Organization, Basis of Presentation and Significant Accounting Policies (Details) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2012 community home state	Jun. 30, 2011	Jun. 30, 2012 community home state	Jun. 30, 2011
Organization and Basis of Presentation
Number of operating apartment communities owned	181		181
Number of apartment homes included in operating apartment communities owned	53,144		53,144
Number of states where operating apartment communities owned are located	9		9
Number of communities with apartments under reconstruction	11		11
Number of apartment homes under reconstruction	2,721		2,721
Number of owned communities under construction	20		20
Expected number of apartment homes under construction	6,114		6,114
Communities under development rights	33		33
Estimated number of apartment homes in communities to be developed	9,036		9,036
Basic and diluted shares outstanding
Weighted average common shares - basic	95,308,163	87,317,602	95,082,172	86,746,992
Weighted average DownREIT units outstanding (in shares)	7,500	7,707	7,500	8,992
Effect of dilutive securities (in shares)	677,162	871,129	730,531	841,997
Weighted average common shares - diluted	95,992,825	88,196,438	95,820,203	87,597,981
Calculation of Earnings per Share - basic
Net income attributable to common stockholders	$ 156,909	$ 43,373	$ 214,667	$ 73,713
Net income allocated to unvested restricted shares	(547)	(123)	(845)	(206)
Net income attributable to common stockholders, adjusted	156,362	43,250	213,822	73,507
Weighted average common shares - basic	95,308,163	87,317,602	95,082,172	86,746,992
Earnings per common share - basic	$ 1.64	$ 0.50	$ 2.25	$ 0.85
Calculation of Earnings per Share - diluted
Net income attributable to common stockholders	156,909	43,373	214,667	73,713
Add: noncontrolling interests of Down REIT unitholders in consolidated partnerships, including discontinued operations	7	7	13	13
Adjusted net income attributable to common stockholders	$ 156,916	$ 43,380	$ 214,680	$ 73,726
Weighted average common shares - diluted	95,992,825	88,196,438	95,820,203	87,597,981
Earnings per common share - diluted (in dollars per share)	$ 1.63	$ 0.49	$ 2.24	$ 0.84
Options to purchase shares of common stock excluded from computation of earnings per share amount			424,357	320,698
Estimated forfeiture rate of stock options (as a percent)			0.30%