Reportable Segments (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012	Dec. 31, 2012
Rental income:
Same store	$ 461,195	$ 439,744	$ 906,902	$ 864,071
Non-same store/other	171,210	6,395	245,016	10,267
Total rental income	632,405	446,139	1,151,918	874,338
Operating expenses:
Same store	158,469	153,009	317,324	306,986
Non-same store/other	59,928	6,773	93,013	15,377
Total operating expenses	218,397	159,782	410,337	322,363
NOI:
Same store	302,726	286,735	589,578	557,085
Non-same store/other	111,282	(378)	152,003	(5,110)
Total NOI	414,008	286,357	741,581	551,975
Total assets	23,482,383		23,482,383		17,201,000
Segment Reporting Information, Expenditures for Additions to Long-Lived Assets			57,253
Reportable Segments (Textuals) [Abstract]
Rental income	632,405	446,139	1,151,918	874,338
Cost of Property Repairs and Maintenance	(117,819)	(85,423)	(220,417)	(173,819)
Real Estate Taxes and Insurance	(78,547)	(53,422)	(145,400)	(104,268)
Property management	(22,031)	(20,937)	(44,520)	(44,276)
Total Operating Expenses	(218,397)	(159,782)	(410,337)	(322,363)
Units in same store properties	85,509		84,965
Northeast [Member]
Rental income:
Same store	185,729	179,400	367,912	353,749
Non-same store/other	105,007	5,279	148,738	5,580
Total rental income	290,736	184,679	516,650	359,329
Operating expenses:
Same store	64,923	62,557	132,525	127,586
Non-same store/other	34,235	1,903	48,299	2,806
Total operating expenses	99,158	64,460	180,824	130,392
NOI:
Same store	120,806	116,843	235,387	226,163
Non-same store/other	70,772	3,376	100,439	2,774
Total NOI	191,578	120,219	335,826	228,937
Total assets	11,418,096		11,418,096
Segment Reporting Information, Expenditures for Additions to Long-Lived Assets			21,826
Northwest [Member]
Rental income:
Same store	106,008	98,281	204,680	189,969
Non-same store/other	38,541	1,127	57,614	3,912
Total rental income	144,549	99,408	262,294	193,881
Operating expenses:
Same store	35,481	34,254	69,442	67,360
Non-same store/other	12,872	350	19,942	1,388
Total operating expenses	48,353	34,604	89,384	68,748
NOI:
Same store	70,527	64,027	135,238	122,609
Non-same store/other	25,669	777	37,672	2,524
Total NOI	96,196	64,804	172,910	125,133
Total assets	4,610,358		4,610,358
Segment Reporting Information, Expenditures for Additions to Long-Lived Assets			14,465
Southeast [Member]
Rental income:
Same store	60,776	57,953	120,456	114,924
Non-same store/other	760	0	1,017	0
Total rental income	61,536	57,953	121,473	114,924
Operating expenses:
Same store	22,732	22,352	45,190	44,249
Non-same store/other	302	0	406	0
Total operating expenses	23,034	22,352	45,596	44,249
NOI:
Same store	38,044	35,601	75,266	70,675
Non-same store/other	458	0	611	0
Total NOI	38,502	35,601	75,877	70,675
Total assets	1,576,965		1,576,965
Segment Reporting Information, Expenditures for Additions to Long-Lived Assets			8,276
Southwest [Member]
Rental income:
Same store	108,682	104,110	213,854	205,429
Non-same store/other	21,296	50	30,141	818
Total rental income	129,978	104,160	243,995	206,247
Operating expenses:
Same store	35,333	33,846	70,167	67,791
Non-same store/other	9,285	2	12,956	299
Total operating expenses	44,618	33,848	83,123	68,090
NOI:
Same store	73,349	70,264	143,687	137,638
Non-same store/other	12,011	48	17,185	519
Total NOI	85,360	70,312	160,872	138,157
Total assets	3,785,624		3,785,624
Segment Reporting Information, Expenditures for Additions to Long-Lived Assets			8,921
Other [Member]
Rental income:
Same store	0	0	0	0
Non-same store/other	5,606	(61)	7,506	(43)
Total rental income	5,606	(61)	7,506	(43)
Operating expenses:
Same store	0	0	0	0
Non-same store/other	3,234	4,518	11,410	10,884
Total operating expenses	3,234	4,518	11,410	10,884
NOI:
Same store	0	0	0	0
Non-same store/other	2,372	(4,579)	(3,904)	(10,927)
Total NOI	2,372	(4,579)	(3,904)	(10,927)
Total assets	2,091,340		2,091,340
Segment Reporting Information, Expenditures for Additions to Long-Lived Assets			$ 3,765