Organization and Basis of Presentation (Details 2) (USD $)	3 Months Ended								12 Months Ended
	Dec. 31, 2012 community	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012 property community	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2008
Basic and diluted shares outstanding
Weighted average common shares - basic (in dollars per share)									97,416,401	89,922,465	83,859,936
Weighted average DownREIT units outstanding (in shares)									7,500	8,322	15,321
Effect of dilutive securities (in shares)									601,251	846,675	757,612
Weighted average common shares - diluted									98,025,152	90,777,462	84,632,869
Calculation of Earnings per Share - basic
Net income attributable to common stockholders	$ 122,356,000	$ 86,844,000	$ 156,909,000	$ 57,758,000	$ 323,085,000	$ 44,824,000	$ 43,373,000	$ 30,341,000	$ 423,869,000	$ 441,622,000	$ 175,331,000
Net income allocated to unvested restricted shares									(1,264,000)	(1,631,000)	(498,000)
Net income attributable to common stockholders, adjusted									422,605,000	439,991,000	174,833,000
Weighted average common shares - basic (in dollars per share)									97,416,401	89,922,465	83,859,936
Earnings per common share - basic (in dollars per share)	$ 1.19	$ 0.89	$ 1.64	$ 0.61	$ 3.40	$ 0.49	$ 0.50	$ 0.35	$ 4.34	$ 4.89	$ 2.08
Calculation of Earnings per Share - diluted
Net income attributable to common stockholders	122,356,000	86,844,000	156,909,000	57,758,000	323,085,000	44,824,000	43,373,000	30,341,000	423,869,000	441,622,000	175,331,000
Add: noncontrolling interests of Down REIT unitholders in consolidated partnerships, including discontinued operations									28,000	27,000	55,000
Adjusted net income attributable to common stockholders									423,897,000	441,649,000	175,386,000
Weighted average common shares - diluted (in dollars per share)									98,025,152	90,777,462	84,632,869
Earnings per common share - diluted (in dollars per share)	$ 1.19	$ 0.89	$ 1.63	$ 0.60	$ 3.38	$ 0.49	$ 0.49	$ 0.35	$ 4.32	$ 4.87	$ 2.07
Dividends per common share									$ 3.88	$ 3.57	$ 3.57
Options to purchase shares of common stock excluded from computation of earnings per share amount									396,346	457,419
Casualty Loss, Abandoned Pursuit Costs and Impairment of Long-Lived Assets
Casualty and impairment loss	1,449,000								1,449,000	14,052,000
Abandoned pursuits costs									1,757,000	1,957,000	2,741,000
Number of impaired parcels of land									2
Impairment of land parcels										12,097,000		9,952,000
Impairment of investment in an unconsolidated joint venture										1,955,000
Assets Held for Sale & Discontinued Operations
Number of operating communities that qualified for held for sale presentation	1								1
Noncontrolling interests
Maximum ownership interest percentage in joint venture	100.00%								100.00%
Interest Rate Caps | Cash Flow Hedges
Organization and basis of presentation
Derivative financial instruments	179,851,000								179,851,000
Variable rate debt | Cash Flow Hedges
Organization and basis of presentation
Derivative financial instruments	410,200,000				640,006,000				410,200,000	640,006,000
Variable rate debt | Interest Rate Caps
Organization and basis of presentation
Derivative financial instruments	46,735,000				79,835,000				46,735,000	79,835,000
Fixed rate debt | Cash Flow Hedges
Organization and basis of presentation
Derivative financial instruments					$ 75,000,000					$ 75,000,000