Equity (Details) (USD $)	1 Months Ended				3 Months Ended		9 Months Ended			1 Months Ended					1 Months Ended			1 Months Ended
	Oct. 31, 2012	Jul. 31, 2012	Apr. 30, 2012	Jan. 31, 2012	Sep. 30, 2012 item	Sep. 30, 2011	Sep. 30, 2012 item	Sep. 30, 2011	Dec. 31, 2011	Jun. 30, 2012 Public offering completed in June 2012	Mar. 31, 2012 Public offering completed in March 2012	Mar. 31, 2011 Public offering completed in March 2011	Apr. 30, 2012 Preferred Stock	Apr. 23, 2012 Preferred Stock	Apr. 30, 2012 Preferred Stock Series E cumulative redeemable preferred stock	Jan. 31, 2012 Preferred Stock Series E cumulative redeemable preferred stock	Apr. 23, 2012 Preferred Stock Series E cumulative redeemable preferred stock	Apr. 30, 2012 Preferred Stock Series F cumulative redeemable preferred stock	Jan. 31, 2012 Preferred Stock Series F cumulative redeemable preferred stock	Apr. 23, 2012 Preferred Stock Series F cumulative redeemable preferred stock
Class of Stock
Shares redeemed																	4,000,000			7,820,000
Dividend Rate (as a percent)															7.25%			7.10%
Preferred stock redemption value (in dollars per share)																	$ 25.00			$ 25.00
Aggregate redemption amount														$ 295,500,000
Preferred stock issuance costs write-off													10,400,000
Dividends declared per preferred share (in dollars per share)																$ 0.45313			$ 0.44375
Dividends declared per common share (in dollars per share)	$ 0.50	$ 0.50	$ 0.50	$ 0.50	$ 0.50	$ 0.48	$ 1.50	$ 1.44
Issuance of common stock, net							761,070,000	1,288,332,000		376,000,000	359,000,000	1,273,000,000
Issuance of common stock, net (in shares)										8,970,000	9,000,000	34,500,000
Stock issued under new issues (in dollars per share)										$ 41.88	$ 39.93	$ 36.90
Proceeds from issuance of common stock												1,235,000,000
Repayment of senior unsecured notes										250,000,000
Interest rate on repaid maturing debt (as a percent)										6.45%
Company's common stock issuances
Dividend Reinvestment and Stock Purchase Plan (in shares)							675,000	1,533,000
Conversion of DownREIT units (in shares)							72,000	30,000
Exercise of stock options (in shares)							2,451,000	733,000
Vesting of restricted stock units (in shares)							385,000	228,000
Accumulated Other Comprehensive Loss
Unrealized gains on available for sale securities					5,716,000		5,716,000
Unrealized losses on cash flow hedges, net					(18,917,000)		(18,917,000)		(15,712,000)
Supplemental Executive Retirement Plan minimum liability					(2,658,000)		(2,658,000)		(2,794,000)
Cumulative foreign currency translation adjustment					(787,000)		(787,000)		(1,076,000)
Total accumulated other comprehensive loss					(16,646,000)		(16,646,000)		(19,582,000)
Noncontrolling interests
DownREIT units outstanding					4,400,000		4,400,000
Number of DownREIT LLCs					5		5
DownREIT unit, carrying value					194,970,000		194,970,000		170,169,000
DownREIT unit, fair value					$ 285,000,000		$ 285,000,000