per share basis. 2018 Fair value of RSUs granted $ 37.13 Expected term (years) 4 Expected volatility — Expected dividend yield 1.68 % Risk-free rate 2.26 %

Fair Value of Assets and Liab_2

Fair Value of Assets and Liabilities (Tables)	9 Months Ended
	Sep. 30, 2018
Fair Value Disclosures [Abstract]
Schedule of Assets Measured at Fair Value	The table below presents assets measured at fair value during 2018 , categorized by the level of inputs used in the valuation of the assets (dollars in thousands): Fair Value at September 30, 2018 Using Quoted Prices in Active Markets for Identical Assets Significant Other Observable Inputs Significant Unobservable Inputs Description Total (Level 1) (Level 2) (Level 3) Recurring Fair Value Measurements: Marketable securities $ 248,838 $ 248,838 $ — $ —
Schedule of Fair Value and Carrying Value of Financial Instruments	At September 30, 2018 and December 31, 2017 , the fair value of these additional financial instruments were not materially different from their carrying values, except as follows (in thousands): September 30, 2018 December 31, 2017 Carrying Amount Fair Value Carrying Amount Fair Value Senior unsecured debt and mortgage notes payable $ 281,149 $ 289,047 $ 856,940 $ 874,280

Earnings Per Common Share (Tabl

Earnings Per Common Share (Tables)	9 Months Ended
	Sep. 30, 2018
Earnings Per Share [Abstract]
Schedule of Computation of Basic and Diluted Earnings Per Share	The following table sets forth the computation of basic and diluted earnings per share (amounts in thousands except per share amounts): Three Months Ended September 30, Nine Months Ended September 30, 2018 2017 2018 2017 Numerator for earnings per common share - basic: Net income $ 32,777 $ 33,224 $ 257,486 $ 51,235 Net income attributable to noncontrolling interest (13 ) (12 ) (90 ) (18 ) Preferred distributions (1,997 ) (1,997 ) (5,991 ) (5,991 ) Numerator for net income per share - basic $ 30,767 $ 31,215 $ 251,405 $ 45,226 Numerator for earnings per common share - diluted: Net income $ 32,777 $ 33,224 $ 257,486 $ 51,235 Net income attributable to noncontrolling interests (13 ) — (90 ) — Preferred distributions (1,997 ) (1,997 ) (5,991 ) (5,991 ) Numerator for net income per share - diluted $ 30,767 $ 31,227 $ 251,405 $ 45,244 Denominator for earnings per common share - basic and diluted: Weighted average number of common shares outstanding - basic (1) 121,845 124,089 122,504 124,068 RSUs (2) 894 983 795 1,023 LTIP Units (3) 112 103 90 103 Weighted average number of common shares outstanding - diluted 122,851 125,175 123,389 125,194 Net income per common share attributable to Equity Commonwealth common shareholders: Basic $ 0.25 $ 0.25 $ 2.05 $ 0.36 Diluted $ 0.25 $ 0.25 $ 2.04 $ 0.36 Anti-dilutive securities: Effect of Series D preferred shares; 6 1/2% cumulative convertible (4) 2,363 2,363 2,363 2,363 Effect of LTIP Units 45 — 43 — Effect of OP Units (5) 1 — 1 — (1) The three months ended September 30, 2018 and 2017 , includes 362 and 0 weighted-average, unvested, earned RSUs, respectively, and the nine months ended September 30, 2018 and 2017 , includes 344 and 0 weighted-average, unvested, earned RSUs, respectively. (2) Represents weighted-average number of common shares that would have been issued if the quarter-end was the measurement date for RSUs. (3) Represents the weighted-average dilutive shares issuable from LTIP Units if the quarter-end was the measurement date for the periods shown. (4) The Series D preferred shares are excluded from the diluted earnings per share calculation because including the Series D preferred shares would also require that the preferred distributions be added back to net income, resulting in anti-dilution. (5) Beneficial interests in the Operating Trust.

