The Combination Fair Value of Assets Acquired and Liabilities Assumed (Details)			3 Months Ended			12 Months Ended
	Jul. 25, 2017 USD ($)	Jul. 18, 2017 USD ($)	Jun. 30, 2018 USD ($)	Dec. 31, 2017 USD ($)	Sep. 30, 2017 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Noncontrolling interests in consolidated subsidiaries						$ (5,550,000)
Proceeds from Collection of Notes Receivable						0	$ 50,934,000	$ 0
Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustment, Bargain Purchase, Reduction Of Gain			$ (7,600,000)	$ (3,400,000)	$ 27,800,000	(7,606,000)	24,376,000	$ 0
JBG Companies [Member]
Fair value of common shares and OP Units purchase consideration		$ 1,224,885,000
Cash		20,573,000
Total consideration paid		1,245,458,000
Land and improvements		338,072,000
Building and improvements		609,156,000
Construction in progress, including land		699,800,000
Leasehold improvements and equipment		7,890,000
Real estate		1,654,918,000
Cash		104,529,000
Restricted cash		13,460,000
Investments in and advances to unconsolidated real estate ventures		241,142,000
Identified intangible assets		138,371,000
Notes receivable (1)		50,934,000
Identified intangible liabilities		(8,687,000)
Mortgages payable assumed (2)		(768,523,000)
Capital lease obligations assumed (3)		(33,543,000)
Lease assumption liabilities (4)		(48,127,000)
Deferred tax liability (5)		(18,610,000)
Other liabilities acquired, net		(60,048,000)
Noncontrolling interests in consolidated subsidiaries		(3,588,000)
Net assets acquired		1,262,228,000
Gain on bargain purchase (6)		$ 16,770,000
Proceeds from Collection of Notes Receivable							$ 50,900,000
Capital Leased Assets, Number of Units		2.0
Business Combination, Recognized Indentifiable Acquired and Liabilities Assumed, Lease Liabilities, Future Payments		$ 14,000,000
Business Combination, Recognized Indentifiable Acquired and Liabilities Assumed, Lease Liabilities, Relocation of Tenants		$ 34,100,000
Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustment, Real Estate Investments, Unconsolidated Real Estate And Other Joint Ventures						468,000
Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustment, Lease Liabilities						(4,700,000)
Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustment, Current Liabilities, Other						$ 2,400,000
Reston, Virginia [Member] | JBG Companies [Member]
Capital Leased Assets, Number of Units	1.0
Payments to Acquire Land	$ 19,500,000