Schedule III - Real Estate and Accumulated Depreciation (Summary) (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation [Line Items]
Initial Cost of Encumbrances	$ 2,993,341
Initial Cost of Land	3,428,034
Initial Cost of Buildings & Improvements	30,427,473
Cost Capitalized Subsequent to Acquisition	2,583,182
Carrying Amount of Land	3,486,620
Carrying Amount of Buildings and Improvements	32,541,295
Accumulated Depreciation	5,715,459	$ 5,499,958	$ 4,838,370	$ 4,093,494
Seniors Housing Operating
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation [Line Items]
Initial Cost of Encumbrances	1,990,607
Initial Cost of Land	1,383,927
Initial Cost of Buildings & Improvements	13,886,675
Cost Capitalized Subsequent to Acquisition	1,879,176
Carrying Amount of Land	1,469,078
Carrying Amount of Buildings and Improvements	15,680,700
Accumulated Depreciation	3,194,057
Triple-net
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation [Line Items]
Initial Cost of Encumbrances	306,038
Initial Cost of Land	1,036,151
Initial Cost of Buildings & Improvements	7,894,992
Cost Capitalized Subsequent to Acquisition	351,136
Carrying Amount of Land	1,057,708
Carrying Amount of Buildings and Improvements	8,224,571
Accumulated Depreciation	1,272,903
Outpatient Medical
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation [Line Items]
Initial Cost of Encumbrances	572,266
Initial Cost of Land	885,789
Initial Cost of Buildings & Improvements	6,626,075
Cost Capitalized Subsequent to Acquisition	323,055
Carrying Amount of Land	959,834
Carrying Amount of Buildings and Improvements	6,875,085
Accumulated Depreciation	1,248,499
Construction in progress
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation [Line Items]
Initial Cost of Encumbrances	0
Initial Cost of Land	0
Initial Cost of Buildings & Improvements	507,931
Cost Capitalized Subsequent to Acquisition	0
Carrying Amount of Land	0
Carrying Amount of Buildings and Improvements	507,931
Accumulated Depreciation	0
Total continuing operating properties
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation [Line Items]
Initial Cost of Encumbrances	2,868,911
Initial Cost of Land	3,305,867
Initial Cost of Buildings & Improvements	28,915,673
Cost Capitalized Subsequent to Acquisition	2,553,367
Carrying Amount of Land	3,486,620
Carrying Amount of Buildings and Improvements	31,288,287
Accumulated Depreciation	5,715,459
Assets held for sale
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation [Line Items]
Initial Cost of Encumbrances	124,430
Initial Cost of Land	122,167
Initial Cost of Buildings & Improvements	1,511,800
Cost Capitalized Subsequent to Acquisition	29,815
Carrying Amount of Land	0
Carrying Amount of Buildings and Improvements	1,253,008
Accumulated Depreciation	$ 0