Reportable Segments (Details) (USD $)	3 Months Ended
	Mar. 31, 2012	Mar. 31, 2011	Dec. 31, 2011
Rental income:
Same store	$ 473,957,000	$ 449,232,000
Non-same store/other	51,638,000	15,318,000
Total rental income	525,595,000	464,550,000
Operating expenses:
Same store	173,054,000	170,017,000
Non-same store/other	18,721,000	9,550,000
Total operating expenses	191,775,000	179,567,000
NOI:
Same store	300,903,000	279,215,000
Non-same store/other	32,917,000	5,768,000
Total NOI	333,820,000	284,983,000
Total assets	16,481,247,000		16,659,303,000
Segment Reporting Information, Expenditures for Additions to Long-Lived Assets	30,225,000
Reconciliation of NOI
Rental income	525,595,000	464,550,000
Property and maintenance expense	(112,379,000)	(105,047,000)
Real estate taxes and insurance expense	(55,987,000)	(52,139,000)
Property management expense	(23,409,000)	(22,381,000)
Total operating expenses	(191,775,000)	(179,567,000)
Reportable Segments (Textuals) [Abstract]
Units in same store properties	105,612	105,612
Overhead Costs		100,000
Northeast [Member]
Rental income:
Same store	174,263,000	164,859,000
Non-same store/other	20,378,000	7,658,000
Total rental income	194,641,000	172,517,000
Operating expenses:
Same store	66,000,000	65,195,000
Non-same store/other	4,583,000	2,486,000
Total operating expenses	70,583,000	67,681,000
NOI:
Same store	108,263,000	99,664,000
Non-same store/other	15,795,000	5,172,000
Total NOI	124,058,000	104,836,000
Total assets	6,458,496,000
Segment Reporting Information, Expenditures for Additions to Long-Lived Assets	9,843,000
Northwest [Member]
Rental income:
Same store	95,751,000	88,197,000
Non-same store/other	10,166,000	899,000
Total rental income	105,917,000	89,096,000
Operating expenses:
Same store	33,303,000	32,279,000
Non-same store/other	5,469,000	268,000
Total operating expenses	38,772,000	32,547,000
NOI:
Same store	62,448,000	55,918,000
Non-same store/other	4,697,000	631,000
Total NOI	67,145,000	56,549,000
Total assets	2,931,942,000
Segment Reporting Information, Expenditures for Additions to Long-Lived Assets	6,506,000
Southeast [Member]
Rental income:
Same store	91,757,000	88,061,000
Non-same store/other	4,770,000	2,924,000
Total rental income	96,527,000	90,985,000
Operating expenses:
Same store	36,170,000	35,496,000
Non-same store/other	1,664,000	1,134,000
Total operating expenses	37,834,000	36,630,000
NOI:
Same store	55,587,000	52,565,000
Non-same store/other	3,106,000	1,790,000
Total NOI	58,693,000	54,355,000
Total assets	2,483,091,000
Segment Reporting Information, Expenditures for Additions to Long-Lived Assets	7,440,000
Southwest [Member]
Rental income:
Same store	112,186,000	108,115,000
Non-same store/other	13,930,000	3,715,000
Total rental income	126,116,000	111,830,000
Operating expenses:
Same store	37,581,000	37,047,000
Non-same store/other	4,662,000	1,927,000
Total operating expenses	42,243,000	38,974,000
NOI:
Same store	74,605,000	71,068,000
Non-same store/other	9,268,000	1,788,000
Total NOI	83,873,000	72,856,000
Total assets	3,392,065,000
Segment Reporting Information, Expenditures for Additions to Long-Lived Assets	5,419,000
Other [Member]
Rental income:
Same store	0	0
Non-same store/other	2,394,000	122,000
Total rental income	2,394,000	122,000
Operating expenses:
Same store	0	0
Non-same store/other	2,343,000	3,735,000
Total operating expenses	2,343,000	3,735,000
NOI:
Same store	0	0
Non-same store/other	51,000	(3,613,000)
Total NOI	51,000	(3,613,000)
Total assets	1,215,653,000
Segment Reporting Information, Expenditures for Additions to Long-Lived Assets	$ 1,017,000