Organization and Basis of Presentation Pro Forma Information (Details) - USD ($)	3 Months Ended			9 Months Ended
	Sep. 30, 2018	Dec. 31, 2017	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Transaction and other costs	$ 4,126,000		$ 104,095,000	$ 12,134,000	$ 115,173,000
Total revenue			160,428,000		481,314,000
Net loss attributable to common shareholders			2,283,000		(13,741,000)
Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustment, Bargain Purchase, Reduction Of Gain	0	$ 3,400,000	27,771,000	(7,606,000)	27,771,000
JBG Companies
Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustment, Lease Liabilities				(4,700,000)
Business Combination, Pro Forma Information, Revenue of Acquiree since Acquisition Date, Actual			34,900,000		34,900,000
Transaction and other costs	$ 4,126,000		104,095,000		115,200,000
Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustment, Real Estate Investments, Unconsolidated Real Estate And Other Joint Ventures				(468,000)
Business Combination, Pro Forma Information, Earnings or Loss of Acquiree since Acquisition Date, Actual			$ 7,800,000		$ 7,800,000
Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustment, Current Liabilities, Other				$ (2,400,000)