Business (Details)

Business (Details) ft² in Millions, $ in Billions	Sep. 30, 2018USD ($)ft²buildingproperty
Noncontrolling Interest [Line Items]
Cash, cash equivalents, and short-term investments | $	$ 2.9
Consolidated Properties
Noncontrolling Interest [Line Items]
Number of real estate properties | property	11
Number of buildings | building	19
Square footage (in sqft) | ft²	5.4
Operating Trust
Noncontrolling Interest [Line Items]
Noncontrolling interest, ownership percentage by parent	99.96%

Summary of Significant Accoun_4

Summary of Significant Accounting Policies (Details) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Jan. 01, 2018	Dec. 31, 2017
New Accounting Pronouncement, Early Adoption [Line Items]
Cash provided by operating activities	$ 50,547	$ 62,818
Cash used in investing activities	$ 943,591	375,351
Accounting Standards Update 2016-18 and Accounting Standards Update 2016-15
New Accounting Pronouncement, Early Adoption [Line Items]
Cash provided by operating activities		62,818
Cash used in investing activities		375,351
Accounting Standards Update 2016-01 | Cumulative other comprehensive loss
New Accounting Pronouncement, Early Adoption [Line Items]
Cumulative effect of new accounting principle in period of adoption			$ (1,900)	$ (1,902)
Accounting Standards Update 2016-01 | Cumulative net income
New Accounting Pronouncement, Early Adoption [Line Items]
Cumulative effect of new accounting principle in period of adoption			$ 1,900	$ 1,902
Originally Reported | Accounting Standards Update 2016-18 and Accounting Standards Update 2016-15
New Accounting Pronouncement, Early Adoption [Line Items]
Cash provided by operating activities		64,326
Cash used in investing activities		372,718
Effect of Change | Accounting Standards Update 2016-18 and Accounting Standards Update 2016-15
New Accounting Pronouncement, Early Adoption [Line Items]
Cash provided by operating activities		(1,508)
Cash used in investing activities		$ 2,633

Real Estate Properties - Narrat

Real Estate Properties - Narrative (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017
Real Estate [Abstract]
Real estate improvements	$ 43.3	$ 43.3

Real Estate Properties - Summar

Real Estate Properties - Summary of Balance Sheet Information for all Properties Classified as Held for sale (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Liabilities related to properties held for sale	$ 0	$ 1,840
Held-for-sale
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Real estate properties		76,066
Rents receivable		13,270
Other assets, net		8,352
Assets held for sale		97,688
Accounts payable and accrued expenses		1,021
Rent collected in advance		408
Security deposits		411
Liabilities related to properties held for sale		$ 1,840

