Reportable Segments - NOI for Each Segment from Our Rental Real Estate Specific to Continuing Operations (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Segment Reporting Information [Line Items]
Rental income	$ 623,951	$ 605,856	$ 1,840,170	$ 1,816,960
Operating income	217,786	209,373	632,707	632,016
Total Assets	20,697,833		20,697,833		$ 20,704,148
Capital Expenditures			143,258
Same store
Segment Reporting Information [Line Items]
Rental income	587,142	574,594	1,685,303	1,647,988
Operating expenses	173,844	171,019	496,232	482,410
Operating income	413,298	403,575	1,189,071	1,165,578
Total Assets	16,731,045		16,731,045
Capital Expenditures			139,638
Non-same store/other
Segment Reporting Information [Line Items]
Rental income	36,809	31,262	154,867	168,972
Operating expenses	14,964	14,540	63,731	66,232
Operating income	21,845	16,722	91,136	102,740
Total Assets	3,966,788		3,966,788
Capital Expenditures			3,620
Same Store, Non-same Store and Other
Segment Reporting Information [Line Items]
Rental income	623,951	605,856	1,840,170	1,816,960
Operating expenses	188,808	185,559	559,963	548,642
Operating income	435,143	420,297	1,280,207	1,268,318
Other markets | Same store
Segment Reporting Information [Line Items]
Rental income	459	457	1,384	1,342
Operating expenses	158	136	499	428
Operating income	301	321	885	914
Total Assets	12,778		12,778
Capital Expenditures			83
Los Angeles | Same store
Segment Reporting Information [Line Items]
Rental income	102,566	98,542	296,345	285,463
Operating expenses	27,768	29,362	83,927	83,459
Operating income	74,798	69,180	212,418	202,004
Total Assets	2,620,672		2,620,672
Capital Expenditures			18,952
Orange County | Same store
Segment Reporting Information [Line Items]
Rental income	22,427	21,455	66,081	63,005
Operating expenses	5,500	5,362	16,355	15,529
Operating income	16,927	16,093	49,726	47,476
Total Assets	330,580		330,580
Capital Expenditures			6,671
San Diego | Same store
Segment Reporting Information [Line Items]
Rental income	22,432	21,516	66,052	63,171
Operating expenses	5,867	5,740	17,386	16,884
Operating income	16,565	15,776	48,666	46,287
Total Assets	425,241		425,241
Capital Expenditures			3,776
Subtotal - Southern California | Same store
Segment Reporting Information [Line Items]
Rental income	147,425	141,513	428,478	411,639
Operating expenses	39,135	40,464	117,668	115,872
Operating income	108,290	101,049	310,810	295,767
Total Assets	3,376,493		3,376,493
Capital Expenditures			29,399
Washington DC | Same store
Segment Reporting Information [Line Items]
Rental income	108,763	107,340	322,307	317,879
Operating expenses	33,459	32,817	98,043	95,187
Operating income	75,304	74,523	224,264	222,692
Total Assets	3,836,992		3,836,992
Capital Expenditures			26,308
New York | Same store
Segment Reporting Information [Line Items]
Rental income	125,351	124,709	345,656	345,434
Operating expenses	46,219	44,223	126,226	120,390
Operating income	79,132	80,486	219,430	225,044
Total Assets	4,181,890		4,181,890
Capital Expenditures			23,547
San Francisco | Same store
Segment Reporting Information [Line Items]
Rental income	98,805	97,259	283,654	277,666
Operating expenses	24,875	24,487	69,053	68,246
Operating income	73,930	72,772	214,601	209,420
Total Assets	2,480,069		2,480,069
Capital Expenditures			25,896
Boston | Same store
Segment Reporting Information [Line Items]
Rental income	57,071	56,368	170,545	168,118
Operating expenses	16,351	16,399	47,538	47,308
Operating income	40,720	39,969	123,007	120,810
Total Assets	1,670,467		1,670,467
Capital Expenditures			18,906
Seattle | Same store
Segment Reporting Information [Line Items]
Rental income	49,268	46,948	133,279	125,910
Operating expenses	13,647	12,493	37,205	34,979
Operating income	35,621	34,455	96,074	90,931
Total Assets	1,172,356		1,172,356
Capital Expenditures			15,499
Non-same store | Non-same store/other
Segment Reporting Information [Line Items]
Rental income	33,974	14,128	144,975	78,287
Operating expenses	10,371	6,120	53,367	29,173
Operating income	23,603	8,008	91,608	49,114
Total Assets	3,080,697		3,080,697
Capital Expenditures			3,206
Other | Non-same store/other
Segment Reporting Information [Line Items]
Rental income	2,835	17,134	9,892	90,685
Operating expenses	4,593	8,420	10,364	37,059
Operating income	(1,758)	$ 8,714	(472)	$ 53,626
Total Assets	$ 886,091		886,091
Capital Expenditures			$ 414