Reportable Segments (Details) (USD $)	3 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011	Dec. 31, 2011
Rental income:
Same store	$ 489,865,000	$ 464,313,000	$ 950,505,000	$ 901,181,000
Non-same store/other	51,704,000	14,106,000	112,657,000	37,940,000
Total rental income	541,569,000	478,419,000	1,063,162,000	939,121,000
Operating expenses:
Same store	170,209,000	166,862,000	338,380,000	331,925,000
Non-same store/other	20,949,000	7,279,000	42,809,000	19,990,000
Total operating expenses	191,158,000	174,141,000	381,189,000	351,915,000
NOI:
Same store	319,656,000	297,451,000	612,125,000	569,256,000
Non-same store/other	30,755,000	6,827,000	69,848,000	17,950,000
Total NOI	350,411,000	304,278,000	681,973,000	587,206,000
Total assets	16,714,412,000		16,714,412,000		16,659,303,000
Segment Reporting Information, Expenditures for Additions to Long-Lived Assets			68,319,000
Reconciliation of NOI
Rental income	541,569,000	478,419,000	1,063,162,000	939,121,000
Property and maintenance expense	(107,024,000)	(98,571,000)	(218,004,000)	(202,157,000)
Real estate taxes and insurance expense	(63,126,000)	(54,803,000)	(118,768,000)	(106,610,000)
Property management expense	(21,008,000)	(20,767,000)	(44,417,000)	(43,148,000)
Total operating expenses	(191,158,000)	(174,141,000)	(381,189,000)	(351,915,000)
Reportable Segments (Textuals) [Abstract]
Units in same store properties	105,604		103,950
Overhead Costs		100,000		200,000
Northeast [Member]
Rental income:
Same store	180,583,000	170,855,000	352,015,000	332,964,000
Non-same store/other	23,587,000	9,546,000	45,941,000	19,106,000
Total rental income	204,170,000	180,401,000	397,956,000	352,070,000
Operating expenses:
Same store	63,461,000	60,644,000	128,239,000	124,633,000
Non-same store/other	7,912,000	3,900,000	13,188,000	7,035,000
Total operating expenses	71,373,000	64,544,000	141,427,000	131,668,000
NOI:
Same store	117,122,000	110,211,000	223,776,000	208,331,000
Non-same store/other	15,675,000	5,646,000	32,753,000	12,071,000
Total NOI	132,797,000	115,857,000	256,529,000	220,402,000
Total assets	6,918,894,000		6,918,894,000
Segment Reporting Information, Expenditures for Additions to Long-Lived Assets			23,199,000
Northwest [Member]
Rental income:
Same store	99,300,000	91,969,000	193,858,000	179,133,000
Non-same store/other	12,054,000	245,000	23,425,000	2,175,000
Total rental income	111,354,000	92,214,000	217,283,000	181,308,000
Operating expenses:
Same store	33,264,000	32,495,000	66,223,000	64,450,000
Non-same store/other	5,809,000	32,000	11,658,000	626,000
Total operating expenses	39,073,000	32,527,000	77,881,000	65,076,000
NOI:
Same store	66,036,000	59,474,000	127,635,000	114,683,000
Non-same store/other	6,245,000	213,000	11,767,000	1,549,000
Total NOI	72,281,000	59,687,000	139,402,000	116,232,000
Total assets	2,919,857,000		2,919,857,000
Segment Reporting Information, Expenditures for Additions to Long-Lived Assets			15,225,000
Southeast [Member]
Rental income:
Same store	94,829,000	90,446,000	181,102,000	173,488,000
Non-same store/other	1,363,000	0	9,572,000	5,977,000
Total rental income	96,192,000	90,446,000	190,674,000	179,465,000
Operating expenses:
Same store	36,255,000	35,800,000	70,308,000	69,175,000
Non-same store/other	751,000	0	3,644,000	2,323,000
Total operating expenses	37,006,000	35,800,000	73,952,000	71,498,000
NOI:
Same store	58,574,000	54,646,000	110,794,000	104,313,000
Non-same store/other	612,000	0	5,928,000	3,654,000
Total NOI	59,186,000	54,646,000	116,722,000	107,967,000
Total assets	2,432,052,000		2,432,052,000
Segment Reporting Information, Expenditures for Additions to Long-Lived Assets			14,837,000
Southwest [Member]
Rental income:
Same store	115,153,000	111,043,000	223,530,000	215,596,000
Non-same store/other	12,361,000	3,695,000	29,015,000	9,941,000
Total rental income	127,514,000	114,738,000	252,545,000	225,537,000
Operating expenses:
Same store	37,229,000	37,923,000	73,610,000	73,667,000
Non-same store/other	4,569,000	1,726,000	9,995,000	4,474,000
Total operating expenses	41,798,000	39,649,000	83,605,000	78,141,000
NOI:
Same store	77,924,000	73,120,000	149,920,000	141,929,000
Non-same store/other	7,792,000	1,969,000	19,020,000	5,467,000
Total NOI	85,716,000	75,089,000	168,940,000	147,396,000
Total assets	3,355,321,000		3,355,321,000
Segment Reporting Information, Expenditures for Additions to Long-Lived Assets			13,196,000
Other [Member]
Rental income:
Same store	0	0	0	0
Non-same store/other	2,339,000	620,000	4,704,000	741,000
Total rental income	2,339,000	620,000	4,704,000	741,000
Operating expenses:
Same store	0	0	0	0
Non-same store/other	1,908,000	1,621,000	4,324,000	5,532,000
Total operating expenses	1,908,000	1,621,000	4,324,000	5,532,000
NOI:
Same store	0	0	0	0
Non-same store/other	431,000	(1,001,000)	380,000	(4,791,000)
Total NOI	431,000	(1,001,000)	380,000	(4,791,000)
Total assets	1,088,288,000		1,088,288,000
Segment Reporting Information, Expenditures for Additions to Long-Lived Assets			$ 1,862,000