Real Estate Properties - Summ_2

Real Estate Properties - Summary of Properties Sold (Details) $ in Thousands	1 Months Ended					3 Months Ended		9 Months Ended
	Sep. 30, 2018USD ($)ft²buildingproperty	Aug. 31, 2018USD ($)ft²buildingproperty	May 31, 2018USD ($)ft²buildingproperty	Mar. 31, 2018USD ($)ft²buildingproperty	Feb. 28, 2018USD ($)ft²buildingproperty	Sep. 30, 2018USD ($)	Sep. 30, 2017USD ($)	Sep. 30, 2018USD ($)ft²buildingproperty	Sep. 30, 2017USD ($)
Real Estate Properties [Line Items]
Gain on Sale						$ 20,877	$ 25,080	$ 253,025	$ 44,670
Disposed of by Sale
Real Estate Properties [Line Items]
Number of Properties | property								6
Number of Buildings | building								8
Square Footage (in sqft) | ft²								4,115,411
Gross Sales Price								$ 1,024,186
Gain on Sale								253,060
Net income included in continued operations						$ 100	$ 2,400	$ 110,400	$ 7,400
Disposed of by Sale | 1600 Market Street
Real Estate Properties [Line Items]
Number of Properties | property					1
Number of Buildings | building					1
Square Footage (in sqft) | ft²					825,968
Gross Sales Price					$ 160,000
Gain on Sale					$ 54,599
Disposed of by Sale | 600 West Chicago Avenue
Real Estate Properties [Line Items]
Number of Properties | property					1
Number of Buildings | building					2
Square Footage (in sqft) | ft²					1,561,477
Gross Sales Price					$ 510,000
Gain on Sale					$ 107,790
Disposed of by Sale | 5073, 5075, & 5085 S. Syracuse Street
Real Estate Properties [Line Items]
Number of Properties | property				1
Number of Buildings | building				1
Square Footage (in sqft) | ft²				248,493
Gross Sales Price				$ 115,186
Gain on Sale				$ 42,762
Disposed of by Sale | 1601 Dry Creek Drive
Real Estate Properties [Line Items]
Number of Properties | property			1
Number of Buildings | building			1
Square Footage (in sqft) | ft²			552,865
Gross Sales Price			$ 68,500
Gain on Sale			$ 26,979
Disposed of by Sale | 777 East Eisenhower Parkway
Real Estate Properties [Line Items]
Number of Properties | property		1
Number of Buildings | building		1
Square Footage (in sqft) | ft²		290,530
Gross Sales Price		$ 29,500
Gain on Sale		$ 5,321
Disposed of by Sale | 8750 Bryn Mawr Avenue
Real Estate Properties [Line Items]
Number of Properties | property	1
Number of Buildings | building	2
Square Footage (in sqft) | ft²	636,078
Gross Sales Price	$ 141,000
Gain on Sale	$ 15,609

Marketable Securities (Details)

Marketable Securities (Details) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2018	Jan. 01, 2018	Dec. 31, 2017
Debt Securities, Available-for-sale [Line Items]
Recognized loss on sale of common stock included in interest and other income	$ 5,000
Estimated Fair Value	248,838		$ 276,928
Marketable securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	249,844		276,567
Unrealized Loss	(1,006)
Unrealized Gain, Net			361
Estimated Fair Value	$ 248,838		276,928
Accounting Standards Update 2016-01 | Cumulative other comprehensive loss
Debt Securities, Available-for-sale [Line Items]
Cumulative effect of new accounting principle in period of adoption		$ (1,900)	(1,902)
Accounting Standards Update 2016-01 | Cumulative net income
Debt Securities, Available-for-sale [Line Items]
Cumulative effect of new accounting principle in period of adoption		$ 1,900	$ 1,902

