Stockholders' Equity - Additional Information (Detail) (USD $)	6 Months Ended			3 Months Ended	6 Months Ended	8 Months Ended
	Jun. 30, 2011	Jun. 30, 2010	Dec. 31, 2010	Jun. 30, 2011 Continuous Equity Program CEP II	Jun. 30, 2011 Continuous Equity Program CEP II	Jun. 30, 2011 Continuous Equity Program CEP II Month	Jun. 30, 2011 Continuous Equity Program CEP II Maximum
Class of Stock [Line Items]
Common stock shares issued through public offerings (in shares)	1,801,766
Common stock shares issued in connection with stock options exercised (in shares)	597,436
Common stock shares issued through dividend reinvestment plan (in shares)	1,809
Common stock shares issued in connection with stock grants (in shares)	498,810
Common stock shares issued for DownREIT OP units conversion (in shares)	7,500
Number of shares of stock grants withheld (in shares)	126,928
Restricted common stock shares forfeited (in shares)	505
Common stock options granted to employees (in shares)	144,827
Maximum amount of common stock that can be sold under 2009 CEP II	140,000,000		140,000,000				500,000,000
Number of months common stock can be sold under 2009 CEP II						36
Shares sold under 2009 CEP II (in shares)				553,856	1,801,766	2,234,598
Shares sold under 2009 CEP II, average sales price				$ 130.56	$ 120.47	$ 118.91
Net proceeds from sales of shares sold under 2009 CEP II	$ 264,134,000	$ 306,817,000		$ 71,225,000	$ 213,794,000	$ 261,729,000