Equity (Details) (USD $)	0 Months Ended				3 Months Ended		9 Months Ended			0 Months Ended	3 Months Ended	0 Months Ended		1 Months Ended			9 Months Ended
	Oct. 24, 2013	Jul. 25, 2013	Apr. 25, 2013	Jan. 24, 2013	Sep. 30, 2013 item	Sep. 30, 2012	Sep. 30, 2013 item	Sep. 30, 2012	Dec. 31, 2012	Apr. 23, 2012 Preferred Stock	Mar. 31, 2012 Preferred Stock	Apr. 23, 2012 Preferred Stock Series E cumulative redeemable preferred stock	Apr. 23, 2012 Preferred Stock Series F cumulative redeemable preferred stock	Oct. 31, 2012 Common Stock	Jun. 30, 2012 Common Stock	Mar. 31, 2012 Common Stock	Sep. 30, 2013 Common Stock	Sep. 30, 2012 Common 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	$ 25
Aggregate redemption amount								$ 295,500,000		$ 295,500,000
Preferred stock issuance costs write-off											10,400,000
Dividends declared per common share (in dollars per share)	$ 0.525	$ 0.525	$ 0.525	$ 0.525	$ 0.525	$ 0.50	$ 1.575	$ 1.50
Issuance of common stock, net							77,509,000	761,070,000						979,000,000	376,000,000	359,000,000	1,859,000	19,096,000
Issuance of common stock, net (in shares)														22,000,000	8,970,000	9,000,000	1,859,000	19,096,000
Stock issued under new issues (in dollars per share)														$ 44.50	$ 41.88	$ 39.93
Repayment of senior unsecured notes															250,000,000
Dividend Reinvestment and Stock Purchase Plan (in shares)							1,681,000	675,000
Conversion of DownREIT units (in shares)																	85,000	72,000
Exercise of stock options (in shares)							875,000	2,451,000									875,000	2,451,000
Vesting of restricted stock units (in shares)																	110,000	385,000
Accumulated Other Comprehensive Loss
Unrealized gains on available for sale securities									7,776,000
Unrealized losses on cash flow hedges, net					(12,066,000)		(12,066,000)		(18,452,000)
Supplemental Executive Retirement Plan minimum liability					(2,983,000)		(2,983,000)		(3,150,000)
Cumulative foreign currency translation adjustment					(830,000)		(830,000)		(827,000)
Total accumulated other comprehensive loss					(15,879,000)		(15,879,000)		(14,653,000)
Noncontrolling interests
Non-managing members DownREIT units outstanding					4,000,000		4,000,000
Number of DownREIT LLCs					4		4
DownREIT unit, carrying value					184,791,000		184,791,000		187,788,000
DownREIT unit, fair value					$ 246,000,000		$ 246,000,000