Indebtedness - Narrative (Detai

Indebtedness - Narrative (Details)	Sep. 30, 2018USD ($)property	May 04, 2018USD ($)	Mar. 07, 2018USD ($)	Sep. 30, 2018USD ($)property	Sep. 30, 2017USD ($)	Sep. 30, 2018USD ($)optionproperty	Sep. 30, 2017USD ($)	Dec. 31, 2017USD ($)
Debt Instrument [Line Items]
Loss on early extinguishment of debt				$ 0	$ 203,000	$ 6,403,000	$ 266,000
Senior unsecured notes, net	$ 250,000,000			250,000,000		250,000,000
Real estate properties, net	767,297,000			767,297,000		767,297,000		$ 1,296,893,000
Mortgage notes payable, net	31,643,000			31,643,000		$ 31,643,000		$ 32,594,000
Loan facility, 5-year term | LIBOR
Debt Instrument [Line Items]
Basis spread on variable rate						1.15%
Loan facility, 5-year term | LIBOR | Minimum
Debt Instrument [Line Items]
Basis spread on variable rate						0.90%
Loan facility, 5-year term | LIBOR | Maximum
Debt Instrument [Line Items]
Basis spread on variable rate						1.80%
Loan facility, 7-year term
Debt Instrument [Line Items]
Term loan amount outstanding	200,000,000			200,000,000		$ 200,000,000
Loan facility, 7-year term | LIBOR
Debt Instrument [Line Items]
Basis spread on variable rate						1.55%
Loan facility, 7-year term | LIBOR | Minimum
Debt Instrument [Line Items]
Basis spread on variable rate						1.40%
Loan facility, 7-year term | LIBOR | Maximum
Debt Instrument [Line Items]
Basis spread on variable rate						2.35%
Revolving credit facility and term loans
Debt Instrument [Line Items]
Additional increases in maximum amount of borrowings available	$ 1,150,000,000			$ 1,150,000,000		$ 1,150,000,000
5.75% Senior Unsecured Notes Due 2042
Debt Instrument [Line Items]
Debt redeemed at par			$ 175,000,000
Loss on early extinguishment of debt			$ 4,900,000
Debt instrument, interest rate			5.75%
Mortgage notes
Debt Instrument [Line Items]
Number of real estate properties secured by mortgage | property	2			2		2
Real estate properties, net	$ 51,700,000			$ 51,700,000		$ 51,700,000
Mortgage notes payable, net	31,600,000			31,600,000		$ 31,600,000
Revolving credit facility
Debt Instrument [Line Items]
Interest rate payable percentage						3.31%
Amount outstanding	0			0		$ 0
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 spread on variable rate						1.05%
Revolving credit facility | LIBOR | Minimum
Debt Instrument [Line Items]
Basis spread on variable rate						0.875%
Revolving credit facility | LIBOR | Maximum
Debt Instrument [Line Items]
Basis spread on variable rate						1.55%
Revolving credit facility | Base rate | Minimum
Debt Instrument [Line Items]
Basis spread on variable rate						0.00%
Revolving credit facility | Base rate | Maximum
Debt Instrument [Line Items]
Basis spread on variable rate						0.55%
Revolving credit facility | Unsecured revolving credit facility
Debt Instrument [Line Items]
Maximum borrowing capacity	750,000,000			750,000,000		$ 750,000,000
Debt instrument, number of extension options | option						2
Debt instrument, extension option term						6 months
Debt instrument, extension option fee, percent						0.075%
Amount available for borrowing	$ 750,000,000			750,000,000		$ 750,000,000
Commitment fee percentage	0.20%
Revolving credit facility | Loan facility, 5-year term
Debt Instrument [Line Items]
Term loan amount outstanding	$ 200,000,000			200,000,000		$ 200,000,000
Debt instrument, term						5 years
Revolving credit facility | Loan facility, 7-year term
Debt Instrument [Line Items]
Term loan amount outstanding	200,000,000			200,000,000		$ 200,000,000
Debt instrument, term						7 years
Revolving credit facility | Loan facility, 5-year and 7-year term
Debt Instrument [Line Items]
Debt redeemed at par		$ 400,000,000
Revolving credit facility | Loan facility, 5-year and 7-year term | Line of Credit
Debt Instrument [Line Items]
Loss on early extinguishment of debt		$ 1,500,000
Letter of credit | Unsecured revolving credit facility
Debt Instrument [Line Items]
Maximum borrowing capacity	$ 100,000,000			$ 100,000,000		$ 100,000,000

Shareholders' Equity - Narrativ

Shareholders' Equity - Narrative (Details) - USD ($)	Oct. 23, 2018	Oct. 09, 2018	Mar. 14, 2018	Mar. 15, 2017	Mar. 31, 2018	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Class of Stock [Line Items]
Stock repurchase program, period in force			12 months
Common stock, dividend declared (in dollars per share)						$ 2.50	$ 0	$ 2.50	$ 0
Subsequent event
Class of Stock [Line Items]
Common stock, dividend declared (in dollars per share)		$ 2.5
Common stock, cash dividends paid	$ 304,700,000
Common Shares
Class of Stock [Line Items]
Stock repurchase program, authorized amount (up to)				$ 150,000,000
Stock repurchase program, period in force				12 months
Stock repurchase program, amount not utilized					$ 81,000,000
Stock repurchase program, additional authorized amount (up to)			$ 150,000,000
Stock repurchased and retired during period (in shares)								2,970,209
Stock repurchased and retired during period, price per share (in dollars per share)								$ 29.67
Stock repurchased and retired during period, value			$ 19,100,000	$ 69,000,000				$ 88,100,000
Stock repurchase program, remaining authorized repurchase amount						$ 130,900,000		$ 130,900,000
Number of shares surrendered to satisfy tax withholding obligations (in shares)								57,348	6,694

