Schedule III - Real Estate and Accumulated Depreciation - Assets Held for Sale (Details) - USD ($) $ in Thousands	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation [Line Items]
Initial cost of encumbrances	$ 2,202,312
Initial cost of land and land improvements	3,629,663
Initial cost of building and improvements	28,865,012
Cost Capitalized Subsequent to Acquisition	5,167,480
Carrying amount of land and land improvements	3,968,430
Carrying amount of building and improvements	33,637,317
Accumulated depreciation	6,910,114	$ 6,104,297	$ 5,715,459	$ 5,499,958
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 and land improvements	30,940
Initial cost of building and improvements	156,620
Cost Capitalized Subsequent to Acquisition	2,945
Carrying amount of land and land improvements	0
Carrying amount of building and improvements	134,097
Accumulated depreciation	0
Assets held for sale | Brookline, MA
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 and land improvements	0
Initial cost of building and improvements	3,799
Cost Capitalized Subsequent to Acquisition	0
Carrying amount of land and land improvements	0
Carrying amount of building and improvements	3,799
Accumulated depreciation	0
Assets held for sale | Concord, NH
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 and land improvements	720
Initial cost of building and improvements	3,041
Cost Capitalized Subsequent to Acquisition	0
Carrying amount of land and land improvements	0
Carrying amount of building and improvements	2,974
Accumulated depreciation	0
Assets held for sale | Concord, NH
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 and land improvements	1,760
Initial cost of building and improvements	43,179
Cost Capitalized Subsequent to Acquisition	0
Carrying amount of land and land improvements	0
Carrying amount of building and improvements	21,811
Accumulated depreciation	0
Assets held for sale | Fort Collins, CO
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 and land improvements	890
Initial cost of building and improvements	4,532
Cost Capitalized Subsequent to Acquisition	0
Carrying amount of land and land improvements	0
Carrying amount of building and improvements	4,478
Accumulated depreciation	0
Assets held for sale | Fountain Valley, CA
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 and land improvements	5,259
Initial cost of building and improvements	9,379
Cost Capitalized Subsequent to Acquisition	0
Carrying amount of land and land improvements	0
Carrying amount of building and improvements	13,952
Accumulated depreciation	0
Assets held for sale | Franconia, NH
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 and land improvements	360
Initial cost of building and improvements	8,609
Cost Capitalized Subsequent to Acquisition	0
Carrying amount of land and land improvements	0
Carrying amount of building and improvements	8,609
Accumulated depreciation	0
Assets held for sale | Gig Harbor, WA
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 and land improvements	3,000
Initial cost of building and improvements	4,463
Cost Capitalized Subsequent to Acquisition	0
Carrying amount of land and land improvements	0
Carrying amount of building and improvements	7,062
Accumulated depreciation	0
Assets held for sale | Hemet, CA
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 and land improvements	6,224
Initial cost of building and improvements	8,414
Cost Capitalized Subsequent to Acquisition	0
Carrying amount of land and land improvements	0
Carrying amount of building and improvements	14,001
Accumulated depreciation	0
Assets held for sale | Irving, TX
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 and land improvements	1,030
Initial cost of building and improvements	6,823
Cost Capitalized Subsequent to Acquisition	0
Carrying amount of land and land improvements	0
Carrying amount of building and improvements	2,785
Accumulated depreciation	0
Assets held for sale | Las Vegas, NV
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 and land improvements	0
Initial cost of building and improvements	0
Cost Capitalized Subsequent to Acquisition	2,945
Carrying amount of land and land improvements	0
Carrying amount of building and improvements	2,945
Accumulated depreciation	0
Assets held for sale | Louisville, KY
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 and land improvements	490
Initial cost of building and improvements	10,010
Cost Capitalized Subsequent to Acquisition	0
Carrying amount of land and land improvements	0
Carrying amount of building and improvements	7,780
Accumulated depreciation	0
Assets held for sale | Morrison, CO
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 and land improvements	2,720
Initial cost of building and improvements	16,261
Cost Capitalized Subsequent to Acquisition	0
Carrying amount of land and land improvements	0
Carrying amount of building and improvements	13,433
Accumulated depreciation	0
Assets held for sale | Owensboro, KY
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 and land improvements	225
Initial cost of building and improvements	13,275
Cost Capitalized Subsequent to Acquisition	0
Carrying amount of land and land improvements	0
Carrying amount of building and improvements	7,687
Accumulated depreciation	0
Assets held for sale | Owenton, KY
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 and land improvements	100
Initial cost of building and improvements	2,400
Cost Capitalized Subsequent to Acquisition	0
Carrying amount of land and land improvements	0
Carrying amount of building and improvements	1,282
Accumulated depreciation	0
Assets held for sale | Palm Desert, CA
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 and land improvements	6,195
Initial cost of building and improvements	8,922
Cost Capitalized Subsequent to Acquisition	0
Carrying amount of land and land improvements	0
Carrying amount of building and improvements	14,451
Accumulated depreciation	0
Assets held for sale | Rexburg, ID
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 and land improvements	1,267
Initial cost of building and improvements	3,213
Cost Capitalized Subsequent to Acquisition	0
Carrying amount of land and land improvements	0
Carrying amount of building and improvements	67
Accumulated depreciation	0
Assets held for sale | Shelbyville, KY
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 and land improvements	630
Initial cost of building and improvements	3,870
Cost Capitalized Subsequent to Acquisition	0
Carrying amount of land and land improvements	0
Carrying amount of building and improvements	3,315
Accumulated depreciation	0
Assets held for sale | Williamstown, KY
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 and land improvements	70
Initial cost of building and improvements	6,430
Cost Capitalized Subsequent to Acquisition	0
Carrying amount of land and land improvements	0
Carrying amount of building and improvements	3,666
Accumulated depreciation	$ 0