Acquisitions, Dispositions and Other Significant Transactions - Dispositions of Owned Communities and Assets Held for Sale (Details) $ in Thousands		3 Months Ended		9 Months Ended		12 Months Ended
	Nov. 01, 2018 USD ($) community	Sep. 30, 2018 USD ($)	Sep. 30, 2017 USD ($)	Sep. 30, 2018 USD ($) community	Sep. 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		$ 9,833	$ (233)	$ 76,586	$ (1,383)
Assets held for sale		241,854		241,854		$ 106,435
Revenue		840,179		$ 2,642,414
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
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of additional communities held for sale | community				18
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
Impairment of assets held for sale		3,000		$ 15,000
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
Disposal Group, Held-for-sale or Disposed of by Sale, Not Discontinued Operations | 32 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				32
Assets held for sale		241,900		$ 241,900
Retirement Centers
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Revenue		144,631		$ 462,321
Retirement Centers | Disposal Group, Held-for-sale or Disposed of by Sale, Not Discontinued Operations | 32 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				4
Assisted Living
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Revenue		483,125		$ 1,537,432
Assisted Living | Disposal Group, Held-for-sale or Disposed of by Sale, Not Discontinued Operations | 32 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				26
CCRCs Rental
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Revenue		104,147		$ 314,012
CCRCs Rental | Disposal Group, Held-for-sale or Disposed of by Sale, Not Discontinued Operations | 32 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						$ 30,100
Mortgage notes payable | Disposal Group, Held-for-sale or Disposed of by Sale, Not Discontinued Operations | 32 Communities Held For Sale
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Mortgage debt		158,600		$ 158,600
Resident fees
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Revenue		840,179	922,892	2,642,414	2,873,889
Resident fees | Disposal Group, Held-for-sale or Disposed of by Sale, Not Discontinued Operations | 32 Communities Held For Sale
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Revenue		26,400	27,000	81,100	81,700
Facility operating expense
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Costs of services		607,076	650,654	1,866,477	1,967,601
Facility operating expense | Disposal Group, Held-for-sale or Disposed of by Sale, Not Discontinued Operations | 32 Communities Held For Sale
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Costs of services		$ 20,000	$ 19,100	$ 58,900	$ 56,600
Subsequent event | Sale Of One Community
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of communities disposed of | community	1
Net cash proceeds	$ 144,000