Reportable Segments (Details) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2015 USD ($)	Sep. 30, 2014 USD ($)	Sep. 30, 2015 USD ($)	Sep. 30, 2014 USD ($)	Dec. 31, 2014 USD ($)
Reportable Segments - Reconciliation of NOI
Rental income	$ 694,245	$ 662,001	$ 2,035,359	$ 1,942,492
Property and maintenance expense	(122,383)	(120,139)	(364,948)	(361,087)
Real estate taxes and insurance expense	(84,962)	(80,568)	(254,513)	(245,717)
Property management expense	(18,925)	(18,407)	(60,887)	(61,080)
Total operating expenses	(226,270)	(219,114)	(680,348)	(667,884)
Net operating income	467,975	442,887	1,355,011	1,274,608
Rental income:
Rental Income - Same Store	660,728	626,979	1,915,727	1,822,727
Rental Income - Non-same store/other	33,517	35,022	119,632	119,765
Total Rental Income	694,245	662,001	2,035,359	1,942,492
Operating expenses:
Operating Expenses - Same Store	215,606	209,658	635,123	618,532
Operating Expenses - Non-same store/other	10,664	9,456	45,225	49,352
Total operating expenses	226,270	219,114	680,348	667,884
NOI:
Net operating income - Same Store	445,122	417,321	1,280,604	1,204,195
Net operating income - Non-same store/other	22,853	25,566	74,407	70,413
Net operating income	467,975	442,887	1,355,011	1,274,608
Total Assets	$ 23,008,804		23,008,804		$ 22,950,614
Capital Expenditures			$ 134,438
Units in same store properties	97,737		96,432
Boston [Member] | Same Store [Member]
Rental income:
Rental Income - Same Store	$ 63,673	61,407	$ 188,091	182,085
Operating expenses:
Operating Expenses - Same Store	20,159	19,056	61,890	58,769
NOI:
Net operating income - Same Store	43,514	42,351	126,201	123,316
Total Assets	1,854,057		1,854,057
Capital Expenditures			12,948
Boston [Member] | Non-same store [Member]
Rental income:
Rental Income - Non-same store/other	2,858	932	6,192	2,750
Operating expenses:
Operating Expenses - Non-same store/other	916	222	1,795	633
NOI:
Net operating income - Non-same store/other	1,942	710	4,397	2,117
Total Assets	173,368		173,368
Capital Expenditures			308
New York [Domain] | Same Store [Member]
Rental income:
Rental Income - Same Store	120,149	114,968	354,704	340,374
Operating expenses:
Operating Expenses - Same Store	43,578	41,961	133,433	129,006
NOI:
Net operating income - Same Store	76,571	73,007	221,271	211,368
Total Assets	4,593,457		4,593,457
Capital Expenditures			13,213
New York [Domain] | Non-same store [Member]
Rental income:
Rental Income - Non-same store/other	3,079	6	4,302	16
Operating expenses:
Operating Expenses - Non-same store/other	2,258	0	3,660	1
NOI:
Net operating income - Non-same store/other	821	6	642	15
Total Assets	366,978		366,978
Capital Expenditures			(12)
San Francisco [Member] | Same Store [Member]
Rental income:
Rental Income - Same Store	96,115	86,468	278,514	251,812
Operating expenses:
Operating Expenses - Same Store	27,512	27,124	81,469	80,087
NOI:
Net operating income - Same Store	68,603	59,344	197,045	171,725
Total Assets	2,666,256		2,666,256
Capital Expenditures			19,932
San Francisco [Member] | Non-same store [Member]
Rental income:
Rental Income - Non-same store/other	2,216	4	2,709	10
Operating expenses:
Operating Expenses - Non-same store/other	899	12	1,465	24
NOI:
Net operating income - Non-same store/other	1,317	(8)	1,244	(14)
Total Assets	261,039		261,039
Capital Expenditures			3
Seattle [Member] | Same Store [Member]
Rental income:
Rental Income - Same Store	43,481	40,538	124,623	116,327
Operating expenses:
Operating Expenses - Same Store	13,206	13,082	38,724	38,706
NOI:
Net operating income - Same Store	30,275	27,456	85,899	77,621
