Reportable Segments (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Rental income:
Same store	$ 1,769,280	$ 1,693,239	$ 1,771,449
Segment Reporting Segment Non Same Store Rental Income	608,724	44,690	(255,255)
Segment Reporting Information Revenues	2,378,004	1,737,929	1,516,194
Operating expenses:
Same store	607,243	587,037	638,671
Non-same store/other	220,559	33,778	(74,018)
Segment Reporting Segment Operating Expenses	827,802	620,815	564,653
NOI:
Same store	1,162,037	1,106,202	1,132,778
Non-same store/other	388,165	10,912	(181,237)
Total NOI	1,550,202	1,117,114	951,541
Assets	22,834,545	17,201,000
Payments to Acquire Other Property, Plant, and Equipment	135,816	152,828
Reportable Segments (Textuals) [Abstract]
Units in same store properties	80,247	98,577
Rental income	2,378,004	1,737,929	1,516,194
Property and maintenance expense	(449,461)	(332,190)	(304,380)
Real estate taxes and insurance expense	(293,999)	(206,723)	(178,406)
Property management expense	(84,342)	(81,902)	(81,867)
Total operating expenses	(827,802)	(620,815)	(564,653)
Same Store [Domain]
NOI:
Assets	11,945,273	12,341,635
Payments to Acquire Other Property, Plant, and Equipment	94,492	122,024
Non-same store [Domain]
NOI:
Assets	10,889,272	4,859,365
Payments to Acquire Other Property, Plant, and Equipment	41,324	30,804
Boston [Domain]
Rental income:
Same store	175,031	168,063	142,514
Segment Reporting Segment Non Same Store Rental Income	61,139	0	8,115
Operating expenses:
Same store	57,261	54,888	49,317
Non-same store/other	18,238	0	2,361
NOI:
Same store	117,770	113,175	93,197
Non-same store/other	42,901	0	5,754
Boston [Domain] | Same Store [Domain]
NOI:
Assets	1,087,370	1,133,098
Payments to Acquire Other Property, Plant, and Equipment	15,630	22,592
Boston [Domain] | Non-same store [Domain]
NOI:
Assets	946,747	0
Payments to Acquire Other Property, Plant, and Equipment	2,097	0
Denver [Domain]
Rental income:
Same store	100,425	93,571	99,681
Segment Reporting Segment Non Same Store Rental Income	2,805	1,325	0
Operating expenses:
Same store	30,028	28,204	32,564
Non-same store/other	744	429	1
NOI:
Same store	70,397	65,367	67,117
Non-same store/other	2,061	896	(1)
Denver [Domain] | Same Store [Domain]
NOI:
Assets	520,999	542,243
Payments to Acquire Other Property, Plant, and Equipment	5,330	4,593
Denver [Domain] | Non-same store [Domain]
NOI:
Assets	20,481	20,974
Payments to Acquire Other Property, Plant, and Equipment	54	5
New York Domain]
Rental income:
Same store	286,345	274,683	254,441
Segment Reporting Segment Non Same Store Rental Income	136,182	14,611	6,794
Operating expenses:
Same store	114,587	109,667	104,492
Non-same store/other	43,055	5,988	366
NOI:
Same store	171,758	165,016	149,949
Non-same store/other	93,127	8,623	6,428
New York Domain] | Same Store [Domain]
NOI:
Assets	2,678,546	2,742,346
Payments to Acquire Other Property, Plant, and Equipment	8,982	12,437
New York Domain] | Non-same store [Domain]
NOI:
Assets	2,092,454	406,013
Payments to Acquire Other Property, Plant, and Equipment	3,024	142
San Francisco [Domain]
Rental income:
Same store	173,011	159,535	126,951
Segment Reporting Segment Non Same Store Rental Income	119,749	7,268	3,889
Operating expenses:
Same store	59,104	57,373	43,627
Non-same store/other	42,851	3,022	1,796
NOI:
Same store	113,907	102,162	83,324
Non-same store/other	76,898	4,246	2,093
San Francisco [Domain] | Same Store [Domain]
NOI:
Assets	968,840	999,267
Payments to Acquire Other Property, Plant, and Equipment	11,767	18,010