Shareholders' Equity - Schedule

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

Noncontrolling Interest (Detail

Noncontrolling Interest (Details) - shares	3 Months Ended	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2018
Noncontrolling Interest [Abstract]
Common stock, conversion term		6 months
Common stock, conversion basis (in shares)		1
Operating Trust
Noncontrolling Interest [Line Items]
Noncontrolling interest, weighted average ownership percentage by parent	99.96%	99.96%

Noncontrolling Interest - Commo

Noncontrolling Interest - Common Shares and Units Activity (Details)	9 Months Ended
	Sep. 30, 2018shares
Increase (Decrease) in Stockholders' Equity
Ownership percentage by noncontrolling owners	0.04%
Common Shares
Increase (Decrease) in Stockholders' Equity
Outstanding, beginning balance (in shares)	124,217,616
Repurchase of shares (in shares)	(2,970,209)
Restricted share, time-based LTIP Unit grants and vested restricted stock units, net of forfeitures (in shares)	235,266
Outstanding, ending balance (in shares)	121,482,673
OP Units and LTIP Units | Noncontrolling Interest
Increase (Decrease) in Stockholders' Equity
Outstanding, beginning balance (in shares)	42,520
Repurchase of shares (in shares)	0
Restricted share, time-based LTIP Unit grants and vested restricted stock units, net of forfeitures (in shares)	3,200
Outstanding, ending balance (in shares)	45,720
Common Stock, LTIP Units, and OP Units
Increase (Decrease) in Stockholders' Equity
Outstanding, beginning balance (in shares)	124,260,136
Repurchase of shares (in shares)	(2,970,209)
Restricted share, time-based LTIP Unit grants and vested restricted stock units, net of forfeitures (in shares)	238,466
Outstanding, ending balance (in shares)	121,528,393

Cumulative Other Comprehensiv_3

Cumulative Other Comprehensive Loss - Schedule of Amounts Recognized in Cumulative Other Comprehensive Loss (Details) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2018
Amounts recognized in cumulative other comprehensive income (Loss) by component
Balance as of beginning of period		$ 3,300,495
Amounts reclassified from cumulative other comprehensive loss to cumulative net income pursuant to a change in accounting principle		(1,902)
Other comprehensive income before reclassifications		619
Amounts reclassified from cumulative other comprehensive [loss] to net income [(loss)]		372
Net current period other comprehensive income		991
Balance as of end of period	$ 3,169,511	3,169,511
Unrealized Loss on Derivative Instruments
Amounts recognized in cumulative other comprehensive income (Loss) by component
Balance as of beginning of period		(456)
Amounts reclassified from cumulative other comprehensive loss to cumulative net income pursuant to a change in accounting principle		0
Other comprehensive income before reclassifications		84
Amounts reclassified from cumulative other comprehensive [loss] to net income [(loss)]		372
Net current period other comprehensive income		456
Balance as of end of period	0	0
Unrealized Gain (Loss) on Marketable Securities
Amounts recognized in cumulative other comprehensive income (Loss) by component
Balance as of beginning of period	(1,469)	361
Amounts reclassified from cumulative other comprehensive loss to cumulative net income pursuant to a change in accounting principle		(1,902)
Other comprehensive income before reclassifications	463	535
Amounts reclassified from cumulative other comprehensive [loss] to net income [(loss)]		0
Net current period other comprehensive income	463	535
Balance as of end of period	(1,006)	(1,006)
AOCI Including Portion Attributable to Noncontrolling Interest
Amounts recognized in cumulative other comprehensive income (Loss) by component
Balance as of beginning of period		(95)
Balance as of end of period	$ (1,006)	$ (1,006)

