Schedule III - Real Estate and Accumulated Depreciation (Assets Held for Sale) (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Initial Cost of Encumbrances	$ 2,485,711
Initial Cost of Land	3,141,623
Initial Cost of Buildings & Improvements	28,882,284
Cost Capitalized Subsequent to Acquisition	1,868,125
Carrying Amount of Land	3,205,091
Carrying Amount of Buildings and Improvements	30,385,297
Accumulated Depreciation	5,499,958	$ 4,838,370	$ 4,093,494	$ 3,796,297
Assets Held-for-sale [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Initial Cost of Encumbrances	0
Initial Cost of Land	68,392
Initial Cost of Buildings & Improvements	821,723
Cost Capitalized Subsequent to Acquisition	1,997
Carrying Amount of Land	0
Carrying Amount of Buildings and Improvements	590,271
Accumulated Depreciation	0
Agawam, MA
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Initial Cost of Encumbrances	0
Initial Cost of Land	880
Initial Cost of Buildings & Improvements	10,044
Cost Capitalized Subsequent to Acquisition	0
Carrying Amount of Land	0
Carrying Amount of Buildings and Improvements	8,696
Accumulated Depreciation	0
Agawam, MA
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Initial Cost of Encumbrances	0
Initial Cost of Land	1,230
Initial Cost of Buildings & Improvements	13,618
Cost Capitalized Subsequent to Acquisition	0
Carrying Amount of Land	0
Carrying Amount of Buildings and Improvements	6,074
Accumulated Depreciation	0
Agawam, MA
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Initial Cost of Encumbrances	0
Initial Cost of Land	930
Initial Cost of Buildings & Improvements	15,304
Cost Capitalized Subsequent to Acquisition	0
Carrying Amount of Land	0
Carrying Amount of Buildings and Improvements	6,511
Accumulated Depreciation	0
Agawam, MA
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Initial Cost of Encumbrances	0
Initial Cost of Land	920
Initial Cost of Buildings & Improvements	10,661
Cost Capitalized Subsequent to Acquisition	0
Carrying Amount of Land	0
Carrying Amount of Buildings and Improvements	4,592
Accumulated Depreciation	0
Agawam, MA
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Initial Cost of Encumbrances	0
Initial Cost of Land	920
Initial Cost of Buildings & Improvements	10,562
Cost Capitalized Subsequent to Acquisition	0
Carrying Amount of Land	0
Carrying Amount of Buildings and Improvements	4,537
Accumulated Depreciation	0
Ayer, MA
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Initial Cost of Encumbrances	0
Initial Cost of Land	0
Initial Cost of Buildings & Improvements	22,074
Cost Capitalized Subsequent to Acquisition	0
Carrying Amount of Land	0
Carrying Amount of Buildings and Improvements	8,691
Accumulated Depreciation	0
Beachwood, OH
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Initial Cost of Encumbrances	0
Initial Cost of Land	1,260
Initial Cost of Buildings & Improvements	23,478
Cost Capitalized Subsequent to Acquisition	0
Carrying Amount of Land	0
Carrying Amount of Buildings and Improvements	10,503
Accumulated Depreciation	0
Birmingham, UKG
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Initial Cost of Encumbrances	0
Initial Cost of Land	4
Initial Cost of Buildings & Improvements	21,321
Cost Capitalized Subsequent to Acquisition	0
Carrying Amount of Land	0
Carrying Amount of Buildings and Improvements	13,200
Accumulated Depreciation	0
Bridgewater, NJ
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Initial Cost of Encumbrances	0
Initial Cost of Land	1,850
Initial Cost of Buildings & Improvements	3,050
Cost Capitalized Subsequent to Acquisition	0
Carrying Amount of Land	0
Carrying Amount of Buildings and Improvements	3,342
Accumulated Depreciation	0
Broadview Heights, OH
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Initial Cost of Encumbrances	0
Initial Cost of Land	920
