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, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Segment Reporting Information [Line Items]
Rental income	$ 652,677	$ 623,951	$ 1,925,128	$ 1,840,170
Operating income	224,731	217,786	639,254	632,707
Total Assets	20,542,150		20,542,150		$ 20,570,599
Capital Expenditures			138,119
Same store
Segment Reporting Information [Line Items]
Rental income	605,412	591,714	1,767,717	1,729,293
Operating expenses	182,220	175,796	531,532	513,836
Operating income	423,192	415,918	1,236,185	1,215,457
Total Assets	17,095,643		17,095,643
Capital Expenditures			133,683
Non-same store/other
Segment Reporting Information [Line Items]
Rental income	47,265	32,237	157,411	110,877
Operating expenses	15,709	13,012	59,739	46,127
Operating income	31,556	19,225	97,672	64,750
Total Assets	3,446,507		3,446,507
Capital Expenditures			4,436
Same Store, Non-same Store and Other
Segment Reporting Information [Line Items]
Rental income	652,677	623,951	1,925,128	1,840,170
Operating expenses	197,929	188,808	591,271	559,963
Operating income	454,748	435,143	1,333,857	1,280,207
Other markets | Same store
Segment Reporting Information [Line Items]
Rental income	485	459	1,453	1,384
Operating expenses	165	158	506	498
Operating income	320	301	947	886
Total Assets	12,820		12,820
Capital Expenditures			135
Los Angeles | Same store
Segment Reporting Information [Line Items]
Rental income	108,050	104,190	310,972	300,702
Operating expenses	30,848	29,470	89,211	85,972
Operating income	77,202	74,720	221,761	214,730
Total Assets	2,631,459		2,631,459
Capital Expenditures			21,438
Orange County | Same store
Segment Reporting Information [Line Items]
Rental income	23,182	22,427	68,513	66,082
Operating expenses	5,615	5,491	16,706	16,330
Operating income	17,567	16,936	51,807	49,752
Total Assets	321,551		321,551
Capital Expenditures			6,327
San Diego | Same store
Segment Reporting Information [Line Items]
Rental income	23,215	22,432	68,640	66,052
Operating expenses	6,233	5,881	17,984	17,415
Operating income	16,982	16,551	50,656	48,637
Total Assets	410,450		410,450
Capital Expenditures			3,665
Subtotal - Southern California | Same store
Segment Reporting Information [Line Items]
Rental income	154,447	149,049	448,125	432,836
Operating expenses	42,696	40,842	123,901	119,717
Operating income	111,751	108,207	324,224	313,119
Total Assets	3,363,460		3,363,460
Capital Expenditures			31,430
San Francisco | Same store
Segment Reporting Information [Line Items]
Rental income	116,937	113,154	331,067	321,894
Operating expenses	29,271	29,527	80,377	81,942
Operating income	87,666	83,627	250,690	239,952
Total Assets	2,989,059		2,989,059
Capital Expenditures			34,617
Washington DC | Same store
Segment Reporting Information [Line Items]
Rental income	109,637	108,764	325,480	322,310
Operating expenses	34,722	33,474	101,620	98,046
Operating income	74,915	75,290	223,860	224,264
Total Assets	3,734,007		3,734,007
Capital Expenditures			21,722
New York | Same store
Segment Reporting Information [Line Items]
Rental income	115,757	114,799	343,239	341,295
Operating expenses	45,156	42,620	135,700	128,076
Operating income	70,601	72,179	207,539	213,219
Total Assets	4,108,581		4,108,581
Capital Expenditures			17,217
Boston | Same store
Segment Reporting Information [Line Items]
Rental income	57,531	56,022	171,042	167,010
Operating expenses	16,073	15,726	47,666	45,871
Operating income	41,458	40,296	123,376	121,139
Total Assets	1,599,983		1,599,983
Capital Expenditures			16,477
Seattle | Same store
Segment Reporting Information [Line Items]
Rental income	50,618	49,467	147,311	142,564
Operating expenses	14,137	13,449	41,762	39,686
Operating income	36,481	36,018	105,549	102,878
Total Assets	1,287,733		1,287,733
Capital Expenditures			12,085
Non-same store | Non-same store/other
Segment Reporting Information [Line Items]
Rental income	42,702	18,252	134,308	60,446
Operating expenses	13,605	6,870	45,997	21,096
Operating income	29,097	11,382	88,311	39,350
Total Assets	2,884,686		2,884,686
Capital Expenditures			4,171
Other [Member] | Non-same store/other
Segment Reporting Information [Line Items]
Rental income	4,563	13,985	23,103	50,431
Operating expenses	2,104	6,142	13,742	25,031
Operating income	2,459	$ 7,843	9,361	$ 25,400
Total Assets	$ 561,821		561,821
Capital Expenditures			$ 265