Segments Disclosure - Schedule of NOI for Each Reportable Segment Information (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Segment Reporting Information [Line Items]
Property-related revenues	$ 167,342	$ 165,641	$ 504,971	$ 486,480
Property-related operating expenses	(64,025)	(63,354)	(186,964)	(183,019)
NOI from unconsolidated joint ventures	3,974	4,303	12,938	12,362
NOI	107,291	106,590	330,945	315,823
New York [Member]
Segment Reporting Information [Line Items]
Property-related revenues	113,029	114,747	346,683	339,418
Property-related operating expenses	(45,748)	(45,709)	(132,428)	(131,412)
NOI from unconsolidated joint ventures	3,893	4,210	12,696	12,084
NOI	71,174	73,248	226,951	220,090
Washington, D.C. [Member]
Segment Reporting Information [Line Items]
Property-related revenues	22,229	21,117	63,689	61,341
Property-related operating expenses	(8,322)	(8,285)	(24,691)	(23,989)
NOI	13,907	12,832	38,998	37,352
San Francisco [Member]
Segment Reporting Information [Line Items]
Property-related revenues	31,441	29,156	92,762	83,820
Property-related operating expenses	(7,994)	(7,267)	(22,426)	(21,061)
NOI	23,447	21,889	70,336	62,759
Other [Member]
Segment Reporting Information [Line Items]
Property-related revenues	643	621	1,837	1,901
Property-related operating expenses	(1,961)	(2,093)	(7,419)	(6,557)
NOI from unconsolidated joint ventures	81	93	242	278
NOI	$ (1,237)	$ (1,379)	$ (5,340)	$ (4,378)