Reportable Segments (Details) (USD $)	3 Months Ended		6 Months Ended
	Jun. 30, 2011	Jun. 30, 2010	Jun. 30, 2011	Jun. 30, 2010	Dec. 31, 2010
Rental income:
Rental income	$ 496,111,000	$ 444,333,000	$ 974,096,000	$ 871,091,000
Segment Reporting Information Operating Expenses Abstract
Operating expenses	180,560,000	172,659,000	364,898,000	349,053,000
NOI:
Net Operating Income	315,551,000	271,674,000	609,198,000	522,038,000
Total assets	16,398,122,000		16,398,122,000		16,184,194,000
Rental income	496,111,000	444,333,000	974,096,000	871,091,000
Reconciliation of NOI
Rental income	496,111,000	444,333,000	974,096,000	871,091,000
Property and maintenance expense	103,092,000	100,045,000	211,418,000	202,801,000
Real estate taxes and insurance expense	56,701,000	52,350,000	110,332,000	105,496,000
Property management expense	20,767,000	20,264,000	43,148,000	40,756,000
Operating expenses	(180,560,000)	(172,659,000)	(364,898,000)	(349,053,000)
Net Operating Income	315,551,000	271,674,000	609,198,000	522,038,000
Reportable Segments (Textuals) [Abstract]
Units in same store properties	105,730	105,730	104,163	104,163
Overhead Costs	100,000	100,000	200,000	300,000
Threshold limit of major customers	10.00%	10.00%	10.00%	10.00%
Number of customers accounted for threshold limit of major customers	0	0	0	0
Northeast [Member]
Rental income:
Rental income	181,899,000	161,821,000	355,329,000	312,524,000
Segment Reporting Information Operating Expenses Abstract
Operating expenses	65,560,000	61,543,000	133,575,000	122,612,000
NOI:
Net Operating Income	116,339,000	100,278,000	221,754,000	189,912,000
Total assets	6,216,580,000		6,216,580,000
Rental income	181,899,000	161,821,000	355,329,000	312,524,000
Reconciliation of NOI
Rental income	181,899,000	161,821,000	355,329,000	312,524,000
Operating expenses	(65,560,000)	(61,543,000)	(133,575,000)	(122,612,000)
Net Operating Income	116,339,000	100,278,000	221,754,000	189,912,000
Northeast [Member] | Same Store [Member]
Rental income:
Rental income	161,318,000	152,989,000	290,625,000	275,609,000
Segment Reporting Information Operating Expenses Abstract
Operating expenses	58,024,000	57,136,000	107,507,000	106,286,000
NOI:
Net Operating Income	103,294,000	95,853,000	183,118,000	169,323,000
Rental income	161,318,000	152,989,000	290,625,000	275,609,000
Reconciliation of NOI
Rental income	161,318,000	152,989,000	290,625,000	275,609,000
Operating expenses	(58,024,000)	(57,136,000)	(107,507,000)	(106,286,000)
Net Operating Income	103,294,000	95,853,000	183,118,000	169,323,000
Northeast [Member] | Non Same Store [Member]
Rental income:
Rental income	20,581,000	8,832,000	64,704,000	36,915,000
Segment Reporting Information Operating Expenses Abstract
Operating expenses	7,536,000	4,407,000	26,068,000	16,326,000
NOI:
Net Operating Income	13,045,000	4,425,000	38,636,000	20,589,000
Rental income	20,581,000	8,832,000	64,704,000	36,915,000
Reconciliation of NOI
Rental income	20,581,000	8,832,000	64,704,000	36,915,000
Operating expenses	(7,536,000)	(4,407,000)	(26,068,000)	(16,326,000)
Net Operating Income	13,045,000	4,425,000	38,636,000	20,589,000
Northwest [Member]
Rental income:
Rental income	95,661,000	84,475,000	188,115,000	165,766,000
Segment Reporting Information Operating Expenses Abstract
Operating expenses	33,880,000	31,696,000	67,908,000	62,476,000
NOI:
Net Operating Income	61,781,000	52,779,000	120,207,000	103,290,000
Total assets	2,664,432,000		2,664,432,000
Rental income	95,661,000	84,475,000	188,115,000	165,766,000
Reconciliation of NOI
Rental income	95,661,000	84,475,000	188,115,000	165,766,000
Operating expenses	(33,880,000)	(31,696,000)	(67,908,000)	(62,476,000)
