Reportable Segments - NOI for Each Segment from Our Rental Real Estate Specific to Continuing Operations (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Segment Reporting Information [Line Items]
Rental income	$ 2,577,681	$ 2,470,689	$ 2,422,233
Operating income	858,560	847,471	856,086
Total Assets	20,394,209	20,570,599
Capital Expenditures	188,501	202,607
Same store
Segment Reporting Information [Line Items]
Rental income	2,363,491	2,311,240	2,200,094
Operating expenses	705,890	681,198	639,342
Operating income	1,657,601	1,630,042	1,560,752
Total Assets	16,945,585	17,412,173
Capital Expenditures	181,559	195,960
Non-same store/other
Segment Reporting Information [Line Items]
Rental income	214,190	159,449	222,139
Operating expenses	81,259	59,578	84,868
Operating income	132,931	99,871	137,271
Total Assets	3,448,624	3,158,426
Capital Expenditures	6,942	6,647
Same Store, Non-same Store and Other
Segment Reporting Information [Line Items]
Rental income	2,577,681	2,470,689	2,422,233
Operating expenses	787,149	740,776	724,210
Operating income	1,790,532	1,729,913	1,698,023
Other markets | Same store
Segment Reporting Information [Line Items]
Rental income	1,940	1,839	1,797
Operating expenses	657	652	567
Operating income	1,283	1,187	1,230
Total Assets	12,781	12,768
Capital Expenditures	163	103
Los Angeles | Same store
Segment Reporting Information [Line Items]
Rental income	416,688	402,192	382,425
Operating expenses	119,473	114,055	111,055
Operating income	297,215	288,137	271,370
Total Assets	2,608,347	2,692,602
Capital Expenditures	28,306	26,921
Orange County | Same store
Segment Reporting Information [Line Items]
Rental income	91,650	88,527	84,590
Operating expenses	22,071	21,544	20,600
Operating income	69,579	66,983	63,990
Total Assets	318,472	328,593
Capital Expenditures	8,192	9,237
San Diego | Same store
Segment Reporting Information [Line Items]
Rental income	91,971	88,507	84,646
Operating expenses	24,023	23,073	22,366
Operating income	67,948	65,434	62,280
Total Assets	405,449	421,267
Capital Expenditures	4,525	4,922
Subtotal - Southern California | Same store
Segment Reporting Information [Line Items]
Rental income	600,309	579,226	551,661
Operating expenses	165,567	158,672	154,021
Operating income	434,742	420,554	397,640
Total Assets	3,332,268	3,442,462
Capital Expenditures	41,023	41,080
San Francisco | Same store
Segment Reporting Information [Line Items]
Rental income	442,773	430,501	371,633
Operating expenses	105,686	108,689	90,393
Operating income	337,087	321,812	281,240
Total Assets	2,967,735	3,039,552
Capital Expenditures	44,837	38,618
Washington DC | Same store
Segment Reporting Information [Line Items]
Rental income	434,545	430,060	424,350
Operating expenses	134,427	129,720	126,154
Operating income	300,118	300,340	298,196
Total Assets	3,702,368	3,803,185
Capital Expenditures	29,616	35,423
New York | Same store
Segment Reporting Information [Line Items]
Rental income	458,366	454,945	456,664
Operating expenses	181,040	170,064	159,302
Operating income	277,326	284,881	297,362
Total Assets	4,063,568	4,160,603
Capital Expenditures	23,472	30,403
Boston | Same store
Segment Reporting Information [Line Items]
Rental income	229,079	223,595	225,014
Operating expenses	63,348	60,931	62,650
Operating income	165,731	162,664	162,364
Total Assets	1,589,000	1,636,925
Capital Expenditures	24,623	28,687
Seattle | Same store
Segment Reporting Information [Line Items]
Rental income	196,479	191,074	168,975
Operating expenses	55,165	52,470	46,255
Operating income	141,314	138,604	122,720
Total Assets	1,277,865	1,316,678
Capital Expenditures	17,825	21,646
Non-same store | Non-same store/other
Segment Reporting Information [Line Items]
Rental income	190,814	95,016	117,143
Operating expenses	63,907	30,742	41,877
Operating income	126,907	64,274	75,266
Total Assets	2,940,770	2,276,312
Capital Expenditures	6,665	3,704
Other [Member] | Non-same store/other
Segment Reporting Information [Line Items]
Rental income	23,376	64,433	104,996
Operating expenses	17,352	28,836	42,991
Operating income	6,024	35,597	$ 62,005
Total Assets	507,854	882,114
Capital Expenditures	$ 277	$ 2,943