Earnings per Common Share (Detail) (USD $) In Thousands, except Share data	3 Months Ended		6 Months Ended
	Jun. 30, 2011	Jun. 30, 2010	Jun. 30, 2011	Jun. 30, 2010
Basic and diluted shares outstanding
Weighted average common shares - basic	87,317,602	83,517,908	86,746,992	82,583,638
Weighted average DownREIT units outstanding	7,707	15,351	8,992	15,351
Effect of dilutive securities	871,129	711,846	841,997	649,006
Weighted average common shares - diluted	88,196,438	84,245,105	87,597,981	83,247,995
Calculation of Earnings per Share - basic
Net income attributable to common stockholders	$ 43,373	$ 51,125	$ 73,713	$ 123,648
Net income allocated to unvested restricted shares	(123)	(143)	(206)	(368)
Net income attributable to common stockholders, adjusted	43,250	50,982	73,507	123,280
Weighted average common shares - basic	87,317,602	83,517,908	86,746,992	82,583,638
Earnings per common share - basic	$ 0.50	$ 0.61	$ 0.85	$ 1.49
Calculation of Earnings per Share - diluted
Net income attributable to common stockholders	43,373	51,125	73,713	123,648
Add: noncontrolling interests of DownREIT unitholders in consolidated partnerships, including discontinued operations	7	14	13	27
Adjusted net income attributable to common stockholders	$ 43,380	$ 51,139	$ 73,726	$ 123,675
Weighted average common shares - diluted	88,196,438	84,245,105	87,597,981	83,247,995
Earnings per common share - diluted	$ 0.49	$ 0.61	$ 0.84	$ 1.49