Cumulative Other Comprehensiv_4

Cumulative Other Comprehensive Loss - Schedule of Reclassifications Out of Cumulative Other Comprehensive Loss (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Reclassifications out of cumulative other comprehensive income (loss)
Interest expense	$ 5,085	$ 11,510	$ 21,550	$ 41,387
Interest and other income, net	(12,626)	$ (7,596)	(31,074)	$ (17,987)
Amount reclassified			(372)
Amounts Reclassified from Cumulative Other Comprehensive Loss to Net Income
Reclassifications out of cumulative other comprehensive income (loss)
Amount reclassified	0		372
Amounts Reclassified from Cumulative Other Comprehensive Loss to Net Income | Interest rate cap
Reclassifications out of cumulative other comprehensive income (loss)
Interest expense	0		79
Interest and other income, net	$ 0		$ 293

Income Taxes - Schedule of Prov

Income Taxes - Schedule of Provision for Income Taxes (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Current:
State and local	$ (65)	$ (335)	$ (2,616)	$ (550)
Federal	0	0	0	(5)
Income tax expense	$ (65)	$ (335)	$ (2,616)	$ (555)

Derivative Instruments - Narrat

Derivative Instruments - Narrative (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	May 08, 2018
Derivative [Line Items]
Interest expense	$ 5,085,000	$ 11,510,000	$ 21,550,000	$ 41,387,000
LIBOR | Interest rate cap
Derivative [Line Items]
Derivative, cap interest rate, percent					2.50%
Notional amount					$ 400,000,000
Interest expense			$ 300,000

Derivative Instruments - Schedu

Derivative Instruments - Schedule of Fair Value of Derivative Financial Instruments (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Interest rate cap | Cash flow hedging | Designated as hedging instrument | Other assets
Derivatives, Fair Value [Line Items]
Interest Rate Derivative Designated as Hedging Instrument	$ 0	$ 17

Derivative Instruments - Sche_2

Derivative Instruments - Schedule of Gain or Loss Recognized on Interest Rate Derivatives Designated as Cash Flow Hedges (Details) - Cash flow hedging - Designated as hedging instrument - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Derivative Instruments, Gain (Loss) [Line Items]
Amount of (loss) gain recognized in cumulative other comprehensive loss (effective portion)	$ 0	$ (32)	$ 84	$ (300)
Amount of loss reclassified from cumulative other comprehensive loss into interest expense (effective portion)	0	13	79	20
Amount of loss recognized in income (ineffective portion and amount excluded from effectiveness testing)	$ 0	$ 0	$ 293	$ 0

