Segments - Schedule of NOI for Each Reportable Segment Information (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Segment Reporting Information [Line Items]
Property-related revenues	$ 694,755	$ 666,410	$ 652,160
Property-related operating expenses	(266,136)	(250,040)	(244,754)
NOI from unconsolidated joint ventures	19,643	17,195	16,580
NOI	448,262	433,565	423,986
New York [Member]
Segment Reporting Information [Line Items]
Property-related revenues	430,548	449,794	452,842
Property-related operating expenses	(180,855)	(176,445)	(174,273)
NOI from unconsolidated joint ventures	19,143	16,874	16,210
NOI	268,836	290,223	294,779
Washington, D.C. [Member]
Segment Reporting Information [Line Items]
Property-related revenues	72,143	86,389	82,366
Property-related operating expenses	(27,342)	(32,721)	(32,482)
NOI	44,801	53,668	49,884
San Francisco [Member]
Segment Reporting Information [Line Items]
Property-related revenues	191,677	127,813	114,472
Property-related operating expenses	(50,906)	(30,889)	(29,277)
NOI	140,771	96,924	85,195
Other [Member]
Segment Reporting Information [Line Items]
Property-related revenues	387	2,414	2,480
Property-related operating expenses	(7,033)	(9,985)	(8,722)
NOI from unconsolidated joint ventures	500	321	370
NOI	$ (6,146)	$ (7,250)	$ (5,872)