Reportable Segments - NOI for Each Segment from Our Rental Real Estate Specific to Continuing Operations (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018
Segment Reporting Information [Line Items]
Rental income	$ 669,374	$ 639,620	$ 1,331,676	$ 1,272,451
Operating income	369,260	217,603	579,229	556,685
Total Assets	20,990,711		20,990,711		$ 20,394,209
Capital Expenditures			81,528
Same store
Segment Reporting Information [Line Items]
Rental income	630,569	609,272	1,241,077	1,201,296
Operating expenses	186,521	180,488	374,457	360,407
Operating income	444,048	428,784	866,620	840,889
Total Assets	17,664,156		17,664,156
Capital Expenditures			76,267
Non-same store/other
Segment Reporting Information [Line Items]
Rental income	38,805	30,348	90,599	71,155
Operating expenses	13,386	12,738	31,962	32,935
Operating income	25,419	17,610	58,637	38,220
Total Assets	3,326,555		3,326,555
Capital Expenditures			5,261
Same Store, Non-same Store and Other
Segment Reporting Information [Line Items]
Rental income	669,374	639,620	1,331,676	1,272,451
Operating expenses	199,907	193,226	406,419	393,342
Operating income	469,467	446,394	925,257	879,109
Other markets | Same store
Segment Reporting Information [Line Items]
Rental income	518	481	1,040	968
Operating expenses	192	175	377	339
Operating income	326	306	663	629
Total Assets	12,887		12,887
Capital Expenditures			126
Los Angeles | Same store
Segment Reporting Information [Line Items]
Rental income	117,147	111,230	232,546	222,227
Operating expenses	33,776	31,845	68,259	63,887
Operating income	83,371	79,385	164,287	158,340
Total Assets	3,027,063		3,027,063
Capital Expenditures			15,775
Orange County | Same store
Segment Reporting Information [Line Items]
Rental income	26,058	25,193	51,886	49,996
Operating expenses	5,996	6,140	12,201	12,231
Operating income	20,062	19,053	39,685	37,765
Total Assets	412,837		412,837
Capital Expenditures			4,812
San Diego | Same store
Segment Reporting Information [Line Items]
Rental income	23,720	22,932	47,023	45,425
Operating expenses	5,958	5,790	12,084	11,754
Operating income	17,762	17,142	34,939	33,671
Total Assets	398,072		398,072
Capital Expenditures			1,749
Subtotal - Southern California | Same store
Segment Reporting Information [Line Items]
Rental income	166,925	159,355	331,455	317,648
Operating expenses	45,730	43,775	92,544	87,872
Operating income	121,195	115,580	238,911	229,776
Total Assets	3,837,972		3,837,972
Capital Expenditures			22,336
San Francisco | Same store
Segment Reporting Information [Line Items]
Rental income	123,341	118,416	245,103	235,767
Operating expenses	29,317	28,591	59,735	58,001
Operating income	94,024	89,825	185,368	177,766
Total Assets	3,375,932		3,375,932
Capital Expenditures			14,222
Washington DC | Same store
Segment Reporting Information [Line Items]
Rental income	112,694	110,096	223,479	218,624
Operating expenses	34,066	33,553	68,742	67,463
Operating income	78,628	76,543	154,737	151,161
Total Assets	3,716,255		3,716,255
Capital Expenditures			9,007
New York | Same store
Segment Reporting Information [Line Items]
Rental income	113,579	110,691	225,942	220,456
Operating expenses	46,304	43,683	94,472	88,496
Operating income	67,275	67,008	131,470	131,960
Total Assets	3,917,085		3,917,085
Capital Expenditures			10,679
Boston | Same store
Segment Reporting Information [Line Items]
Rental income	58,224	56,379	112,127	108,348
Operating expenses	15,859	15,475	31,075	30,152
Operating income	42,365	40,904	81,052	78,196
Total Assets	1,490,866		1,490,866
Capital Expenditures			11,924
Seattle | Same store
Segment Reporting Information [Line Items]
Rental income	55,288	53,854	101,931	99,485
Operating expenses	15,053	15,236	27,512	28,084
Operating income	40,235	38,618	74,419	71,401
Total Assets	1,313,159		1,313,159
Capital Expenditures			7,973
Non-same store | Non-same store/other
Segment Reporting Information [Line Items]
Rental income	36,712	17,393	81,933	40,438
Operating expenses	11,676	6,465	26,738	15,393
Operating income	25,036	10,928	55,195	25,045
Total Assets	2,548,854		2,548,854
Capital Expenditures			4,961
Other [Member] | Non-same store/other
Segment Reporting Information [Line Items]
Rental income	2,093	12,955	8,666	30,717
Operating expenses	1,710	6,273	5,224	17,542
Operating income	383	$ 6,682	3,442	$ 13,175
Total Assets	$ 777,701		777,701
Capital Expenditures			$ 300