Equity (Details) (USD $)	0 Months Ended		3 Months Ended			1 Months Ended			0 Months Ended	3 Months Ended	0 Months Ended
	Apr. 25, 2013	Jan. 24, 2013	Mar. 31, 2013 item	Mar. 31, 2012	Dec. 31, 2012	Mar. 31, 2012 Public offering completed in March 2012	Jun. 30, 2012 Public offering completed in June 2012	Oct. 31, 2012 Public offering completed in October 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
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
Preferred stock issuance costs write-off										10,400,000
Dividends declared per common share (in dollars per share)	$ 0.525	$ 0.525	$ 0.525	$ 0.50
Issuance of common stock, net			12,277,000	366,922,000		359,000,000	376,000,000	979,000,000
Issuance of common stock, net (in shares)						9,000,000	8,970,000	22,000,000
Stock issued under new issues (in dollars per share)						$ 39.93	$ 41.88	$ 45.50
Repayment of senior unsecured notes							250,000,000
Company's common stock issuances
Dividend Reinvestment and Stock Purchase Plan (in shares)			382,000	210,000
Conversion of DownREIT units (in shares)			51,000	36,000
Exercise of stock options (in shares)			796,000	1,412,000
Vesting of restricted stock units (in shares)			17,000	314,000
Accumulated Other Comprehensive Loss
Unrealized gains on available for sale securities					7,776,000
Unrealized losses on cash flow hedges, net			(12,860,000)		(18,452,000)
Supplemental Executive Retirement Plan minimum liability			(3,095,000)		(3,150,000)
Cumulative foreign currency translation adjustment			(649,000)		(827,000)
Total accumulated other comprehensive loss			(16,604,000)		(14,653,000)
Noncontrolling interests
DownREIT units outstanding			4,000,000
Number of DownREIT LLCs			4
DownREIT unit, carrying value			185,608,000		187,788,000
DownREIT unit, fair value			$ 301,000,000