Organization, Basis of Presentation and Significant Accounting Policies (Details) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2012 community home state	Sep. 30, 2011	Sep. 30, 2012 community home state	Sep. 30, 2011
Organization and Basis of Presentation
Number of operating apartment communities owned	183		183
Number of apartment homes included in operating apartment communities owned	53,487		53,487
Number of states where operating apartment communities owned are located	9		9
Number of communities with apartments under reconstruction	7		7
Number of apartment homes under reconstruction	1,802		1,802
Number of owned communities under construction	22		22
Expected number of apartment homes under construction	6,614		6,614
Communities under development rights	31		31
Estimated number of apartment homes in communities to be developed	8,837		8,837
Basic and diluted shares outstanding
Weighted average common shares - basic	97,044,603	91,388,357	95,742,676	88,312,930
Weighted average DownREIT units outstanding (in shares)	7,500	7,707	7,500	8,559
Effect of dilutive securities (in shares)	494,466	944,304	651,382	878,009
Weighted average common shares - diluted	97,546,569	92,340,368	96,401,558	89,199,498
Calculation of Earnings per Share - basic
Net income attributable to common stockholders	$ 86,844	$ 44,824	$ 301,512	$ 118,537
Net income allocated to unvested restricted shares	(186)	(206)	(1,003)	(406)
Net income attributable to common stockholders, adjusted	86,658	44,618	300,509	118,131
Weighted average common shares - basic	97,044,603	91,388,357	95,742,676	88,312,930
Earnings per common share - basic	$ 0.89	$ 0.49	$ 3.14	$ 1.34
Calculation of Earnings per Share - diluted
Net income attributable to common stockholders	86,844	44,824	301,512	118,537
Add: noncontrolling interests of Down REIT unitholders in consolidated partnerships, including discontinued operations	7	7	21	20
Adjusted net income attributable to common stockholders	$ 86,851	$ 44,831	$ 301,533	$ 118,557
Weighted average common shares - diluted	97,546,569	92,340,368	96,401,558	89,199,498
Earnings per common share - diluted (in dollars per share)	$ 0.89	$ 0.49	$ 3.13	$ 1.33
Options to purchase shares of common stock excluded from computation of earnings per share amount			418,177	320,698
Estimated forfeiture rate of stock options (as a percent)			1.50%