Share-Based Compensation - Narr

Share-Based Compensation - Narrative (Details) $ / shares in Units, $ in Millions	Jun. 20, 2018USD ($)trustee$ / sharesshares	Jan. 29, 2018$ / sharesshares	Jun. 20, 2017USD ($)trustee$ / sharesshares	Jan. 24, 2017$ / sharesshares	Sep. 30, 2018USD ($)shares	Sep. 30, 2017USD ($)	Sep. 30, 2018USD ($)tranche$ / sharesshares	Sep. 30, 2017USD ($)
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of independent trustees | trustee	9		9
Compensation expense, accelerated vesting due to a staffing reduction | $							$ 0.4
Share award plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common shares available for issuance (in shares)					839,470		839,470
General and Administrative Expense | Share award plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Compensation expense | $					$ 5.2	$ 5.6	$ 15.7	$ 16.3
Restricted shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period					4 years
Number of equity awards granted (in shares)	25,600	125,409	25,248	76,424
Granted shares value (in dollars per share) | $ / shares		$ 29.78
RSUs
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period					3 years
Number of tranches | tranche							2
Granted shares value (in dollars per share) | $ / shares							$ 37.13
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 period							1 year
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)		254,615		155,168
RSUs at maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of equity awards granted (in shares)		634,628		386,756
LTIP Units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Conversion basis (in shares)					1		1
Time-based LTIP units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of equity awards granted (in shares)	3,200		3,156	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
Market-based LTIP units at maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of equity awards granted (in shares)				199,211
Restricted shares and time-based LTIP Units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period	1 year
Share based compensation amount | $	$ 0.1		$ 0.1
Exercise price (in dollars per share) | $ / shares	$ 31.25		$ 31.69	$ 31.47
Estimated future compensation expense for unvested shares | $					$ 7.5		$ 7.5
Weighted average period over which compensation expense will be recorded							2 years 3 months 18 days
Restricted shares and time-based LTIP Units | Independent Trustee
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of equity awards granted (in shares)	3,200		3,156
RSUs and market-based LTIP units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Exercise price (in dollars per share) | $ / shares				$ 39.81
Estimated future compensation expense for unvested shares | $					$ 15.1		$ 15.1
Weighted average period over which compensation expense will be recorded							2 years 2 months 12 days

Share-Based Compensation - Summ

Share-Based Compensation - Summary of Assumptions and Fair Values for Restricted Stock Units Granted in the Period (Details) - RSUs	9 Months Ended
	Sep. 30, 2018$ / shares
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions and Methodology [Abstract]
Fair value of RSUs granted (in dollars per share)	$ 37.13
Expected term	4 years
Expected volatility	0.00%
Expected dividend yield	1.68%
Risk-free rate	2.26%

Fair Value of Assets and Liab_3

Fair Value of Assets and Liabilities - Schedule of Assets and Liabilities Measure at Fair Value (Details) - Recurring - Marketable securities $ in Thousands	Sep. 30, 2018USD ($)
Recurring Fair Value Measurements:
Assets, fair value	$ 248,838
Quoted Prices in Active Markets for Identical Assets (Level 1)
Recurring Fair Value Measurements:
Assets, fair value	248,838
Significant Other Observable Inputs (Level 2)
Recurring Fair Value Measurements:
Assets, fair value	0
Significant Unobservable Inputs (Level 3)
Recurring Fair Value Measurements:
Assets, fair value	$ 0

Fair Value of Assets and Liab_4

Fair Value of Assets and Liabilities - Narrative (Details) $ in Thousands	3 Months Ended			9 Months Ended
	Sep. 30, 2018USD ($)	Mar. 31, 2018USD ($)	Sep. 30, 2017USD ($)	Sep. 30, 2018USD ($)	Sep. 30, 2017USD ($)	Dec. 31, 2017USD ($)
Fair value of assets and liabilities
Loss on asset impairment	$ 0	$ 12,100	$ 0	$ 12,087	$ 19,714
Real estate properties, net	$ 767,297			$ 767,297		$ 1,296,893
Total rents | Customer concentration
Fair value of assets and liabilities
Concentration risk, benchmark percentage				0.075
Total rents | Customer concentration | Tenant
Fair value of assets and liabilities
Concentration risk, percent				13.90%
Real Estate Held-for-sale | Significant Unobservable Inputs (Level 3) | Market Approach Valuation Technique
Fair value of assets and liabilities
Assets, fair value		29,700
Fair Value, Measurements, Nonrecurring | Disposition Plan | Carrying Amount
Fair value of assets and liabilities
Real estate properties, net		$ 41,800

Fair Value of Assets and Liab_5

Fair Value of Assets and Liabilities - Schedule of Fair Value and Carrying Value of Financial Instruments (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Carrying Amount
Fair value of financial instruments
Senior unsecured debt and mortgage notes payable	$ 281,149	$ 856,940
Fair Value
Fair value of financial instruments
Senior unsecured debt and mortgage notes payable	$ 289,047	$ 874,280

