Reportable Segments (Details) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2015 USD ($)	Jun. 30, 2014 USD ($)	Jun. 30, 2015 USD ($)	Jun. 30, 2014 USD ($)	Dec. 31, 2014 USD ($)
Reportable Segments
Assets	$ 23,081,830		$ 23,081,830		$ 22,950,614
Payments to Acquire Other Property, Plant, and Equipment			81,962
Rental income:
Total Rental Income	676,508	$ 649,766	1,341,114	$ 1,280,491
Operating expenses:
Total Operating Expenses	221,642	218,992	454,078	448,770
NOI:
Total NOI	$ 454,866	430,774	$ 887,036	831,721
Reportable Segments (Textuals) [Abstract]
Units in same store properties	97,580		96,761
Rental income	$ 676,508	649,766	$ 1,341,114	1,280,491
Property and maintenance expense	(118,005)	(115,382)	(242,565)	(240,948)
Real estate taxes and insurance expense	(83,119)	(83,055)	(169,551)	(165,149)
Property management expense	(20,518)	(20,555)	(41,962)	(42,673)
Total Operating Expenses	(221,642)	(218,992)	(454,078)	(448,770)
Non-same store [Domain]
Reportable Segments
Assets	3,889,003		3,889,003
Payments to Acquire Other Property, Plant, and Equipment			2,649
Same Store [Domain]
Rental income:
Same store - Rental Income	645,198	614,933	1,267,098	1,207,334
Operating expenses:
Same store - Operating Expenses	211,126	203,515	423,791	413,053
NOI:
Same store - NOI	434,072	411,418	843,307	794,281
Same Store [Domain] | Boston [Domain]
Reportable Segments
Assets	1,868,144		1,868,144
Payments to Acquire Other Property, Plant, and Equipment			6,525
Rental income:
Same store - Rental Income	62,556	60,779	124,419	120,678
Operating expenses:
Same store - Operating Expenses	20,061	18,442	41,741	39,725
NOI:
Same store - NOI	42,495	42,337	82,678	80,953
Same Store [Domain] | Denver [Domain]
Reportable Segments
Assets	510,123		510,123
Payments to Acquire Other Property, Plant, and Equipment			2,958
Rental income:
Same store - Rental Income	29,829	27,511	58,826	54,123
Operating expenses:
Same store - Operating Expenses	8,168	7,635	15,627	15,097
NOI:
Same store - NOI	21,661	19,876	43,199	39,026
Same Store [Domain] | New York [Domain]
Reportable Segments
Assets	4,602,213		4,602,213
Payments to Acquire Other Property, Plant, and Equipment			8,365
Rental income:
Same store - Rental Income	118,485	114,221	234,555	225,407
Operating expenses:
Same store - Operating Expenses	44,091	41,410	89,855	87,045
NOI:
Same store - NOI	74,394	72,811	144,700	138,362
Same Store [Domain] | San Francisco [Domain]
Reportable Segments
Assets	2,683,829		2,683,829
Payments to Acquire Other Property, Plant, and Equipment			12,676
Rental income:
Same store - Rental Income	92,549	84,082	182,399	165,343
Operating expenses:
Same store - Operating Expenses	26,854	26,354	53,957	52,964
NOI:
Same store - NOI	65,695	57,728	128,442	112,379
Same Store [Domain] | Seattle [Domain]
Reportable Segments
Assets	1,080,755		1,080,755
Payments to Acquire Other Property, Plant, and Equipment			6,505
Rental income:
Same store - Rental Income	41,515	38,890	81,906	76,511
Operating expenses:
Same store - Operating Expenses	13,030	13,036	25,697	25,770
NOI:
Same store - NOI	28,485	25,854	56,209	50,741
Same Store [Domain] | South Florida [Domain]
Reportable Segments
Assets	1,118,604		1,118,604
Payments to Acquire Other Property, Plant, and Equipment			7,453
Rental income:
Same store - Rental Income	50,527	47,860	99,828	94,755
Operating expenses:
Same store - Operating Expenses	18,256	17,634	36,224	35,159
NOI:
Same store - NOI	32,271	30,226	63,604	59,596
Same Store [Domain] | Southern California [Domain]
Reportable Segments
Assets	2,768,169		2,768,169
Payments to Acquire Other Property, Plant, and Equipment			14,390
Rental income:
Same store - Rental Income	110,357	104,319	212,517	201,450
