Segments - Schedule of NOI for Each Reportable Segment Information (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Segment Reporting Information [Line Items]
Property-related revenues	$ 188,517	$ 173,936	$ 555,333	$ 518,872
Property-related operating expenses	(69,811)	(68,264)	(206,435)	(197,696)
NOI from unconsolidated joint ventures	4,448	4,993	13,757	14,774
NOI	123,154	110,665	362,655	335,950
New York [Member]
Segment Reporting Information [Line Items]
Property-related revenues	118,539	109,493	347,720	321,419
Property-related operating expenses	(48,257)	(46,609)	(140,710)	(134,657)
NOI from unconsolidated joint ventures	4,356	4,815	13,514	14,406
NOI	74,638	67,699	220,524	201,168
Washington, D.C. [Member]
Segment Reporting Information [Line Items]
Property-related revenues	12,685	14,986	43,569	56,911
Property-related operating expenses	(4,782)	(5,887)	(16,363)	(21,376)
NOI	7,903	9,099	27,206	35,535
San Francisco [Member]
Segment Reporting Information [Line Items]
Property-related revenues	57,568	49,758	164,811	139,898
Property-related operating expenses	(15,206)	(14,164)	(44,370)	(35,889)
NOI	42,362	35,594	120,441	104,009
Other [Member]
Segment Reporting Information [Line Items]
Property-related revenues	(275)	(301)	(767)	644
Property-related operating expenses	(1,566)	(1,604)	(4,992)	(5,774)
NOI from unconsolidated joint ventures	92	178	243	368
NOI	$ (1,749)	$ (1,727)	$ (5,516)	$ (4,762)