Initial Cost of Buildings & Improvements	12,400
Cost Capitalized Subsequent to Acquisition	0
Carrying Amount of Land	0
Carrying Amount of Buildings and Improvements	9,590
Accumulated Depreciation	0
Canton, MA
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Initial Cost of Encumbrances	0
Initial Cost of Land	820
Initial Cost of Buildings & Improvements	8,201
Cost Capitalized Subsequent to Acquisition	0
Carrying Amount of Land	0
Carrying Amount of Buildings and Improvements	2,626
Accumulated Depreciation	0
Centerville, MA
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Initial Cost of Encumbrances	0
Initial Cost of Land	1,300
Initial Cost of Buildings & Improvements	27,357
Cost Capitalized Subsequent to Acquisition	0
Carrying Amount of Land	0
Carrying Amount of Buildings and Improvements	23,139
Accumulated Depreciation	0
Charles Town, WV
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Initial Cost of Encumbrances	0
Initial Cost of Land	230
Initial Cost of Buildings & Improvements	22,834
Cost Capitalized Subsequent to Acquisition	0
Carrying Amount of Land	0
Carrying Amount of Buildings and Improvements	18,509
Accumulated Depreciation	0
Cinnaminson, NJ
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Initial Cost of Encumbrances	0
Initial Cost of Land	860
Initial Cost of Buildings & Improvements	6,663
Cost Capitalized Subsequent to Acquisition	0
Carrying Amount of Land	0
Carrying Amount of Buildings and Improvements	6,014
Accumulated Depreciation	0
Cloquet, MN
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Initial Cost of Encumbrances	0
Initial Cost of Land	340
Initial Cost of Buildings & Improvements	4,660
Cost Capitalized Subsequent to Acquisition	0
Carrying Amount of Land	0
Carrying Amount of Buildings and Improvements	4,285
Accumulated Depreciation	0
Concord, NH
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Initial Cost of Encumbrances	0
Initial Cost of Land	720
Initial Cost of Buildings & Improvements	3,041
Cost Capitalized Subsequent to Acquisition	0
Carrying Amount of Land	0
Carrying Amount of Buildings and Improvements	3,344
Accumulated Depreciation	0
Dallas, TX
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Initial Cost of Encumbrances	0
Initial Cost of Land	1,080
Initial Cost of Buildings & Improvements	9,655
Cost Capitalized Subsequent to Acquisition	0
Carrying Amount of Land	0
Carrying Amount of Buildings and Improvements	4,412
Accumulated Depreciation	0
Gettysburg, PA
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Initial Cost of Encumbrances	0
Initial Cost of Land	590
Initial Cost of Buildings & Improvements	8,913
Cost Capitalized Subsequent to Acquisition	0
Carrying Amount of Land	0
Carrying Amount of Buildings and Improvements	7,501
Accumulated Depreciation	0
Glastonbury, CT
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Initial Cost of Encumbrances	0
Initial Cost of Land	1,950
Initial Cost of Buildings & Improvements	9,532
Cost Capitalized Subsequent to Acquisition	0
Carrying Amount of Land	0
Carrying Amount of Buildings and Improvements	5,500
Accumulated Depreciation	0
Hamburg, PA
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Initial Cost of Encumbrances	0
Initial Cost of Land	840
Initial Cost of Buildings & Improvements	10,543
Cost Capitalized Subsequent to Acquisition	0
Carrying Amount of Land	0
Carrying Amount of Buildings and Improvements	8,994
Accumulated Depreciation	0
Houston, TX
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Initial Cost of Encumbrances	0
Initial Cost of Land	5,090
Initial Cost of Buildings & Improvements	9,471
Cost Capitalized Subsequent to Acquisition	0
Carrying Amount of Land	0
Carrying Amount of Buildings and Improvements	7,840
Accumulated Depreciation	0
Lancaster, NH
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Initial Cost of Encumbrances	0
