Equity (Details) (USD $)	0 Months Ended	12 Months Ended			3 Months Ended		6 Months Ended	1 Months Ended				12 Months Ended			0 Months Ended			12 Months Ended
	Jan. 30, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011	Mar. 31, 2013 Senior housing Blackstone JV item	Dec. 31, 2012 Senior housing Blackstone JV item	Mar. 31, 2013 Senior housing Blackstone JV item	Oct. 31, 2012 Common Stock	Jun. 30, 2012 Common Stock	Mar. 31, 2012 Common Stock	Mar. 31, 2011 Common Stock	Dec. 31, 2013 Common Stock	Dec. 31, 2012 Common Stock	Dec. 31, 2011 Common Stock	Apr. 23, 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	Dec. 31, 2013 Noncontrolling Interests item	Dec. 31, 2012 Noncontrolling Interests	Dec. 31, 2011 Noncontrolling Interests
Equity
Dividends declared per common share (in dollars per share)	$ 0.545	$ 2.10	$ 2.00	$ 1.92
Issuance of common stock, net		$ 107,018,000	$ 1,756,796,000	$ 1,302,008,000				$ 979,000,000	$ 376,000,000	$ 359,000,000	$ 1,273,000,000	$ 3,136,000	$ 42,468,000	$ 36,683,000				$ (3,683,000)	$ (25,029,000)	$ (3,456,000)
Number of senior housing communities					4	129	133
Stock issued under new issues (in dollars per share)									$ 41.88	$ 39.93	$ 36.90
Repayment of senior unsecured notes									250,000,000
Interest rate on repaid maturing debt (as a percent)									6.45%
Proceeds from issuance of common stock											1,235,000,000
Company's common stock issuances
Dividend Reinvestment and Stock Purchase Plan (in shares)												2,441,000	1,064,000	1,910,000
Conversion of DownREIT units (in shares)												100,000	736,000	80,000				6,000,000
Exercise of stock options (in shares)		876,000	2,455,000									876,000	2,455,000	1,157,000
Vesting of restricted stock units (in shares)												471,000	707,000	228,000
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
Issuance of common stock, net (in shares)								22,000,000	8,970,000	9,000,000	34,500,000	3,136,000	42,468,000	36,683,000
Noncontrolling interests
Non-managing members DownREIT units outstanding																		4,000,000
Number of DownREIT LLCs																		4
DownREIT unit, carrying value		184,105,000	187,788,000															184,000,000
DownREIT unit, fair value																		217,000,000
Accumulated Other Comprehensive Loss
Unrealized gains on available for sale securities			7,776,000
Unrealized losses on cash flow hedges, net		(10,797,000)	(18,452,000)
Supplemental Executive Retirement Plan minimum liability		(2,910,000)	(3,150,000)
Cumulative foreign currency translation adjustment		(780,000)	(827,000)
Total accumulated other comprehensive loss		$ (14,487,000)	$ (14,653,000)