Acquisitions, Dispositions and Other Significant Transactions - HCP Master Lease Transaction and RIDEA Ventures Restructuring (Details) $ in Thousands			1 Months Ended				3 Months Ended	12 Months Ended
	Jan. 01, 2018 USD ($) community	Nov. 01, 2017 community joint_venture	Apr. 30, 2018 USD ($) community	Mar. 31, 2018 USD ($)	Jan. 31, 2018 USD ($) community	Dec. 31, 2017 USD ($) joint_venture	Dec. 31, 2020 lease community	Dec. 31, 2020 USD ($) community lease	Dec. 31, 2019 USD ($) community	Dec. 31, 2018 USD ($) community
Business Acquisition [Line Items]
Number of communities leased | lease							235	235
Ownership Percentage							50.00%	50.00%
Gain (loss) on sale of assets, net								$ 374,532	$ 7,245	$ 293,246
Non-cash management contract termination fee								0	969	8,724
Number of communities previously subject to sale leaseback transactions | community							8
Gain on facility lease termination								2,303	$ (3,388)	$ (162,001)
Master Lease Transactions
Business Acquisition [Line Items]
Annual rent reduction	$ 2,500
Number of communities with rent reduction | community	2
RIDEA Ventures Restructuring
Business Acquisition [Line Items]
Number of communities management is retained | community			18
Reduction in lease liabilities								(9,700)
Deferred revenue								$ 9,700
RIDEA Ventures Restructuring | 37 Communities Disposed Of Through Management Agreement
Business Acquisition [Line Items]
Number of communities disposed of | community								37
HCP, Inc. | Master Lease Transactions
Business Acquisition [Line Items]
Number of communities acquired | community										2
Payment to acquire leased assets										$ 35,400
Number of communities leased | community									43
Reduction in capital lease obligations and assets under lease										$ (145,600)
Gain on facility lease termination								$ (2,300)
HCP, Inc. | RIDEA Ventures Restructuring
Business Acquisition [Line Items]
Number of communities acquired | community			3		1
Payments to acquire asset from joint venture			$ 207,400		$ 32,100
RIDEA Ventures | RIDEA Ventures Restructuring
Business Acquisition [Line Items]
Ownership Percentage									10.00%
Number of joint ventures | joint_venture		2				1
Proceeds from sale of joint venture interest				$ 62,300		$ 32,100
Gain (loss) on sale of assets, net				$ 41,700		$ 7,200
Number of communities managed | community		59
33 Communities Disposed Of Through Lease Terminations | HCP, Inc. | Master Lease Transactions
Business Acquisition [Line Items]
Number of communities disposed of | community										33
Assets under leases									$ 332,800
Capital lease obligations									$ 378,300
Number of communities previously subject to sale leaseback transactions | community									20
Gain on facility lease termination									$ 1,500
37 Communities Disposed Of Through Operations and or Management Termination | RIDEA Ventures Restructuring
Business Acquisition [Line Items]
Number of communities disposed of | community								37
37 Communities Disposed Of Through Management Agreement | RIDEA Ventures Restructuring
Business Acquisition [Line Items]
Number of communities disposed of | community								37
37 Communities Disposed Of Through Management Termination | RIDEA Ventures Restructuring
Business Acquisition [Line Items]
Non-cash management contract termination fee								$ 9,700
20 Of The 33 Communities Disposed Of Through Lease Terminations | HCP, Inc. | Master Lease Transactions
Business Acquisition [Line Items]
Gain (loss) on sale of assets, net									$ 44,200