Mortgage Notes Payable, Unsecured Notes and Credit Facility (Details 2)	1 Months Ended							12 Months Ended
	Dec. 31, 2015 USD ($) community	Nov. 30, 2015 USD ($)	Jul. 31, 2015 USD ($)	Jun. 30, 2015 USD ($)	May. 31, 2015 USD ($)	Apr. 30, 2015 USD ($) community loan	Jun. 30, 2014 USD ($)	Dec. 31, 2015 USD ($) community	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)	Mar. 31, 2015 USD ($)
Mortgage Notes Payable, Unsecured Notes and Credit Facility
Repayments of secured mortgages								$ 0	$ 16,341,000	$ 0
Number of real estate properties | community	259							259
Gains (losses) on extinguishment of debt								$ (26,736,000)	412,000	$ 14,921,000
Term loan
Mortgage Notes Payable, Unsecured Notes and Credit Facility
Principal amount of notes issued											$ 300,000,000
Unused borrowing capacity											$ 50,000,000
Unsecured notes | Term loan | LIBOR
Mortgage Notes Payable, Unsecured Notes and Credit Facility
Debt instrument, basis spread on variable rate (as a percent)								1.45%
Mortgage notes payable
Mortgage Notes Payable, Unsecured Notes and Credit Facility
Amount drawn	$ 1,045,182,000							$ 1,045,182,000	$ 1,047,461,000
Repayments of secured mortgages							$ 5,914,000
Mortgage notes payable | Notes payable maturing in 2015
Mortgage Notes Payable, Unsecured Notes and Credit Facility
Fixed rate (as a percent)						3.12%
Repayments of secured mortgages						$ 481,582,000	$ 10,427,000
Mortgage Loans on Real Estate, Number of Loans | loan						8
Number of real estate properties | community						8
Gains (losses) on extinguishment of debt						$ (8,724,000)
Mortgage notes payable | Notes Payable Maturities 2041 [Member]
Mortgage Notes Payable, Unsecured Notes and Credit Facility
Fixed rate (as a percent)				7.50%
Gains (losses) on extinguishment of debt				$ 455,000
Repayments of Debt				$ 15,778,000
Mortgage notes payable | Notes Payable Maturities 2027 [Member]
Mortgage Notes Payable, Unsecured Notes and Credit Facility
Fixed rate (as a percent)				7.84%
Gains (losses) on extinguishment of debt				$ 263,000
Repayments of Debt				$ 7,805,000
Mortgage notes payable | Notes Payable Maturities 2019 [Member]
Mortgage Notes Payable, Unsecured Notes and Credit Facility
Fixed rate (as a percent)				5.95%
Gains (losses) on extinguishment of debt				$ 259,000
Repayments of Debt				$ 74,531,000
Mortgage notes payable | Notes Payable Maturities 2053 [Member]
Mortgage Notes Payable, Unsecured Notes and Credit Facility
Fixed rate (as a percent)			5.56%
Repayments of secured mortgages			$ 140,346,000
Gains (losses) on extinguishment of debt			(18,987,000)
Debt Instrument, Unamortized Premium, Write-off			30,215,000
Prepayment Penalty			$ 11,228,000
Mortgage notes payable | Fixed Rate Secured Mortgage Note 6.23%
Mortgage Notes Payable, Unsecured Notes and Credit Facility
Fixed rate (as a percent)	6.23%							6.23%
Repayments of Debt	$ 46,938,000
Mortgage notes payable | Fixed Rate Secured Mortgage Note 6.13%
Mortgage Notes Payable, Unsecured Notes and Credit Facility
Fixed rate (as a percent)	6.13%							6.13%
Repayments of Debt	$ 56,492,000
Unsecured Notes 3.45 Percent [Member]
Mortgage Notes Payable, Unsecured Notes and Credit Facility
Principal amount of notes issued					$ 525,000,000
Net proceeds from issuance of debt					$ 520,653,000
Stated interest rate (as a percent)					3.45%
Unsecured notes 3.50 percent
Mortgage Notes Payable, Unsecured Notes and Credit Facility
Principal amount of notes issued		$ 300,000,000
Net proceeds from issuance of debt		$ 297,072,000
Stated interest rate (as a percent)		3.50%