Equity (Details) (USD $)	0 Months Ended	12 Months Ended								1 Months Ended				12 Months Ended									0 Months Ended
	Jan. 25, 2013	Dec. 31, 2012 item	Dec. 31, 2011	Dec. 31, 2010	Dec. 04, 2012 Senior housing item	Oct. 31, 2012 Senior housing item	Dec. 31, 2012 Senior housing Blackstone JV item	Dec. 04, 2012 Senior housing Blackstone JV item	Oct. 31, 2012 Senior housing Blackstone JV item	Mar. 31, 2011 Public offering completed in March 2011	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	Dec. 31, 2012 Series E cumulative redeemable preferred stock	Dec. 31, 2011 Series E cumulative redeemable preferred stock	Dec. 31, 2010 Series E cumulative redeemable preferred stock	Dec. 31, 2012 Series F cumulative redeemable preferred stock	Dec. 31, 2011 Series F cumulative redeemable preferred stock	Dec. 31, 2010 Series F cumulative redeemable preferred stock	Dec. 31, 2012 Common Stock	Dec. 31, 2011 Common Stock	Dec. 31, 2010 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
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
Ordinary dividends (in dollars per share)														$ 0.4383	$ 1.4335	$ 1.6695	$ 0.4292	$ 1.4038	$ 1.6350	$ 1.4618	$ 0.9259	$ 1.0935
Capital gain dividends (in dollars per share)														$ 0.0148	$ 0.3790	$ 0.1430	$ 0.0145	$ 0.3712	$ 0.1400	$ 0.0495	$ 0.2448	$ 0.0937
Nondividend Distributions (in dollars per share)																				$ 0.4887	$ 0.7493	$ 0.6728
Total dividends (in dollars per share)														$ 0.4531	$ 1.8125	$ 1.8125	$ 0.4437	$ 1.7750	$ 1.7750	$ 2.0000	$ 1.9200	$ 1.8600
Dividends declared per common share (in dollars per share)	$ 0.525	$ 2.00	$ 1.92	$ 1.86
Issuance of common stock, net		1,756,796,000	1,302,008,000	2,425,110,000						1,273,000,000	359,000,000	376,000,000	979,000,000
Issuance of common stock, net (in shares)										34,500,000	9,000,000	8,970,000	22,000,000
Stock issued under new issues (in dollars per share)										$ 36.90	$ 39.93	$ 41.88
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%
Number of senior housing communities					2	127	129	2	127
Company's common stock issuances
Dividend Reinvestment and Stock Purchase Plan (in shares)		1,064,000	1,910,000
Conversion of DownREIT units (in shares)		736,000	80,000
Exercise of stock options (in shares)		2,455,000	1,157,000
Vesting of restricted stock units (in shares)		707,000	228,000
Accumulated Other Comprehensive Loss
Unrealized gains on available for sale securities		7,776,000
Unrealized losses on cash flow hedges, net		(18,452,000)	(15,712,000)
Supplemental Executive Retirement Plan minimum liability		(3,150,000)	(2,794,000)
Cumulative foreign currency translation adjustment		(827,000)	(1,076,000)
Total accumulated other comprehensive loss		(14,653,000)	(19,582,000)
Noncontrolling interests
Non-managing member units outstanding in DownREIT LLCs (in units)		4,000,000
Issuance of common stock upon conversion		6,000,000
Number of DownREIT LLCs having non-managing member units outstanding		4
Non-managing member unitholders		187,788,000	170,169,000
Non-managing member units outstanding, fair value		$ 275,000,000