Organization, Basis of Presentation and Significant Accounting Policies (Details 2) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Basic and diluted shares outstanding
Weighted average common shares - basic (in dollars per share)	119,680,510	94,855,266
Weighted average DownREIT units outstanding (in shares)	7,500	7,500
Effect of dilutive securities (in shares)	423,118	791,013
Weighted average common shares - diluted	120,111,128	95,653,779
Calculation of Earnings per Share - basic
Net income attributable to common stockholders	$ 75,427	$ 57,758
Net income allocated to unvested restricted shares	(139)	(254)
Net income attributable to common stockholders, adjusted	75,288	57,504
Weighted average common shares - basic (in dollars per share)	119,680,510	94,855,266
Earnings per common share - basic (in dollars per share)	$ 0.63	$ 0.61
Calculation of Earnings per Share - diluted
Net income attributable to common stockholders	75,427	57,758
Add: noncontrolling interests of Down REIT unitholders in consolidated partnerships, including discontinued operations	8	7
Adjusted net income attributable to common stockholders	$ 75,435	$ 57,765
Weighted average common shares - diluted (in dollars per share)	120,111,128	95,653,779
Earnings per common share - diluted (in dollars per share)	$ 0.63	$ 0.60
Options to purchase shares of common stock excluded from computation of earnings per share amount	608,148	427,288
Estimated forfeiture rate of stock options (as a percent)	1.20%