Reportable Segments (Details) (USD $)	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011	Dec. 31, 2011
Rental income:
Same store	$ 508,063,000	$ 480,256,000	$ 1,428,431,000	$ 1,353,690,000
Non-same store/other	45,029,000	10,688,000	174,204,000	63,446,000
Total rental income	553,092,000	490,944,000	1,602,635,000	1,417,136,000
Operating expenses:
Same store	177,463,000	172,382,000	505,516,000	494,509,000
Non-same store/other	15,944,000	5,614,000	64,546,000	30,474,000
Total operating expenses	193,407,000	177,996,000	570,062,000	524,983,000
NOI:
Same store	330,600,000	307,874,000	922,915,000	859,181,000
Non-same store/other	29,085,000	5,074,000	109,658,000	32,972,000
Total NOI	359,685,000	312,948,000	1,032,573,000	892,153,000
Total assets	16,668,545,000		16,668,545,000		16,659,303,000
Segment Reporting Information, Expenditures for Additions to Long-Lived Assets			114,535,000
Reconciliation of NOI
Rental income	553,092,000	490,944,000	1,602,635,000	1,417,136,000
Property and maintenance expense	(110,679,000)	(101,712,000)	(325,071,000)	(300,362,000)
Real estate taxes and insurance expense	(64,235,000)	(57,109,000)	(182,222,000)	(162,430,000)
Property management expense	(18,493,000)	(19,175,000)	(62,769,000)	(62,191,000)
Total operating expenses	(193,407,000)	(177,996,000)	(570,062,000)	(524,983,000)
Reportable Segments (Textuals) [Abstract]
Units in same store properties	105,902		102,241
Overhead Costs		100,000		300,000
Northeast [Member]
Rental income:
Same store	190,140,000	180,074,000	530,972,000	502,724,000
Non-same store/other	21,399,000	6,034,000	76,922,000	33,937,000
Total rental income	211,539,000	186,108,000	607,894,000	536,661,000
Operating expenses:
Same store	66,289,000	64,479,000	190,952,000	185,770,000
Non-same store/other	7,318,000	1,926,000	23,636,000	11,799,000
Total operating expenses	73,607,000	66,405,000	214,588,000	197,569,000
NOI:
Same store	123,851,000	115,595,000	340,020,000	316,954,000
Non-same store/other	14,081,000	4,108,000	53,286,000	22,138,000
Total NOI	137,932,000	119,703,000	393,306,000	339,092,000
Total assets	6,945,739,000		6,945,739,000
Segment Reporting Information, Expenditures for Additions to Long-Lived Assets			42,259,000
Northwest [Member]
Rental income:
Same store	103,926,000	95,679,000	295,518,000	272,851,000
Non-same store/other	12,226,000	86,000	37,945,000	4,217,000
Total rental income	116,152,000	95,765,000	333,463,000	277,068,000
Operating expenses:
Same store	34,805,000	33,844,000	100,384,000	97,617,000
Non-same store/other	5,686,000	195,000	18,197,000	1,680,000
Total operating expenses	40,491,000	34,039,000	118,581,000	99,297,000
NOI:
Same store	69,121,000	61,835,000	195,134,000	175,234,000
Non-same store/other	6,540,000	(109,000)	19,748,000	2,537,000
Total NOI	75,661,000	61,726,000	214,882,000	177,771,000
Total assets	3,029,831,000		3,029,831,000
Segment Reporting Information, Expenditures for Additions to Long-Lived Assets			27,337,000
Southeast [Member]
Rental income:
Same store	93,609,000	89,056,000	264,709,000	253,202,000
Non-same store/other	1,400,000	468,000	14,667,000	9,764,000
Total rental income	95,009,000	89,524,000	279,376,000	262,966,000
Operating expenses:
Same store	36,949,000	34,437,000	103,333,000	100,102,000
Non-same store/other	788,000	154,000	5,793,000	3,528,000
Total operating expenses	37,737,000	34,591,000	109,126,000	103,630,000
NOI:
Same store	56,660,000	54,619,000	161,376,000	153,100,000
Non-same store/other	612,000	314,000	8,874,000	6,236,000
Total NOI	57,272,000	54,933,000	170,250,000	159,336,000
Total assets	2,358,093,000		2,358,093,000
Segment Reporting Information, Expenditures for Additions to Long-Lived Assets			22,280,000
Southwest [Member]
Rental income:
Same store	120,388,000	115,447,000	337,232,000	324,913,000
Non-same store/other	10,126,000	3,791,000	44,834,000	18,902,000
Total rental income	130,514,000	119,238,000	382,066,000	343,815,000
Operating expenses:
Same store	39,420,000	39,622,000	110,847,000	111,020,000
Non-same store/other	3,580,000	1,673,000	15,326,000	8,005,000
Total operating expenses	43,000,000	41,295,000	126,173,000	119,025,000
NOI:
Same store	80,968,000	75,825,000	226,385,000	213,893,000
Non-same store/other	6,546,000	2,118,000	29,508,000	10,897,000
Total NOI	87,514,000	77,943,000	255,893,000	224,790,000
Total assets	3,286,507,000		3,286,507,000
Segment Reporting Information, Expenditures for Additions to Long-Lived Assets			21,732,000
Other [Member]
Rental income:
Same store	0	0	0	0
Non-same store/other	(122,000)	309,000	(164,000)	(3,374,000)
Total rental income	(122,000)	309,000	(164,000)	(3,374,000)
Operating expenses:
Same store	0	0	0	0
Non-same store/other	(1,428,000)	1,666,000	1,594,000	5,462,000
Total operating expenses	(1,428,000)	1,666,000	1,594,000	5,462,000
NOI:
Same store	0	0	0	0
Non-same store/other	1,306,000	(1,357,000)	(1,758,000)	(8,836,000)
Total NOI	1,306,000	(1,357,000)	(1,758,000)	(8,836,000)
Total assets	1,048,375,000		1,048,375,000
Segment Reporting Information, Expenditures for Additions to Long-Lived Assets			$ 927,000