Acquisitions, Dispositions and Other Significant Transactions - Revenue of Disposal Groups (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Resident fees	$ 3,449,211
Cash lease payments	324,870	$ 365,077	$ 384,104
Communities Disposed Of Through Sale And Lease Terminations
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Resident fees	302,486	653,884	1,033,600
Facility operating expense	207,802	453,364	722,448
Cash lease payments	78,264	172,442	259,114
Independent Living
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Resident fees	599,977
Independent Living | Communities Disposed Of Through Sale And Lease Terminations
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Resident fees	81,280	152,190	189,205
Facility operating expense	48,154	90,134	110,014
Assisted Living and Memory Care
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Resident fees	1,995,851
Assisted Living and Memory Care | Communities Disposed Of Through Sale And Lease Terminations
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Resident fees	205,241	423,302	642,782
Facility operating expense	145,106	294,528	443,137
CCRCs
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Resident fees	416,408
CCRCs | Communities Disposed Of Through Sale And Lease Terminations
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Resident fees	15,965	78,392	201,613
Facility operating expense	$ 14,542	$ 68,702	$ 169,297