Organization, Basis of Presentation and Significant Accounting Policies (Details) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended
	Mar. 31, 2012 community state home	Mar. 31, 2011
Organization and Basis of Presentation
Number of operating apartment communities owned	179
Number of apartment homes included in operating apartment communities owned	53,315
Number of states where operating apartment communities owned are located	10
Number of communities with apartments under reconstruction	10
Number of apartment homes under reconstruction	2,580
Number of owned communities under construction	20
Expected number of apartment homes under construction	5,775
Communities under development rights	34
Estimated number of apartment homes in communities to be developed	9,415
Basic and diluted shares outstanding
Weighted average common shares - basic	94,855,266	86,168,732
Weighted average DownREIT units outstanding (in shares)	7,500	10,291
Effect of dilutive securities (in shares)	791,013	818,507
Weighted average common shares - diluted	95,653,779	86,997,530
Calculation of Earnings per Share - basic
Net income attributable to common stockholders	$ 57,758	$ 30,341
Net income allocated to unvested restricted shares	(254)	(83)
Net income attributable to common stockholders, adjusted	57,504	30,258
Weighted average common shares - basic	94,855,266	86,168,732
Earnings per common share - basic	$ 0.61	$ 0.35
Calculation of Earnings per Share - diluted
Net income attributable to common stockholders	57,758	30,341
Add: noncontrolling interests of Down REIT unitholders in consolidated partnerships, including discontinued operations	7	7
Adjusted net income attributable to common stockholders	$ 57,765	$ 30,348
Weighted average common shares - diluted	95,653,779	86,997,530
Earnings per common share - diluted (in dollars per share)	$ 0.60	$ 0.35
Options to purchase shares of common stock excluded from computation of earnings per share amount	427,288	469,647
Estimated forfeiture rate of stock options (as a percent)	0.30%