Net Operating Income	61,781,000	52,779,000	120,207,000	103,290,000
Northwest [Member] | Same Store [Member]
Rental income:
Rental income	87,338,000	82,166,000	169,611,000	160,758,000
Segment Reporting Information Operating Expenses Abstract
Operating expenses	30,819,000	30,498,000	60,614,000	60,215,000
NOI:
Net Operating Income	56,519,000	51,668,000	108,997,000	100,543,000
Rental income	87,338,000	82,166,000	169,611,000	160,758,000
Reconciliation of NOI
Rental income	87,338,000	82,166,000	169,611,000	160,758,000
Operating expenses	(30,819,000)	(30,498,000)	(60,614,000)	(60,215,000)
Net Operating Income	56,519,000	51,668,000	108,997,000	100,543,000
Northwest [Member] | Non Same Store [Member]
Rental income:
Rental income	8,323,000	2,309,000	18,504,000	5,008,000
Segment Reporting Information Operating Expenses Abstract
Operating expenses	3,061,000	1,198,000	7,294,000	2,261,000
NOI:
Net Operating Income	5,262,000	1,111,000	11,210,000	2,747,000
Rental income	8,323,000	2,309,000	18,504,000	5,008,000
Reconciliation of NOI
Rental income	8,323,000	2,309,000	18,504,000	5,008,000
Operating expenses	(3,061,000)	(1,198,000)	(7,294,000)	(2,261,000)
Net Operating Income	5,262,000	1,111,000	11,210,000	2,747,000
Southeast [Member]
Rental income:
Rental income	97,471,000	91,962,000	193,353,000	182,641,000
Segment Reporting Information Operating Expenses Abstract
Operating expenses	38,858,000	37,330,000	77,569,000	76,171,000
NOI:
Net Operating Income	58,613,000	54,632,000	115,784,000	106,470,000
Total assets	2,575,526,000		2,575,526,000
Rental income	97,471,000	91,962,000	193,353,000	182,641,000
Reconciliation of NOI
Rental income	97,471,000	91,962,000	193,353,000	182,641,000
Operating expenses	(38,858,000)	(37,330,000)	(77,569,000)	(76,171,000)
Net Operating Income	58,613,000	54,632,000	115,784,000	106,470,000
Southeast [Member] | Same Store [Member]
Rental income:
Rental income	93,372,000	89,389,000	185,295,000	178,368,000
Segment Reporting Information Operating Expenses Abstract
Operating expenses	37,198,000	36,054,000	74,328,000	74,069,000
NOI:
Net Operating Income	56,174,000	53,335,000	110,967,000	104,299,000
Rental income	93,372,000	89,389,000	185,295,000	178,368,000
Reconciliation of NOI
Rental income	93,372,000	89,389,000	185,295,000	178,368,000
Operating expenses	(37,198,000)	(36,054,000)	(74,328,000)	(74,069,000)
Net Operating Income	56,174,000	53,335,000	110,967,000	104,299,000
Southeast [Member] | Non Same Store [Member]
Rental income:
Rental income	4,099,000	2,573,000	8,058,000	4,273,000
Segment Reporting Information Operating Expenses Abstract
Operating expenses	1,660,000	1,276,000	3,241,000	2,102,000
NOI:
Net Operating Income	2,439,000	1,297,000	4,817,000	2,171,000
Rental income	4,099,000	2,573,000	8,058,000	4,273,000
Reconciliation of NOI
Rental income	4,099,000	2,573,000	8,058,000	4,273,000
Operating expenses	(1,660,000)	(1,276,000)	(3,241,000)	(2,102,000)
Net Operating Income	2,439,000	1,297,000	4,817,000	2,171,000
Southwest [Member]
Rental income:
Rental income	117,303,000	106,346,000	230,604,000	210,854,000
Segment Reporting Information Operating Expenses Abstract
Operating expenses	40,608,000	38,566,000	79,899,000	77,145,000
NOI:
Net Operating Income	76,695,000	67,780,000	150,705,000	133,709,000
Total assets	3,229,298,000		3,229,298,000
Rental income	117,303,000	106,346,000	230,604,000	210,854,000
Reconciliation of NOI
Rental income	117,303,000	106,346,000	230,604,000	210,854,000
Operating expenses	(40,608,000)	(38,566,000)	(79,899,000)	(77,145,000)
Net Operating Income	76,695,000	67,780,000	150,705,000	133,709,000
Southwest [Member] | Same Store [Member]
