Schedule III - Real Estate and Accumulated Depreciation (Summary) (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation [Line Items]
Initial Cost of Encumbrances	$ 2,378,073
Initial Cost of Land	3,142,850
Initial Cost of Buildings & Improvements	26,915,633
Cost Capitalized Subsequent to Acquisition	3,649,943
Carrying Amount of Land	3,440,650
Carrying Amount of Buildings and Improvements	30,229,356
Accumulated Depreciation	6,104,297	$ 5,715,459	$ 5,499,958	$ 4,838,370
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	487,742
Cost Capitalized Subsequent to Acquisition	0
Carrying Amount of Land	0
Carrying Amount of Buildings and Improvements	487,742
Accumulated Depreciation	0
Seniors Housing Operating
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation [Line Items]
Initial Cost of Encumbrances	1,706,192
Initial Cost of Land	1,466,472
Initial Cost of Buildings & Improvements	13,489,025
Cost Capitalized Subsequent to Acquisition	2,648,613
Carrying Amount of Land	1,642,393
Carrying Amount of Buildings and Improvements	15,961,717
Accumulated Depreciation	3,554,697
Seniors Housing Operating | Operating Segments
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation [Line Items]
Initial Cost of Encumbrances	1,706,192
Initial Cost of Land	1,466,472
Initial Cost of Buildings & Improvements	13,489,025
Cost Capitalized Subsequent to Acquisition	2,648,613
Carrying Amount of Land	1,642,393
Carrying Amount of Buildings and Improvements	15,961,717
Accumulated Depreciation	3,554,697
Triple-net
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation [Line Items]
Initial Cost of Encumbrances	123,652
Initial Cost of Land	905,073
Initial Cost of Buildings & Improvements	7,397,004
Cost Capitalized Subsequent to Acquisition	596,731
Carrying Amount of Land	957,163
Carrying Amount of Buildings and Improvements	7,941,645
Accumulated Depreciation	1,432,228
Triple-net | Operating Segments
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation [Line Items]
Initial Cost of Encumbrances	123,652
Initial Cost of Land	905,073
Initial Cost of Buildings & Improvements	7,397,004
Cost Capitalized Subsequent to Acquisition	596,731
Carrying Amount of Land	957,163
Carrying Amount of Buildings and Improvements	7,941,645
Accumulated Depreciation	1,432,228
Outpatient Medical
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation [Line Items]
Initial Cost of Encumbrances	548,229
Initial Cost of Land	765,282
Initial Cost of Buildings & Improvements	5,363,198
Cost Capitalized Subsequent to Acquisition	334,253
Carrying Amount of Land	841,094
Carrying Amount of Buildings and Improvements	5,621,639
Accumulated Depreciation	1,117,372
Outpatient Medical | Operating Segments
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation [Line Items]
Initial Cost of Encumbrances	548,229
Initial Cost of Land	765,282
Initial Cost of Buildings & Improvements	5,363,198
Cost Capitalized Subsequent to Acquisition	334,253
Carrying Amount of Land	841,094
Carrying Amount of Buildings and Improvements	5,621,639
Accumulated Depreciation	1,117,372
Total continuing operating properties
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation [Line Items]
Initial Cost of Encumbrances	2,378,073
Initial Cost of Land	3,136,827
Initial Cost of Buildings & Improvements	26,736,969
Cost Capitalized Subsequent to Acquisition	3,579,597
Carrying Amount of Land	3,440,650
Carrying Amount of Buildings and Improvements	30,012,743
Accumulated Depreciation	6,104,297
Assets held for sale
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	6,023
Initial Cost of Buildings & Improvements	178,664
Cost Capitalized Subsequent to Acquisition	70,346
Carrying Amount of Land	0
Carrying Amount of Buildings and Improvements	216,613
Accumulated Depreciation	$ 0