Acquisitions, Dispositions and Other Significant Transactions - HCP Master Lease Transaction and RIDEA Ventures Restructuring (Details) $ in Thousands	1 Months Ended		3 Months Ended				9 Months Ended		12 Months Ended
	Dec. 31, 2017 USD ($)	Nov. 30, 2017 USD ($)	Sep. 30, 2019 USD ($)	Mar. 31, 2019 community	Sep. 30, 2018 USD ($) community	Mar. 31, 2018 USD ($) community	Sep. 30, 2019 USD ($)	Sep. 30, 2018 USD ($) community	Dec. 31, 2018 community
Business Acquisition [Line Items]
Number of communities disposed through termination of lease | community								16
Gain (loss) on sale of assets, net			$ 579		$ 9,833		$ 2,723	$ 76,586
Gain (loss) on facility lease termination and modification, net			$ 0		(2,337)		(2,006)	(148,804)
Non-cash management contract termination gain							640	5,649
35 Communities Disposed Of Through Management Agreement | RIDEA Ventures Restructuring
Business Acquisition [Line Items]
Number of communities disposed of | community				35
Non-cash management contract termination gain		$ 9,300			$ 600		$ 800	$ 5,600
Variable Interest Entity, Not Primary Beneficiary, Real Estate Company | RIDEA Ventures Restructuring
Business Acquisition [Line Items]
Proceeds from divestiture of interest in joint venture	$ 32,100					$ 62,300
Gain (loss) on sale of assets, net						$ 41,700
Percentage ownership						10.00%
HCP, Inc. | RIDEA Ventures Restructuring
Business Acquisition [Line Items]
Number of communities acquired | community					5	1
Number of communities formerly leases | community					2
Payments to acquire interest in joint venture					$ 242,800
HCP, Inc. | Master Lease Transactions
Business Acquisition [Line Items]
Number of communities | community					7			7
HCP, Inc. | 33 Communities Disposed Of Through Lease Terminations | Master Lease Transactions
Business Acquisition [Line Items]
Number of communities disposed of | community									33
Assets under leases					$ 60,900			$ 147,800
Financing lease obligations, carrying value					67,300			160,800
Gain (loss) on sale of assets, net					6,100			$ 12,600
Number of communities previously subject to sale leaseback transactions | community								6
HCP, Inc. | 7 Communities Operating Under Operating and Capital Leases | Master Lease Transactions
Business Acquisition [Line Items]
Gain (loss) on facility lease termination and modification, net					$ (100)			$ 1,800