Rental income:
Rental income	108,394,000	104,968,000	213,357,000	207,541,000
Segment Reporting Information Operating Expenses Abstract
Operating expenses	36,989,000	38,143,000	72,723,000	75,470,000
NOI:
Net Operating Income	71,405,000	66,825,000	140,634,000	132,071,000
Rental income	108,394,000	104,968,000	213,357,000	207,541,000
Reconciliation of NOI
Rental income	108,394,000	104,968,000	213,357,000	207,541,000
Operating expenses	(36,989,000)	(38,143,000)	(72,723,000)	(75,470,000)
Net Operating Income	71,405,000	66,825,000	140,634,000	132,071,000
Southwest [Member] | Non Same Store [Member]
Rental income:
Rental income	8,909,000	1,378,000	17,247,000	3,313,000
Segment Reporting Information Operating Expenses Abstract
Operating expenses	3,619,000	423,000	7,176,000	1,675,000
NOI:
Net Operating Income	5,290,000	955,000	10,071,000	1,638,000
Rental income	8,909,000	1,378,000	17,247,000	3,313,000
Reconciliation of NOI
Rental income	8,909,000	1,378,000	17,247,000	3,313,000
Operating expenses	(3,619,000)	(423,000)	(7,176,000)	(1,675,000)
Net Operating Income	5,290,000	955,000	10,071,000	1,638,000
Others [Member]
Rental income:
Rental income	3,777,000	(271,000)	6,695,000	(694,000)
Segment Reporting Information Operating Expenses Abstract
Operating expenses	1,654,000	3,524,000	5,947,000	10,649,000
NOI:
Net Operating Income	2,123,000	(3,795,000)	748,000	(11,343,000)
Total assets	1,712,286,000		1,712,286,000
Rental income	3,777,000	(271,000)	6,695,000	(694,000)
Reconciliation of NOI
Rental income	3,777,000	(271,000)	6,695,000	(694,000)
Operating expenses	(1,654,000)	(3,524,000)	(5,947,000)	(10,649,000)
Net Operating Income	2,123,000	(3,795,000)	748,000	(11,343,000)
Others [Member] | Non Same Store [Member]
Rental income:
Rental income	3,777,000	(271,000)	6,695,000	(694,000)
Segment Reporting Information Operating Expenses Abstract
Operating expenses	1,654,000	3,524,000	5,947,000	10,649,000
NOI:
Net Operating Income	2,123,000	(3,795,000)	748,000	(11,343,000)
Rental income	3,777,000	(271,000)	6,695,000	(694,000)
Reconciliation of NOI
Rental income	3,777,000	(271,000)	6,695,000	(694,000)
Operating expenses	(1,654,000)	(3,524,000)	(5,947,000)	(10,649,000)
Net Operating Income	2,123,000	(3,795,000)	748,000	(11,343,000)
Same Store [Member]
Rental income:
Rental income	450,422,000	429,512,000	858,888,000	822,276,000
Segment Reporting Information Operating Expenses Abstract
Operating expenses	163,030,000	161,831,000	315,172,000	316,040,000
NOI:
Net Operating Income	287,392,000	267,681,000	543,716,000	506,236,000
Rental income	450,422,000	429,512,000	858,888,000	822,276,000
Reconciliation of NOI
Rental income	450,422,000	429,512,000	858,888,000	822,276,000
Operating expenses	(163,030,000)	(161,831,000)	(315,172,000)	(316,040,000)
Net Operating Income	287,392,000	267,681,000	543,716,000	506,236,000
Non Same Store [Member]
Rental income:
Rental income	45,689,000	14,821,000	115,208,000	48,815,000
Segment Reporting Information Operating Expenses Abstract
Operating expenses	17,530,000	10,828,000	49,726,000	33,013,000
NOI:
Net Operating Income	28,159,000	3,993,000	65,482,000	15,802,000
Rental income	45,689,000	14,821,000	115,208,000	48,815,000
Reconciliation of NOI
Rental income	45,689,000	14,821,000	115,208,000	48,815,000
Operating expenses	(17,530,000)	(10,828,000)	(49,726,000)	(33,013,000)
Net Operating Income	28,159,000	3,993,000	65,482,000	15,802,000
Material Reconciling Items [Member]
Rental income:
Rental income	2,400,000	2,000,000	2,400,000	2,000,000
NOI:
Rental income	2,400,000	2,000,000	2,400,000	2,000,000
Reconciliation of NOI
Rental income	$ 2,400,000	$ 2,000,000	$ 2,400,000	$ 2,000,000