Indebtedness (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		1 Months Ended			4 Months Ended	12 Months Ended	1 Months Ended
	Mar. 31, 2015 item	Mar. 31, 2014	Apr. 30, 2015 item property building	Feb. 28, 2015 item	May 31, 2014	Sep. 30, 2014	Dec. 31, 2014 community	May 31, 2015 community	Dec. 31, 2014 community	May 05, 2015	Aug. 04, 2014	Jun. 30, 2014
Indebtedness
Number of public issues of unsecured senior notes	6
Principal amount of unsecured senior notes	$ 1,743,983						1,743,628		$ 1,743,628
Interest rate (as a percent)	1.57%
Aggregate principal amount of mortgage debt	609,032
Carrying value of mortgaged properties	5,694,103						5,254,761		5,254,761
Assumption of mortgage notes payable	29,955
Unsecured revolving credit facility							80,000		80,000
Loss on extinguishment of debt	(1,409)
Unsecured revolving credit facility
Indebtedness
Revolving credit facility, available amount	750,000
Extension period of revolving credit facility	1 year
Principal repayment due until maturity	0
Option to increase the borrowing capacity under revolving credit facility	1,500,000
Interest rate, description	LIBOR
Interest rate added to the base rate (as a percent)	1.30%
Revolving credit facility, interest rate payable (as a percent)						1.47%
Debt instrument, facility fee (as a percent)	0.30%
Weighted average interest rate on debt (as a percent)	1.54%	1.42%
Unsecured revolving credit facility	0
Unsecured revolving credit facility | Subsequent event
Indebtedness
Revolving credit facility, available amount										200,000	750,000
Unsecured revolving credit facility										550,000
Senior unsecured notes 4.30%
Indebtedness
Principal amount of unsecured senior notes	250,000
Interest rate (as a percent)	4.30%
Senior unsecured notes 6.75%
Indebtedness
Principal amount of unsecured senior notes	200,000
Interest rate (as a percent)	6.75%
Unsecured senior notes due 2021
Indebtedness
Principal amount of unsecured senior notes	300,000
Interest rate (as a percent)	6.75%
Senior unsecured notes 5.625% due in 2042
Indebtedness
Principal amount of unsecured senior notes	350,000
Interest rate (as a percent)	5.625%
Mortgages
Indebtedness
Aggregate principal amount of mortgage debt	609,032
Number of properties mortgaged	46
Number of buildings mortgaged	48
Carrying value of mortgaged properties	776,980
Weighted average interest rate on debt (as a percent)			5.81%	6.02%
Number of mortgage notes			1	1
Repayment of debt			6,274	29,227
Mortgages | Significant Unobservable Inputs (Level 3)
Indebtedness
Interest rate (as a percent)	4.73%
Assumption of mortgage notes payable	29,955
Fair value of mortgage notes payable	31,029
Capital leases
Indebtedness
Carrying value of mortgaged properties	18,095
Number of properties recorded under capital lease	2
Capital leases	12,621
Senior unsecured notes due 2019
Indebtedness
Principal amount of unsecured senior notes	400,000
Interest rate (as a percent)	3.25%
Senior unsecured notes due 2024
Indebtedness
Principal amount of unsecured senior notes	250,000
Interest rate (as a percent)	4.75%
Unsecured Debt
Indebtedness
Unsecured term loan facility	350,000				350,000							350,000
Interest rate, description					LIBOR	LIBOR
Interest rate added to the base rate (as a percent)					1.40%
Weighted average interest rate on debt (as a percent)												1.59%
Maximum borrowing capacity that may be increased					700,000
Senior living communities
Indebtedness
Number of communities							5
Senior living communities | Agreement to acquire
Indebtedness
Assumption of mortgage notes payable							153,000	139,181
Weighted average interest rate on debt (as a percent)							4.77%	4.59%
Bridge loan commitment received							700,000		$ 700,000
Number of communities								37	38