Acquisitions, Dispositions and Other Significant Transactions - Dispositions of Owned Communities and Assets Held for Sale (Details) $ in Thousands	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2018 USD ($)	Jun. 30, 2017 USD ($)	Jun. 30, 2018 USD ($) community	Jun. 30, 2017 USD ($)	Dec. 31, 2017 USD ($) community
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Gain (loss) on sale of assets, net	$ 23,322	$ (547)	$ 66,753	$ (1,150)
Number of additional communities held for sale | community			1
Assets held for sale	141,104		$ 141,104		$ 106,435
Revenue	895,969	934,070	$ 1,802,235	1,950,997
Sale of 3 communities
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of communities disposed of | community					3
Net cash proceeds					$ 8,200
43 Communities Disposed Of By Lease Terminations
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of communities disposed of | community					43
26 Communities Disposed Of By Lease Terminations
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of communities disposed of | community					26
Disposal Group, Held-for-sale or Disposed of by Sale, Not Discontinued Operations | 15 Communities Held For Sale
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of communities classified as held for sale | community			15		15
Impairment of assets held for sale	9,200	12,000	$ 9,200	12,000
Assets held for sale	141,100		$ 141,100
Disposal Group, Held-for-sale or Disposed of by Sale, Not Discontinued Operations | Three Communities
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of communities disposed of | community			3
Number of communities not previously held for sale | community			2
Net cash proceeds			$ 12,800
Gain (loss) on sale of assets, net			1,900
Retirement Centers
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Revenue	159,293	162,248	$ 317,690	334,868
Retirement Centers | Disposal Group, Held-for-sale or Disposed of by Sale, Not Discontinued Operations | 15 Communities Held For Sale
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of communities classified as held for sale | community			1
Assisted Living
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Revenue	522,027	547,430	$ 1,054,307	1,137,967
Assisted Living | Disposal Group, Held-for-sale or Disposed of by Sale, Not Discontinued Operations | 15 Communities Held For Sale
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of communities classified as held for sale | community			12
CCRCs Rental
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Revenue	104,796	114,202	$ 209,865	256,000
CCRCs Rental | Disposal Group, Held-for-sale or Disposed of by Sale, Not Discontinued Operations | 15 Communities Held For Sale
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of communities classified as held for sale | community			2
Mortgage notes payable | Disposal Group, Held-for-sale or Disposed of by Sale, Not Discontinued Operations | 15 Communities Held For Sale
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Mortgage debt	50,200		$ 50,200		$ 30,100
Resident fees
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Revenue	895,969	934,070	1,802,235	1,950,997
Resident fees | Disposal Group, Held-for-sale or Disposed of by Sale, Not Discontinued Operations | 15 Communities Held For Sale
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Revenue	14,300	15,000	29,000	30,300
Facility operating
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Costs of services	627,076	642,405	1,259,401	1,316,947
Facility operating | Disposal Group, Held-for-sale or Disposed of by Sale, Not Discontinued Operations | 15 Communities Held For Sale
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Costs of services	$ 11,000	$ 10,900	$ 22,000	$ 21,700