Schedule III - Real Estate and Accumulated Depreciation and Amortization Part 4 (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Schedule Of Investments In Real Estate [Line Items]
Total Investment in Real Estate	$ 1,113,651	$ 1,043,106	$ 970,964	$ 782,166
Accumulated Depreciation And Amortization Description [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	1,049,787
Cost Capitalized Subsequent to Initial Investment	63,864
Land	669,351
Building and Improvements	444,300
Total Investment in Real Estate	1,113,651
Accumulated Depreciation and Amortization	165,892
Accumulated Depreciation And Amortization Description [Member] | DISTRICT OF COLUMBIA [Member] | Washington One [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	941
Land	664
Building and Improvements	277
Total Investment in Real Estate	941
Accumulated Depreciation and Amortization	$ 106
Date of Initial Leasehold or Acquisition Investment	2013
Accumulated Depreciation And Amortization Description [Member] | DISTRICT OF COLUMBIA [Member] | Washington Two [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 848
Land	418
Building and Improvements	430
Total Investment in Real Estate	848
Accumulated Depreciation and Amortization	$ 143
Date of Initial Leasehold or Acquisition Investment	2013
Accumulated Depreciation And Amortization Description [Member] | FLORIDA [Member] | Callahan [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 2,894
Land	2,056
Building and Improvements	838
Total Investment in Real Estate	2,894
Accumulated Depreciation and Amortization	$ 124
Date of Initial Leasehold or Acquisition Investment	2017
Accumulated Depreciation And Amortization Description [Member] | FLORIDA [Member] | Fernandina Beach [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 2,137
Land	382
Building and Improvements	1,755
Total Investment in Real Estate	2,137
Accumulated Depreciation and Amortization	$ 226
Date of Initial Leasehold or Acquisition Investment	2017
Accumulated Depreciation And Amortization Description [Member] | FLORIDA [Member] | Largo [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 2,064
Land	1,143
Building and Improvements	921
Total Investment in Real Estate	2,064
Accumulated Depreciation and Amortization	$ 5
Date of Initial Leasehold or Acquisition Investment	2019
Accumulated Depreciation And Amortization Description [Member] | FLORIDA [Member] | Orlando One [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 867
Cost Capitalized Subsequent to Initial Investment	34
Land	401
Building and Improvements	500
Total Investment in Real Estate	901
Accumulated Depreciation and Amortization	$ 400
Date of Initial Leasehold or Acquisition Investment	2000
Accumulated Depreciation And Amortization Description [Member] | FLORIDA [Member] | Yulee [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 1,963
Land	570
Building and Improvements	1,393
Total Investment in Real Estate	1,963
Accumulated Depreciation and Amortization	$ 179
Date of Initial Leasehold or Acquisition Investment	2017
Accumulated Depreciation And Amortization Description [Member] | GEORGIA [Member] | Augusta One [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 3,150
Land	286
Building and Improvements	2,864
Total Investment in Real Estate	3,150
Accumulated Depreciation and Amortization	$ 371
Date of Initial Leasehold or Acquisition Investment	2017
Accumulated Depreciation And Amortization Description [Member] | GEORGIA [Member] | Augusta Two [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 1,843
Land	1,077
Building and Improvements	766
Total Investment in Real Estate	1,843
Accumulated Depreciation and Amortization	$ 19
Date of Initial Leasehold or Acquisition Investment	2019
Accumulated Depreciation And Amortization Description [Member] | GEORGIA [Member] | Columbus [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 1,617
Land	984
Building and Improvements	633
Total Investment in Real Estate	1,617
Accumulated Depreciation and Amortization	$ 22
Date of Initial Leasehold or Acquisition Investment	2019
Accumulated Depreciation And Amortization Description [Member] | GEORGIA [Member] | Hinesville [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 995
Land	245
Building and Improvements	750
Total Investment in Real Estate	995
Accumulated Depreciation and Amortization	$ 11
Date of Initial Leasehold or Acquisition Investment	2019
Accumulated Depreciation And Amortization Description [Member] | GEORGIA [Member] | Perry One [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 1,724
Land	1,312
Building and Improvements	412
Total Investment in Real Estate	1,724
Accumulated Depreciation and Amortization	$ 64
Date of Initial Leasehold or Acquisition Investment	2017
Accumulated Depreciation And Amortization Description [Member] | HAWAII [Member] | Haleiwa One [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 1,522
Land	1,058
Building and Improvements	464
Total Investment in Real Estate	1,522
Accumulated Depreciation and Amortization	$ 335
Date of Initial Leasehold or Acquisition Investment	2007
Accumulated Depreciation And Amortization Description [Member] | HAWAII [Member] | Honolulu One [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 1,539
Land	1,219
Building and Improvements	320
Total Investment in Real Estate	1,539
Accumulated Depreciation and Amortization	$ 194
Date of Initial Leasehold or Acquisition Investment	2007
Accumulated Depreciation And Amortization Description [Member] | HAWAII [Member] | Honolulu Two [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 1,769
Land	1,192
Building and Improvements	577
Total Investment in Real Estate	1,769
Accumulated Depreciation and Amortization	$ 331
Date of Initial Leasehold or Acquisition Investment	2007