Earnings Per Common Share - Sch

Earnings Per Common Share - Schedule of Computation of Basic and Diluted Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Numerator for earnings per common share - basic:
Net income	$ 32,777	$ 33,224	$ 257,486	$ 51,235
Net income attributable to noncontrolling interest	(13)	(12)	(90)	(18)
Preferred distributions	(1,997)	(1,997)	(5,991)	(5,991)
Net income attributable to Equity Commonwealth common shareholders	30,767	31,215	251,405	45,226
Numerator for earnings per common share - diluted:
Net income	32,777	33,224	257,486	51,235
Net income attributable to noncontrolling interests	(13)	0	(90)	0
Preferred distributions	(1,997)	(1,997)	(5,991)	(5,991)
Numerator for net income per share - diluted	$ 30,767	$ 31,227	$ 251,405	$ 45,244
Denominator for earnings per common share - basic and diluted:
Weighted average number of common shares outstanding — basic (in shares)	121,845	124,089	122,504	124,068
Weighted average number of common shares outstanding — diluted (in shares)	122,851	125,175	123,389	125,194
Net income per common share attributable to Equity Commonwealth common shareholders:
Basic (in dollars per share)	$ 0.25	$ 0.25	$ 2.05	$ 0.36
Diluted (in dollars per share)	$ 0.25	$ 0.25	$ 2.04	$ 0.36
Series D
Anti-dilutive securities:
Effect of anti-dilutive securities (in shares)	2,363	2,363	2,363	2,363
Preferred shares, dividend yield			6.50%	6.50%
LTIP Units
Anti-dilutive securities:
Effect of anti-dilutive securities (in shares)	45	0	43	0
OP Units
Anti-dilutive securities:
Effect of anti-dilutive securities (in shares)	1	0	1	0
RSUs
Denominator for earnings per common share - basic and diluted:
Weighted average number of common shares outstanding, dilutive adjustment (in shares)	894	983	795	1,023
LTIP Units
Denominator for earnings per common share - basic and diluted:
Weighted average number of common shares outstanding, dilutive adjustment (in shares)	112	103	90	103

Earnings Per Common Share - S_2

Earnings Per Common Share - Schedule of Computation of Basic and Diluted Earnings Per Share (Footnote) (Details) - shares	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Earnings Per Share [Abstract]
Weighted average number of shares, restricted stock units, unvested (in shares)	362,000	0	344,000	0

Segment Information - Narrative

Segment Information - Narrative (Details)	9 Months Ended
	Sep. 30, 2018segment
Segment Reporting [Abstract]
Number of reportable segments	1
Percentage of revenue from office properties (more than)	90.00%

Related Person Transactions - N

Related Person Transactions - Narrative (Details) - Two North Riverside Plaza Joint Venture Limited Partnership $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018USD ($)	Sep. 30, 2017USD ($)	Sep. 30, 2018USD ($)option	Sep. 30, 2017USD ($)
Related Party Transaction [Line Items]
Lease term			5 years
Number of renewal options of lease arrangement | option			1
Renewal term of lease arrangement			5 years
Tenant improvements	$ 0.7		$ 0.7
Lease termination period duration			30 days
Expenses from transactions with related party	$ 0.2	$ 0.2	$ 0.6	$ 0.6

Subsequent Events - Narrative (

Subsequent Events - Narrative (Details) - USD ($) $ / shares in Units, $ in Millions	Oct. 23, 2018	Oct. 09, 2018	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Subsequent Event [Line Items]
Common stock, dividend declared (in dollars per share)			$ 2.50	$ 0	$ 2.50	$ 0
Subsequent event
Subsequent Event [Line Items]
Common stock, dividend declared (in dollars per share)		$ 2.5
Common stock, cash dividends paid	$ 304.7