Reportable Segments (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012	Dec. 31, 2012
Rental income:
Same store	$ 463,607	$ 445,521	$ 1,329,326	$ 1,269,876
Non-same store/other	163,273	3,126	420,048	25,555
Segment Reporting Information Revenues	626,880	448,647	1,749,374	1,295,431
Operating expenses:
Same store	159,302	154,450	462,509	447,600
Non-same store/other	55,460	3,789	152,865	23,811
Total operating expenses	214,762	158,239	615,374	471,411
NOI:
Same store	304,305	291,071	866,817	822,276
Non-same store/other	107,813	(663)	267,183	1,744
Total NOI	412,118	290,408	1,134,000	824,020
Total assets	23,811,431		23,811,431		17,201,000
Segment Reporting Information, Expenditures for Additions to Long-Lived Assets			96,866
Reportable Segments (Textuals) [Abstract]
Rental income	626,880	448,647	1,749,374	1,295,431
Cost of Property Repairs and Maintenance	(119,632)	(86,682)	(333,202)	(254,009)
Real Estate Taxes and Insurance	(76,255)	(53,064)	(218,777)	(154,633)
Property management	(18,875)	(18,493)	(63,395)	(62,769)
Total Operating Expenses	(214,762)	(158,239)	(615,374)	(471,411)
Units in same store properties	82,553		81,099
Northeast [Member]
Rental income:
Same store	196,089	190,648	555,391	535,830
Non-same store/other	101,001	1,658	263,745	15,876
Segment Reporting Information Revenues	297,090	192,306	819,136	551,706
Operating expenses:
Same store	68,044	65,758	198,068	190,979
Non-same store/other	34,191	994	86,820	6,170
Total operating expenses	102,235	66,752	284,888	197,149
NOI:
Same store	128,045	124,890	357,323	344,851
Non-same store/other	66,810	664	176,925	9,706
Total NOI	194,855	125,554	534,248	354,557
Total assets	11,499,130		11,499,130
Segment Reporting Information, Expenditures for Additions to Long-Lived Assets			39,331
Northwest [Member]
Rental income:
Same store	107,340	100,280	303,680	282,586
Non-same store/other	38,862	1,305	99,689	8,049
Segment Reporting Information Revenues	146,202	101,585	403,369	290,635
Operating expenses:
Same store	35,784	34,443	101,982	98,616
Non-same store/other	13,296	235	34,352	2,581
Total operating expenses	49,080	34,678	136,334	101,197
NOI:
Same store	71,556	65,837	201,698	183,970
Non-same store/other	25,566	1,070	65,337	5,468
Total NOI	97,122	66,907	267,035	189,438
Total assets	4,540,404		4,540,404
Segment Reporting Information, Expenditures for Additions to Long-Lived Assets			23,793
Southeast [Member]
Rental income:
Same store	59,170	56,987	175,039	167,561
Non-same store/other	768	0	1,785	0
Segment Reporting Information Revenues	59,938	56,987	176,824	167,561
Operating expenses:
Same store	22,619	22,435	66,013	64,878
Non-same store/other	289	0	696	0
Total operating expenses	22,908	22,435	66,709	64,878
NOI:
Same store	36,551	34,552	109,026	102,683
Non-same store/other	479	0	1,089	0
Total NOI	37,030	34,552	110,115	102,683
Total assets	1,513,074		1,513,074
Segment Reporting Information, Expenditures for Additions to Long-Lived Assets			13,270
Southwest [Member]
Rental income:
Same store	101,008	97,606	295,216	283,899
Non-same store/other	21,470	284	51,492	1,793
Segment Reporting Information Revenues	122,478	97,890	346,708	285,692
Operating expenses:
Same store	32,855	31,814	96,446	93,127
Non-same store/other	8,801	52	21,656	617
Total operating expenses	41,656	31,866	118,102	93,744
NOI:
Same store	68,153	65,792	198,770	190,772
Non-same store/other	12,669	232	29,836	1,176
Total NOI	80,822	66,024	228,606	191,948
Total assets	3,578,742		3,578,742
Segment Reporting Information, Expenditures for Additions to Long-Lived Assets			15,360
Other [Member]
Rental income:
Same store	0	0	0	0
Non-same store/other	1,172	(121)	3,337	(163)
Segment Reporting Information Revenues	1,172	(121)	3,337	(163)
Operating expenses:
Same store	0	0	0	0
Non-same store/other	(1,117)	2,508	9,341	14,443
Total operating expenses	(1,117)	2,508	9,341	14,443
NOI:
Same store	0	0	0	0
Non-same store/other	2,289	(2,629)	(6,004)	(14,606)
Total NOI	2,289	(2,629)	(6,004)	(14,606)
Total assets	2,680,081		2,680,081
Segment Reporting Information, Expenditures for Additions to Long-Lived Assets			$ 5,112