Operating expenses:
Same store - Operating Expenses	34,984	34,697	68,164	67,298
NOI:
Same store - NOI	75,373	69,622	144,353	134,152
Same Store [Domain] | Washington DC [Domain]
Reportable Segments
Assets	4,222,865		4,222,865
Payments to Acquire Other Property, Plant, and Equipment			17,340
Rental income:
Same store - Rental Income	116,168	115,081	226,819	224,975
Operating expenses:
Same store - Operating Expenses	37,585	36,372	75,620	73,399
NOI:
Same store - NOI	78,583	78,709	151,199	151,576
Same Store [Domain] | Non-Core [Domain]
Reportable Segments
Assets	338,125		338,125
Payments to Acquire Other Property, Plant, and Equipment			3,101
Rental income:
Same store - Rental Income	23,212	22,190	45,829	44,092
Operating expenses:
Same store - Operating Expenses	8,097	7,935	16,906	16,596
NOI:
Same store - NOI	15,115	14,255	28,923	27,496
Same Store [Domain] | Same Store [Domain]
Reportable Segments
Assets	19,192,827		19,192,827
Payments to Acquire Other Property, Plant, and Equipment			79,313
Non-same store [Domain] | Boston [Domain]
Reportable Segments
Assets	176,247		176,247
Payments to Acquire Other Property, Plant, and Equipment			83
Rental income:
Non-same store - Rental Income	2,411	791	3,336	1,819
Operating expenses:
Non-same store - Operating Expenses	652	202	882	412
NOI:
Non-same store - NOI	1,759	589	2,454	1,407
Non-same store [Domain] | New York [Domain]
Reportable Segments
Assets	118,166		118,166
Payments to Acquire Other Property, Plant, and Equipment			0
Rental income:
Non-same store - Rental Income	112	0	115	0
Operating expenses:
Non-same store - Operating Expenses	613	0	609	0
NOI:
Non-same store - NOI	(501)	0	(494)	0
Non-same store [Domain] | Seattle [Domain]
Reportable Segments
Assets	227,930		227,930
Payments to Acquire Other Property, Plant, and Equipment			21
Rental income:
Non-same store - Rental Income	4,816	1,129	9,396	1,240
Operating expenses:
Non-same store - Operating Expenses	1,297	511	2,596	657
NOI:
Non-same store - NOI	3,519	618	6,800	583
Non-same store [Domain] | South Florida [Domain]
Reportable Segments
Assets	66,082		66,082
Payments to Acquire Other Property, Plant, and Equipment			22
Rental income:
Non-same store - Rental Income	1,981	1,246	3,913	2,047
Operating expenses:
Non-same store - Operating Expenses	672	797	1,346	1,251
NOI:
Non-same store - NOI	1,309	449	2,567	796
Non-same store [Domain] | Southern California [Domain]
Reportable Segments
Assets	842,096		842,096
Payments to Acquire Other Property, Plant, and Equipment			1,691
Rental income:
Non-same store - Rental Income	14,193	7,836	35,005	19,392
Operating expenses:
Non-same store - Operating Expenses	5,375	4,019	12,864	8,642
NOI:
Non-same store - NOI	8,818	3,817	22,141	10,750
Non-same store [Domain] | Washington DC [Domain]
Reportable Segments
Assets	239,843		239,843
Payments to Acquire Other Property, Plant, and Equipment			509
Rental income:
Non-same store - Rental Income	3,492	2,287	10,877	7,707
Operating expenses:
Non-same store - Operating Expenses	910	854	3,024	2,989
NOI:
Non-same store - NOI	2,582	1,433	7,853	4,718
Non-same store [Domain] | Other (3) [Domain]
Reportable Segments
Assets	2,218,639		2,218,639
Payments to Acquire Other Property, Plant, and Equipment			323
Rental income:
Non-same store - Rental Income	4,305	21,544	11,374	40,952
Operating expenses:
Non-same store - Operating Expenses	997	9,094	8,966	21,766
NOI:
Non-same store - NOI	3,308	12,450	2,408	19,186
Non-same store [Domain] | Non-same store [Domain]
Rental income:
Non-same store - Rental Income	31,310	34,833	74,016	73,157
Operating expenses:
Non-same store - Operating Expenses	10,516	15,477	30,287	35,717
NOI:
Non-same store - NOI	$ 20,794	$ 19,356	$ 43,729	$ 37,440