Acquisitions, Dispositions and Other Significant Transactions - 2016 HCP Agreements (Details) $ in Thousands		1 Months Ended	3 Months Ended	12 Months Ended
	Nov. 01, 2016 community	Jan. 31, 2017 community	Dec. 31, 2017 USD ($) community	Dec. 31, 2017 USD ($) community	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Mar. 29, 2017 community
Business Acquisition [Line Items]
Gain on sale of assets, net | $				$ 19,273	$ 7,218	$ 1,270
Termination Of Triple Net Leases
Business Acquisition [Line Items]
Number of communities disposed of	97
Termination Of Triple Net Leases, 33 Communities
Business Acquisition [Line Items]
Number of communities disposed of	33
Termination Of 8 Triple Net Leases
Business Acquisition [Line Items]
Number of communities disposed of	8	1
Ownership Percentage	10.00%
Termination Of 25 Triple Net Leases
Business Acquisition [Line Items]
Number of communities disposed of	25			25
Sale Of 21 Communities
Business Acquisition [Line Items]
Number of communities disposed of				21
Gain on sale of assets, net | $				$ 11,400
Assets under financing leases, amount derecognized | $			$ 145,300	145,300
Financing lease obligations, amount derecognized | $			$ 156,700	$ 156,700
HCP, Inc.
Business Acquisition [Line Items]
Number of communities contributed to joint venture	4
Joint Venture | Blackstone Venture | Senior Housing Communities
Business Acquisition [Line Items]
Number of communities acquired							64
CCRCs-Rental
Business Acquisition [Line Items]
Number of communities disposed of			7
CCRCs-Rental | Termination Of 8 Triple Net Leases
Business Acquisition [Line Items]
Number of communities disposed of				1
Assisted Living | Termination Of 8 Triple Net Leases
Business Acquisition [Line Items]
Number of communities disposed of				6
Retirement Centers | Termination Of 8 Triple Net Leases
Business Acquisition [Line Items]
Number of communities disposed of				1