San Francisco [Domain] | Non-same store [Domain]
NOI:
Assets	1,824,550	178,339
Payments to Acquire Other Property, Plant, and Equipment	9,989	1,176
Seattle [Domain]
Rental income:
Same store	129,283	122,267	122,494
Segment Reporting Segment Non Same Store Rental Income	19,462	4,747	6,012
Operating expenses:
Same store	43,718	41,041	45,275
Non-same store/other	6,284	1,510	2,149
NOI:
Same store	85,565	81,226	77,219
Non-same store/other	13,178	3,237	3,863
Seattle [Domain] | Same Store [Domain]
NOI:
Assets	835,584	865,068
Payments to Acquire Other Property, Plant, and Equipment	6,398	6,892
Seattle [Domain] | Non-same store [Domain]
NOI:
Assets	312,240	90,205
Payments to Acquire Other Property, Plant, and Equipment	1,598	67
South Florida [Domain]
Rental income:
Same store	185,361	177,675	174,417
Segment Reporting Segment Non Same Store Rental Income	2,653	0	14,488
Operating expenses:
Same store	69,891	67,811	66,838
Non-same store/other	1,031	0	5,165
NOI:
Same store	115,470	109,864	107,579
Non-same store/other	1,622	0	9,323
South Florida [Domain] | Same Store [Domain]
NOI:
Assets	1,157,283	1,193,506
Payments to Acquire Other Property, Plant, and Equipment	14,550	17,338
South Florida [Domain] | Non-same store [Domain]
NOI:
Assets	50,414	0
Payments to Acquire Other Property, Plant, and Equipment	300	0
Southern California [Domain]
Rental income:
Same store	333,917	320,749	299,508
Segment Reporting Segment Non Same Store Rental Income	74,123	3,040	30,539
Operating expenses:
Same store	107,346	103,925	98,907
Non-same store/other	31,599	1,179	12,144
NOI:
Same store	226,571	216,824	200,601
Non-same store/other	42,524	1,861	18,395
Southern California [Domain] | Same Store [Domain]
NOI:
Assets	2,177,336	2,250,301
Payments to Acquire Other Property, Plant, and Equipment	16,580	17,747
Southern California [Domain] | Non-same store [Domain]
NOI:
Assets	1,078,038	70,389
Payments to Acquire Other Property, Plant, and Equipment	3,975	141
Washington DC [Domain]
Rental income:
Same store	253,056	247,880	240,755
Segment Reporting Segment Non Same Store Rental Income	179,077	13,124	36,657
Operating expenses:
Same store	76,033	75,580	75,492
Non-same store/other	58,759	3,984	11,373
NOI:
Same store	177,023	172,300	165,263
Non-same store/other	120,318	9,140	25,284
Washington DC [Domain] | Same Store [Domain]
NOI:
Assets	1,873,897	1,941,446
Payments to Acquire Other Property, Plant, and Equipment	10,069	15,426
Washington DC [Domain] | Non-same store [Domain]
NOI:
Assets	2,664,702	276,901
Payments to Acquire Other Property, Plant, and Equipment	14,877	1,062
Non-Core [Domain]
Rental income:
Same store	132,851	128,816	310,688
Operating expenses:
Same store	49,275	48,548	122,159
NOI:
Same store	83,576	80,268	188,529
Non-Core [Domain] | Same Store [Domain]
NOI:
Assets	645,418	674,360
Payments to Acquire Other Property, Plant, and Equipment	5,186	6,989
Other [Domain]
Rental income:
Segment Reporting Segment Non Same Store Rental Income	13,534	575	(3,477)
Operating expenses:
Non-same store/other	17,998	17,666	7,326
NOI:
Non-same store/other	(4,464)	(17,091)	(10,803)
Payments to Acquire Other Property, Plant, and Equipment	5,410	28,211
Other [Domain] | Non-same store [Domain]
NOI:
Assets	1,899,646	3,816,544
Properties sold in 2013 [Domain]
Rental income:
Segment Reporting Segment Non Same Store Rental Income	0	0	(358,272)
Operating expenses:
Non-same store/other	0	0	(116,699)
NOI:
Non-same store/other	$ 0	$ 0	$ (241,573)