Segments Disclosure - Schedule of NOI for Each Reportable Segment Information (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Segment Reporting Information [Line Items]
Property-related revenues	$ 168,128	$ 161,148	$ 337,629	$ 320,839
Property-related operating expenses	(59,994)	(57,781)	(122,939)	(119,665)
NOI from unconsolidated joint ventures	4,536	4,278	8,964	8,059
NOI	112,670	107,645	223,654	209,233
New York [Member]
Segment Reporting Information [Line Items]
Property-related revenues	114,351	110,718	233,654	224,671
Property-related operating expenses	(42,543)	(40,919)	(86,680)	(85,703)
NOI from unconsolidated joint ventures	4,456	4,138	8,803	7,874
NOI	76,264	73,937	155,777	146,842
Washington, D.C. [Member]
Segment Reporting Information [Line Items]
Property-related revenues	21,478	21,014	41,460	40,165
Property-related operating expenses	(8,094)	(7,927)	(16,369)	(15,704)
NOI	13,384	13,087	25,091	24,461
San Francisco [Member]
Segment Reporting Information [Line Items]
Property-related revenues	31,702	28,848	61,321	54,664
Property-related operating expenses	(7,259)	(7,017)	(14,432)	(13,794)
NOI	24,443	21,831	46,889	40,870
Other [Member]
Segment Reporting Information [Line Items]
Property-related revenues	597	568	1,194	1,339
Property-related operating expenses	(2,098)	(1,918)	(5,458)	(4,464)
NOI from unconsolidated joint ventures	80	140	161	185
NOI	$ (1,421)	$ (1,210)	$ (4,103)	$ (2,940)