Initial Cost of Land	160
Initial Cost of Buildings & Improvements	434
Cost Capitalized Subsequent to Acquisition	0
Carrying Amount of Land	0
Carrying Amount of Buildings and Improvements	493
Accumulated Depreciation	0
Langhorne, PA
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Initial Cost of Encumbrances	0
Initial Cost of Land	1,350
Initial Cost of Buildings & Improvements	24,881
Cost Capitalized Subsequent to Acquisition	0
Carrying Amount of Land	0
Carrying Amount of Buildings and Improvements	3,551
Accumulated Depreciation	0
Lowell, MA
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Initial Cost of Encumbrances	0
Initial Cost of Land	1,070
Initial Cost of Buildings & Improvements	13,481
Cost Capitalized Subsequent to Acquisition	0
Carrying Amount of Land	0
Carrying Amount of Buildings and Improvements	1,960
Accumulated Depreciation	0
Lowell, MA
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Initial Cost of Encumbrances	0
Initial Cost of Land	680
Initial Cost of Buildings & Improvements	3,378
Cost Capitalized Subsequent to Acquisition	0
Carrying Amount of Land	0
Carrying Amount of Buildings and Improvements	3,155
Accumulated Depreciation	0
Mendham, NJ
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Initial Cost of Encumbrances	0
Initial Cost of Land	1,240
Initial Cost of Buildings & Improvements	27,169
Cost Capitalized Subsequent to Acquisition	0
Carrying Amount of Land	0
Carrying Amount of Buildings and Improvements	23,295
Accumulated Depreciation	0
Merriam, KS
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Initial Cost of Encumbrances	0
Initial Cost of Land	0
Initial Cost of Buildings & Improvements	1,996
Cost Capitalized Subsequent to Acquisition	0
Carrying Amount of Land	0
Carrying Amount of Buildings and Improvements	0
Accumulated Depreciation	0
Merriam, KS
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Initial Cost of Encumbrances	0
Initial Cost of Land	0
Initial Cost of Buildings & Improvements	5,862
Cost Capitalized Subsequent to Acquisition	0
Carrying Amount of Land	0
Carrying Amount of Buildings and Improvements	0
Accumulated Depreciation	0
Middletown, RI
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Initial Cost of Encumbrances	0
Initial Cost of Land	2,480
Initial Cost of Buildings & Improvements	24,628
Cost Capitalized Subsequent to Acquisition	0
Carrying Amount of Land	0
Carrying Amount of Buildings and Improvements	21,727
Accumulated Depreciation	0
Millville, NJ
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Initial Cost of Encumbrances	0
Initial Cost of Land	840
Initial Cost of Buildings & Improvements	29,944
Cost Capitalized Subsequent to Acquisition	0
Carrying Amount of Land	0
Carrying Amount of Buildings and Improvements	24,559
Accumulated Depreciation	0
Monroe Twp, NJ
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Initial Cost of Encumbrances	0
Initial Cost of Land	1,160
Initial Cost of Buildings & Improvements	13,193
Cost Capitalized Subsequent to Acquisition	0
Carrying Amount of Land	0
Carrying Amount of Buildings and Improvements	11,403
Accumulated Depreciation	0
Mystic, CT
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Initial Cost of Encumbrances	0
Initial Cost of Land	1,400
Initial Cost of Buildings & Improvements	18,274
Cost Capitalized Subsequent to Acquisition	0
Carrying Amount of Land	0
Carrying Amount of Buildings and Improvements	15,316
Accumulated Depreciation	0
Niantic, CT
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Initial Cost of Encumbrances	0
Initial Cost of Land	1,320
Initial Cost of Buildings & Improvements	25,986
Cost Capitalized Subsequent to Acquisition	0
Carrying Amount of Land	0
Carrying Amount of Buildings and Improvements	25,167
Accumulated Depreciation	0
North Cape May, NJ
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Initial Cost of Encumbrances	0