Total Assets	1,072,287		1,072,287
Capital Expenditures			9,608
Seattle [Member] | Non-same store [Member]
Rental income:
Rental Income - Non-same store/other	4,692	1,722	14,894	3,712
Operating expenses:
Operating Expenses - Non-same store/other	1,523	505	4,394	1,332
NOI:
Net operating income - Non-same store/other	3,169	1,217	10,500	2,380
Total Assets	368,624		368,624
Capital Expenditures			70
Southern California [Member] | Same Store [Member]
Rental income:
Rental Income - Same Store	115,657	109,054	322,762	305,427
Operating expenses:
Operating Expenses - Same Store	36,979	36,639	102,989	101,924
NOI:
Net operating income - Same Store	78,678	72,415	219,773	203,503
Total Assets	2,748,274		2,748,274
Capital Expenditures			23,320
Southern California [Member] | Non-same store [Member]
Rental income:
Rental Income - Non-same store/other	12,424	8,486	52,833	32,949
Operating expenses:
Operating Expenses - Non-same store/other	4,351	3,383	19,377	14,068
NOI:
Net operating income - Non-same store/other	8,073	5,103	33,456	18,881
Total Assets	837,998		837,998
Capital Expenditures			3,713
Washington DC [Member] | Same Store [Member]
Rental income:
Rental Income - Same Store	116,935	116,449	341,797	339,453
Operating expenses:
Operating Expenses - Same Store	38,714	37,793	113,582	110,357
NOI:
Net operating income - Same Store	78,221	78,656	228,215	229,096
Total Assets	4,199,832		4,199,832
Capital Expenditures			26,697
Washington DC [Member] | Non-same store [Member]
Rental income:
Rental Income - Non-same store/other	3,538	2,932	16,370	12,610
Operating expenses:
Operating Expenses - Non-same store/other	1,013	901	4,855	4,683
NOI:
Net operating income - Non-same store/other	2,525	2,031	11,515	7,927
Total Assets	237,900		237,900
Capital Expenditures			568
Non-core – South Florida | Same Store [Member]
Rental income:
Rental Income - Same Store	51,960	48,850	151,115	143,000
Operating expenses:
Operating Expenses - Same Store	18,927	18,198	54,961	53,188
NOI:
Net operating income - Same Store	33,033	30,652	96,154	89,812
Total Assets	1,111,103		1,111,103
Capital Expenditures			12,856
Non-core – South Florida | Non-same store [Member]
Rental income:
Rental Income - Non-same store/other	1,331	1,042	5,916	3,694
Operating expenses:
Operating Expenses - Non-same store/other	563	561	2,101	2,026
NOI:
Net operating income - Non-same store/other	768	481	3,815	1,668
Total Assets	65,193		65,193
Capital Expenditures			30
Non-core – Denver | Same Store [Member]
Rental income:
Rental Income - Same Store	30,890	28,269	89,716	82,392
Operating expenses:
Operating Expenses - Same Store	8,619	8,150	24,246	23,246
NOI:
Net operating income - Same Store	22,271	20,119	65,470	59,146
Total Assets	506,436		506,436
Capital Expenditures			5,888
Non-core – other | Same Store [Member]
Rental income:
Rental Income - Same Store	21,868	20,976	64,405	61,857
Operating expenses:
Operating Expenses - Same Store	7,912	7,655	23,829	23,249
NOI:
Net operating income - Same Store	13,956	13,321	40,576	38,608
Total Assets	325,649		325,649
Capital Expenditures			4,862
Other [Member] | Non-same store [Member]
Rental income:
Rental Income - Non-same store/other	3,379	19,898	16,416	64,024
Operating expenses:
Operating Expenses - Non-same store/other	(859)	3,872	7,578	26,585
NOI:
Net operating income - Non-same store/other	4,238	$ 16,026	8,838	$ 37,439
Total Assets	1,620,353		1,620,353
Capital Expenditures			434
Same Store [Member]
NOI:
Total Assets	19,077,351		19,077,351
Capital Expenditures			129,324
Non-same store [Member]
NOI:
Total Assets	$ 3,931,453		3,931,453
Capital Expenditures			$ 5,114