Acquisitions, Dispositions and Other Significant Transactions - 2016 Transactions (Details)		1 Months Ended	3 Months Ended			6 Months Ended	12 Months Ended
	Dec. 29, 2014 community	Aug. 31, 2014 community	Dec. 31, 2016 USD ($) community	Sep. 30, 2016 USD ($) community	Mar. 31, 2016 USD ($) community	Jun. 30, 2016 USD ($) community state	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($) community	Dec. 31, 2015 USD ($) community
Business Acquisition [Line Items]
Number of communities purchased or sold					7
Communities classified as held for sale			10					10	17
Proceeds from sale of assets | $					$ 46,700,000
Number of communities under triple net leases agreement		41
Goodwill | $			$ 705,476,000				$ 505,783,000	$ 705,476,000
Impairment of assets held for sale | $							$ 0	$ 15,800,000	$ 33,600,000
Agreement to sell 44 communities
Business Acquisition [Line Items]
Number of communities purchased or sold						44
Communities classified as held for sale			2					2
Proceeds from sale of assets | $						$ 252,500,000
Number of states in portfolio | state						12
Sale of 32 communities
Business Acquisition [Line Items]
Number of communities purchased or sold				32
Proceeds from sale of assets | $				$ 177,500,000
Sale of 9 communities
Business Acquisition [Line Items]
Number of communities purchased or sold	9		9
Proceeds from sale of assets | $			$ 47,700,000
Removal of one community from the agreement to sell 44 communities
Business Acquisition [Line Items]
Number of communities								1
Decrease in sales price | $								$ 4,700,000
Agreements to sell six communities
Business Acquisition [Line Items]
Number of additional communities identified as held for sale								7
Sale of 3 communities
Business Acquisition [Line Items]
Number of communities purchased or sold			3
Proceeds from sale of assets | $			$ 33,000,000
Assets held for sale
Business Acquisition [Line Items]
Communities classified as held for sale									17
Goodwill | $									$ 8,100,000
Assets held for sale | Agreements to sell six communities
Business Acquisition [Line Items]
Goodwill | $			$ 28,600,000					$ 28,600,000
Impairment of assets held for sale | $								$ 15,800,000
Assets held for sale | Sale of 3 communities
Business Acquisition [Line Items]
Communities classified as held for sale			4					4