Initial Cost of Land	77
Initial Cost of Buildings & Improvements	151
Cost Capitalized Subsequent to Acquisition	48
Carrying Amount of Land	0
Carrying Amount of Buildings and Improvements	276
Accumulated Depreciation	0
North Cape May, NJ
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Initial Cost of Encumbrances	0
Initial Cost of Land	600
Initial Cost of Buildings & Improvements	22,266
Cost Capitalized Subsequent to Acquisition	0
Carrying Amount of Land	0
Carrying Amount of Buildings and Improvements	18,270
Accumulated Depreciation	0
Palm Springs, FL
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Initial Cost of Encumbrances	0
Initial Cost of Land	739
Initial Cost of Buildings & Improvements	4,066
Cost Capitalized Subsequent to Acquisition	0
Carrying Amount of Land	0
Carrying Amount of Buildings and Improvements	2,061
Accumulated Depreciation	0
Palm Springs, FL
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Initial Cost of Encumbrances	0
Initial Cost of Land	1,182
Initial Cost of Buildings & Improvements	7,765
Cost Capitalized Subsequent to Acquisition	0
Carrying Amount of Land	0
Carrying Amount of Buildings and Improvements	3,477
Accumulated Depreciation	0
Pennsauken, NJ
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Initial Cost of Encumbrances	0
Initial Cost of Land	900
Initial Cost of Buildings & Improvements	10,780
Cost Capitalized Subsequent to Acquisition	0
Carrying Amount of Land	0
Carrying Amount of Buildings and Improvements	9,172
Accumulated Depreciation	0
Providence, RI
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Initial Cost of Encumbrances	0
Initial Cost of Land	2,655
Initial Cost of Buildings & Improvements	21,910
Cost Capitalized Subsequent to Acquisition	0
Carrying Amount of Land	0
Carrying Amount of Buildings and Improvements	16,021
Accumulated Depreciation	0
Rockville, CT
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Initial Cost of Encumbrances	0
Initial Cost of Land	1,500
Initial Cost of Buildings & Improvements	4,835
Cost Capitalized Subsequent to Acquisition	0
Carrying Amount of Land	0
Carrying Amount of Buildings and Improvements	5,073
Accumulated Depreciation	0
Sanatoga, PA
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Initial Cost of Encumbrances	0
Initial Cost of Land	980
Initial Cost of Buildings & Improvements	30,695
Cost Capitalized Subsequent to Acquisition	0
Carrying Amount of Land	0
Carrying Amount of Buildings and Improvements	14,166
Accumulated Depreciation	0
South Boston, MA
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Initial Cost of Encumbrances	0
Initial Cost of Land	385
Initial Cost of Buildings & Improvements	2,002
Cost Capitalized Subsequent to Acquisition	1,525
Carrying Amount of Land	0
Carrying Amount of Buildings and Improvements	3,912
Accumulated Depreciation	0
South Windsor, CT
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Initial Cost of Encumbrances	0
Initial Cost of Land	3,000
Initial Cost of Buildings & Improvements	29,295
Cost Capitalized Subsequent to Acquisition	0
Carrying Amount of Land	0
Carrying Amount of Buildings and Improvements	26,338
Accumulated Depreciation	0
Swanton, OH
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Initial Cost of Encumbrances	0
Initial Cost of Land	330
Initial Cost of Buildings & Improvements	6,370
Cost Capitalized Subsequent to Acquisition	0
Carrying Amount of Land	0
Carrying Amount of Buildings and Improvements	4,160
Accumulated Depreciation	0
Troy, OH
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Initial Cost of Encumbrances	0
Initial Cost of Land	470
Initial Cost of Buildings & Improvements	16,730
Cost Capitalized Subsequent to Acquisition	0
Carrying Amount of Land	0
Carrying Amount of Buildings and Improvements	10,730
Accumulated Depreciation	0
Trumbull, CT
