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, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016	Dec. 31, 2016
Segment Reporting Information [Line Items]
Rental income	$ 612,299	$ 594,939	$ 1,216,219	$ 1,211,104
Operating income	210,550	206,018	414,921	422,643
Total Assets	20,636,575		20,636,575		$ 20,704,148
Capital Expenditures			89,297
Same store
Segment Reporting Information [Line Items]
Rental income	571,362	559,564	1,118,370	1,092,822
Operating expenses	167,047	160,478	330,252	317,862
Operating income	404,315	399,086	788,118	774,960
Total Assets	16,889,766		16,889,766
Capital Expenditures			86,710
Non-same store/other
Segment Reporting Information [Line Items]
Rental income	40,937	35,375	97,849	118,282
Operating expenses	19,772	13,244	40,903	45,221
Operating income	21,165	22,131	56,946	73,061
Total Assets	3,746,809		3,746,809
Capital Expenditures			2,587
Same Store, Non-same Store and Other
Segment Reporting Information [Line Items]
Rental income	612,299	594,939	1,216,219	1,211,104
Operating expenses	186,819	173,722	371,155	363,083
Operating income	425,480	421,217	845,064	848,021
Other markets | Same store
Segment Reporting Information [Line Items]
Rental income	461	449	924	885
Operating expenses	144	140	340	292
Operating income	317	309	584	593
Total Assets	12,783		12,783
Capital Expenditures			46
Los Angeles | Same store
Segment Reporting Information [Line Items]
Rental income	100,360	96,930	195,685	188,764
Operating expenses	28,731	27,877	56,725	54,533
Operating income	71,629	69,053	138,960	134,231
Total Assets	2,641,868		2,641,868
Capital Expenditures			11,677
San Diego | Same store
Segment Reporting Information [Line Items]
Rental income	22,760	21,876	45,319	43,346
Operating expenses	6,010	5,862	12,073	11,698
Operating income	16,750	16,014	33,246	31,648
Total Assets	464,432		464,432
Capital Expenditures			2,725
Orange County | Same store
Segment Reporting Information [Line Items]
Rental income	21,999	21,028	43,654	41,551
Operating expenses	5,385	5,025	10,856	10,167
Operating income	16,614	16,003	32,798	31,384
Total Assets	331,904		331,904
Capital Expenditures			3,801
Subtotal - Southern California | Same store
Segment Reporting Information [Line Items]
Rental income	145,119	139,834	284,658	273,661
Operating expenses	40,126	38,764	79,654	76,398
Operating income	104,993	101,070	205,004	197,263
Total Assets	3,438,204		3,438,204
Capital Expenditures			18,203
Washington DC | Same store
Segment Reporting Information [Line Items]
Rental income	107,102	106,246	213,544	210,540
Operating expenses	32,252	30,909	64,585	62,371
Operating income	74,850	75,337	148,959	148,169
Total Assets	3,861,391		3,861,391
Capital Expenditures			17,474
New York | Same store
Segment Reporting Information [Line Items]
Rental income	120,775	120,846	229,462	229,454
Operating expenses	43,320	40,723	84,309	79,746
Operating income	77,455	80,123	145,153	149,708
Total Assets	4,226,200		4,226,200
Capital Expenditures			14,585
San Francisco | Same store
Segment Reporting Information [Line Items]
Rental income	94,475	92,553	188,483	183,915
Operating expenses	22,780	22,502	45,532	44,929
Operating income	71,695	70,051	142,951	138,986
Total Assets	2,493,666		2,493,666
Capital Expenditures			17,039
Boston | Same store
Segment Reporting Information [Line Items]
Rental income	57,097	56,089	113,474	111,749
Operating expenses	15,432	15,273	31,186	30,908
Operating income	41,665	40,816	82,288	80,841
Total Assets	1,679,537		1,679,537
Capital Expenditures			10,559
Seattle | Same store
Segment Reporting Information [Line Items]
Rental income	46,333	43,547	87,825	82,618
Operating expenses	12,993	12,167	24,646	23,218
Operating income	33,340	31,380	63,179	59,400
Total Assets	1,177,985		1,177,985
Capital Expenditures			8,804
Non-same store | Non-same store/other
Segment Reporting Information [Line Items]
Rental income	36,824	17,533	92,408	46,064
Operating expenses	15,923	6,392	34,678	17,225
Operating income	20,901	11,141	57,730	28,839
Total Assets	2,403,684		2,403,684
Capital Expenditures			2,208
Other | Non-same store/other
Segment Reporting Information [Line Items]
Rental income	4,113	17,842	5,441	72,218
Operating expenses	3,849	6,852	6,225	27,996
Operating income	264	$ 10,990	(784)	$ 44,222
Total Assets	$ 1,343,125		1,343,125
Capital Expenditures			$ 379