Real Estate Investments - Summary of Properties Sold (Parenthetical) (Details) $ in Thousands		3 Months Ended	12 Months Ended
	Dec. 20, 2018 USD ($) ft² property	Sep. 30, 2018 USD ($) property	Dec. 31, 2018 USD ($) ft²	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Schedule Of Equity Method Investments [Line Items]
Area of Real Estate Property | ft²			16,777,249
Net gain on disposition of real estate			$ 2,932	$ 31,657	$ 116,983
Metropolitan Washington, D.C. - Office [Member]
Schedule Of Equity Method Investments [Line Items]
Number of properties contributed to a joint venture | property		8
Impairment on assets held for sale		$ 56,900
Rockpoint Portfolio [Member] | Metropolitan Washington, D.C. - Office [Member]
Schedule Of Equity Method Investments [Line Items]
Number of properties contributed to a joint venture | property	8
Area of Real Estate Property | ft²	1,293,197
Aggregate gross sales price of a joint venture	$ 312,000
Impairment on assets held for sale		56,900
Closing costs	2,200
Working capital contributions	600
Cash proceeds at settlement	262,400
Net gain on disposition of real estate	$ 400
Herndon Innovation Center Metro Portfolio Venture, LLC [Member] | Metropolitan Washington, D.C. - Office [Member]
Schedule Of Equity Method Investments [Line Items]
Number of properties contributed to a joint venture | property	8
Area of Real Estate Property | ft²	1,293,197
Aggregate gross sales price of a joint venture	$ 312,000
Equity method investment percentage	15.00%
Equity method investment percentage of parnter	85.00%
Impairment on assets held for sale		$ 56,900
Closing costs	$ 2,200
Working capital contributions	600
Net gain on disposition of real estate	400
Cash paid for acquisition	$ 265,200