Schedule III - Real Estate and Accumulated Depreciation (Assets Held for Sale) (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
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
Assets held for sale | Albuquerque, NM
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	1,270
Initial Cost of Buildings & Improvements	20,837
Cost Capitalized Subsequent to Acquisition	0
Carrying Amount of Land	0
Carrying Amount of Buildings and Improvements	13,230
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	0
Initial Cost of Buildings & Improvements	17,435
Cost Capitalized Subsequent to Acquisition	0
Carrying Amount of Land	0
Carrying Amount of Buildings and Improvements	17,435
Accumulated Depreciation	0
Assets held for sale | Castle Rock, 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	80
Initial Cost of Buildings & Improvements	9,667
Cost Capitalized Subsequent to Acquisition	0
Carrying Amount of Land	0
Carrying Amount of Buildings and Improvements	9,667
Accumulated Depreciation	0
Assets held for sale | Castle Rock, 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	0
Initial Cost of Buildings & Improvements	0
Cost Capitalized Subsequent to Acquisition	10,480
Carrying Amount of Land	0
Carrying Amount of Buildings and Improvements	10,480
Accumulated Depreciation	0
Assets held for sale | Glendale, 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	0
Initial Cost of Buildings & Improvements	0
Cost Capitalized Subsequent to Acquisition	11,228
Carrying Amount of Land	0
Carrying Amount of Buildings and Improvements	11,228
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	1,030
Initial Cost of Buildings & Improvements	2,450
Cost Capitalized Subsequent to Acquisition	0
Carrying Amount of Land	0
Carrying Amount of Buildings and Improvements	2,450
Accumulated Depreciation	0
Assets held for sale | Lakewood, 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	0
Initial Cost of Buildings & Improvements	0
Cost Capitalized Subsequent to Acquisition	11,639
Carrying Amount of Land	0
Carrying Amount of Buildings and Improvements	11,639
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	0
Initial Cost of Buildings & Improvements	0
Cost Capitalized Subsequent to Acquisition	2,945
Carrying Amount of Land	0
Carrying Amount of Buildings and Improvements	2,945
Accumulated Depreciation	0
Assets held for sale | Lincoln, NE
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	0
Cost Capitalized Subsequent to Acquisition	19,641
Carrying Amount of Land	0
Carrying Amount of Buildings and Improvements	19,641
Accumulated Depreciation	0
Assets held for sale | Powell, TN
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	25,692
Cost Capitalized Subsequent to Acquisition	0
Carrying Amount of Land	0
Carrying Amount of Buildings and Improvements	25,692
Accumulated Depreciation	0
Assets held for sale | Powell, TN
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	34,994
Cost Capitalized Subsequent to Acquisition	0
Carrying Amount of Land	0
Carrying Amount of Buildings and Improvements	34,994
Accumulated Depreciation	0
Assets held for sale | Reno, 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	0
Initial Cost of Buildings & Improvements	0
Cost Capitalized Subsequent to Acquisition	14,413
Carrying Amount of Land	0
Carrying Amount of Buildings and Improvements	14,413
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	1,267
Initial Cost of Buildings & Improvements	3,213
Cost Capitalized Subsequent to Acquisition	0
Carrying Amount of Land	0
Carrying Amount of Buildings and Improvements	67
Accumulated Depreciation	0
Assets held for sale | St. Louis, MO
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	336
Initial Cost of Buildings & Improvements	17,247
Cost Capitalized Subsequent to Acquisition	0
Carrying Amount of Land	0
Carrying Amount of Buildings and Improvements	11,772
Accumulated Depreciation	0
Assets held for sale | Toledo, OH
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	2,040
Initial Cost of Buildings & Improvements	47,129
Cost Capitalized Subsequent to Acquisition	0
Carrying Amount of Land	0
Carrying Amount of Buildings and Improvements	30,960
Accumulated Depreciation	$ 0