Equity (Details) (USD $)	0 Months Ended	12 Months Ended			3 Months Ended		6 Months Ended	0 Months Ended	1 Months Ended			0 Months Ended
	Jan. 29, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Mar. 31, 2013 item	Dec. 31, 2012 item	Mar. 31, 2013 item	Oct. 19, 2012	Oct. 31, 2012	Jun. 30, 2012	Mar. 31, 2012	Apr. 23, 2012
Equity
Dividends declared per common share (in dollars per share)	$ 0.565	$ 2.18	$ 2.1	$ 2.00
Issuance of common stock, net		$ 92,131,000	$ 107,018,000	$ 1,756,796,000
Company's common stock issuances
Repurchase of common stock		12,703,000	10,438,000	15,632,000
Aggregate redemption amount				295,500,000
Noncontrolling interests
DownREIT unit, carrying value		188,588,000	184,105,000
Accumulated Other Comprehensive Loss
Unrealized losses on available for sale securities		13,000
Unrealized losses on cash flow hedges, net		(9,624,000)	(10,797,000)
Supplemental Executive Retirement Plan minimum liability		(3,537,000)	(2,910,000)
Cumulative foreign currency translation adjustment		(10,747,000)	(780,000)
Total accumulated other comprehensive loss		(23,895,000)	(14,487,000)
Senior housing | Blackstone JV
Equity
Number of senior housing communities					4	129	133
Common Stock
Equity
Issuance of common stock, net		2,939,000	3,136,000	42,468,000				979,000,000		376,000,000	359,000,000
Issuance of common stock, net (in shares)		2,939,000	3,136,000	42,468,000					22,000,000	9,000,000	9,000,000
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)		2,299,000	2,441,000	1,064,000
Conversion of DownREIT units (in shares)		27,000	100,000	736,000
Exercise of stock options (in shares)		169,000	876,000	2,455,000
Vesting of restricted stock units (in shares)		614,000	471,000	707,000
Repurchase of common stock		323,000	242,000	361,000
Preferred Stock
Company's common stock issuances
Aggregate redemption amount												295,500,000
Preferred stock issuance costs write-off												10,400,000
Preferred Stock | Series E cumulative redeemable preferred stock
Company's common stock issuances
Shares redeemed												4,000,000
Dividend Rate (as a percent)												7.25%
Preferred stock redemption value (in dollars per share)												$ 25
Preferred Stock | Series F cumulative redeemable preferred stock
Company's common stock issuances
Shares redeemed												7,820,000
Dividend Rate (as a percent)												7.10%
Preferred stock redemption value (in dollars per share)												$ 25
Noncontrolling Interests
Equity
Issuance of common stock, net		(557,000)	(3,683,000)	(25,029,000)
Company's common stock issuances
Conversion of DownREIT units (in shares)		6,000,000
Noncontrolling interests
Non-managing members DownREIT units outstanding		4,000,000
Number of DownREIT LLCs		5
DownREIT unit, carrying value		189,000,000
DownREIT unit, fair value		$ 268,000,000