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Initial Cost of Encumbrances	0
Initial Cost of Land	2,850
Initial Cost of Buildings & Improvements	37,685
Cost Capitalized Subsequent to Acquisition	0
Carrying Amount of Land	0
Carrying Amount of Buildings and Improvements	32,020
Accumulated Depreciation	0
Wallingford, CT
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Initial Cost of Encumbrances	0
Initial Cost of Land	490
Initial Cost of Buildings & Improvements	1,210
Cost Capitalized Subsequent to Acquisition	0
Carrying Amount of Land	0
Carrying Amount of Buildings and Improvements	727
Accumulated Depreciation	0
Warwick, RI
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Initial Cost of Encumbrances	0
Initial Cost of Land	2,400
Initial Cost of Buildings & Improvements	24,635
Cost Capitalized Subsequent to Acquisition	0
Carrying Amount of Land	0
Carrying Amount of Buildings and Improvements	21,633
Accumulated Depreciation	0
Waterbury, CT
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Initial Cost of Encumbrances	0
Initial Cost of Land	2,460
Initial Cost of Buildings & Improvements	39,547
Cost Capitalized Subsequent to Acquisition	0
Carrying Amount of Land	0
Carrying Amount of Buildings and Improvements	30,909
Accumulated Depreciation	0
West Chester, PA
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Initial Cost of Encumbrances	0
Initial Cost of Land	1,350
Initial Cost of Buildings & Improvements	29,237
Cost Capitalized Subsequent to Acquisition	0
Carrying Amount of Land	0
Carrying Amount of Buildings and Improvements	24,564
Accumulated Depreciation	0
West Orange, NJ
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Initial Cost of Encumbrances	0
Initial Cost of Land	2,280
Initial Cost of Buildings & Improvements	10,687
Cost Capitalized Subsequent to Acquisition	0
Carrying Amount of Land	0
Carrying Amount of Buildings and Improvements	10,571
Accumulated Depreciation	0
Westlake, OH
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Initial Cost of Encumbrances	0
Initial Cost of Land	1,330
Initial Cost of Buildings & Improvements	17,926
Cost Capitalized Subsequent to Acquisition	0
Carrying Amount of Land	0
Carrying Amount of Buildings and Improvements	8,673
Accumulated Depreciation	0
Wilbraham, MA
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Initial Cost of Encumbrances	0
Initial Cost of Land	660
Initial Cost of Buildings & Improvements	17,639
Cost Capitalized Subsequent to Acquisition	0
Carrying Amount of Land	0
Carrying Amount of Buildings and Improvements	14,484
Accumulated Depreciation	0
Wilkes-Barre, PA
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Initial Cost of Encumbrances	0
Initial Cost of Land	570
Initial Cost of Buildings & Improvements	2,301
Cost Capitalized Subsequent to Acquisition	0
Carrying Amount of Land	0
Carrying Amount of Buildings and Improvements	1,176
Accumulated Depreciation	0
Windsor, CT
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Initial Cost of Encumbrances	0
Initial Cost of Land	2,250
Initial Cost of Buildings & Improvements	8,539
Cost Capitalized Subsequent to Acquisition	0
Carrying Amount of Land	0
Carrying Amount of Buildings and Improvements	10,218
Accumulated Depreciation	0
Windsor, CT
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Initial Cost of Encumbrances	0
Initial Cost of Land	1,800
Initial Cost of Buildings & Improvements	600
Cost Capitalized Subsequent to Acquisition	424
Carrying Amount of Land	0
Carrying Amount of Buildings and Improvements	2,824
Accumulated Depreciation	0
Wyncote, PA
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Initial Cost of Encumbrances	0
Initial Cost of Land	2,700
Initial Cost of Buildings & Improvements	22,244
Cost Capitalized Subsequent to Acquisition	0
Carrying Amount of Land	0
Carrying Amount of Buildings and Improvements	20,290
Accumulated Depreciation	$ 0