Accumulated Depreciation And Amortization Description [Member] | HAWAII [Member] | Honolulu Three [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 1,071
Cost Capitalized Subsequent to Initial Investment	30
Land	981
Building and Improvements	120
Total Investment in Real Estate	1,101
Accumulated Depreciation and Amortization	$ 87
Date of Initial Leasehold or Acquisition Investment	2007
Accumulated Depreciation And Amortization Description [Member] | HAWAII [Member] | Honolulu Four [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 9,211
Land	8,194
Building and Improvements	1,017
Total Investment in Real Estate	9,211
Accumulated Depreciation and Amortization	$ 596
Date of Initial Leasehold or Acquisition Investment	2007
Accumulated Depreciation And Amortization Description [Member] | HAWAII [Member] | Kaneohe One [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 1,977
Cost Capitalized Subsequent to Initial Investment	176
Land	1,473
Building and Improvements	680
Total Investment in Real Estate	2,153
Accumulated Depreciation and Amortization	$ 392
Date of Initial Leasehold or Acquisition Investment	2007
Accumulated Depreciation And Amortization Description [Member] | HAWAII [Member] | Kaneohe Two [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 1,364
Land	822
Building and Improvements	542
Total Investment in Real Estate	1,364
Accumulated Depreciation and Amortization	$ 346
Date of Initial Leasehold or Acquisition Investment	2007
Accumulated Depreciation And Amortization Description [Member] | HAWAII [Member] | Waianae One [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 1,997
Land	871
Building and Improvements	1,126
Total Investment in Real Estate	1,997
Accumulated Depreciation and Amortization	$ 648
Date of Initial Leasehold or Acquisition Investment	2007
Accumulated Depreciation And Amortization Description [Member] | HAWAII [Member] | Waianae Two [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 1,520
Land	648
Building and Improvements	872
Total Investment in Real Estate	1,520
Accumulated Depreciation and Amortization	$ 500
Date of Initial Leasehold or Acquisition Investment	2007
Accumulated Depreciation And Amortization Description [Member] | HAWAII [Member] | Waipahu One [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 2,458
Land	945
Building and Improvements	1,513
Total Investment in Real Estate	2,458
Accumulated Depreciation and Amortization	$ 844
Date of Initial Leasehold or Acquisition Investment	2007
Accumulated Depreciation And Amortization Description [Member] | ILLINOIS [Member] | Prospect Heights [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 1,547
Land	698
Building and Improvements	849
Total Investment in Real Estate	1,547
Accumulated Depreciation and Amortization	$ 65
Date of Initial Leasehold or Acquisition Investment	2018
Accumulated Depreciation And Amortization Description [Member] | ILLINOIS [Member] | Roselle One [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 2,851
Land	1,741
Building and Improvements	1,110
Total Investment in Real Estate	2,851
Accumulated Depreciation and Amortization	$ 6
Date of Initial Leasehold or Acquisition Investment	2019
Accumulated Depreciation And Amortization Description [Member] | KENTUCKY [Member] | Louisville One [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 3,356
Land	818
Building and Improvements	2,538
Total Investment in Real Estate	3,356
Accumulated Depreciation and Amortization	$ 36
Date of Initial Leasehold or Acquisition Investment	2019
Accumulated Depreciation And Amortization Description [Member] | KENTUCKY [Member] | Owensboro One [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 3,810
Land	1,011
Building and Improvements	2,799
Total Investment in Real Estate	3,810
Accumulated Depreciation and Amortization	$ 105
Date of Initial Leasehold or Acquisition Investment	2019
Accumulated Depreciation And Amortization Description [Member] | LOUISIANA [Member] | Bossier City [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 2,181
Land	1,333
Building and Improvements	848
Total Investment in Real Estate	2,181
Accumulated Depreciation and Amortization	$ 124
Date of Initial Leasehold or Acquisition Investment	2017
Accumulated Depreciation And Amortization Description [Member] | MASSACHUSETTS [Member] | Arlington One [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 519
Cost Capitalized Subsequent to Initial Investment	27
Land	338
Building and Improvements	208
Total Investment in Real Estate	546
Accumulated Depreciation and Amortization	$ 186
Date of Initial Leasehold or Acquisition Investment	1985
Accumulated Depreciation And Amortization Description [Member] | MASSACHUSETTS [Member] | Auburn One [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 600
Land	600
Total Investment in Real Estate	$ 600
Date of Initial Leasehold or Acquisition Investment	2011
Accumulated Depreciation And Amortization Description [Member] | MASSACHUSETTS [Member] | Auburn Two [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 625
Land	625
Total Investment in Real Estate	$ 625
Date of Initial Leasehold or Acquisition Investment	2011
Accumulated Depreciation And Amortization Description [Member] | MASSACHUSETTS [Member] | Auburn Three [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 725
Land	725
Total Investment in Real Estate	$ 725
Date of Initial Leasehold or Acquisition Investment	2011
Accumulated Depreciation And Amortization Description [Member] | MASSACHUSETTS [Member] | Auburn Four [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 175
Cost Capitalized Subsequent to Initial Investment	244
Land	125
Building and Improvements	294
Total Investment in Real Estate	419
Accumulated Depreciation and Amortization	$ 179
Date of Initial Leasehold or Acquisition Investment	1986
Accumulated Depreciation And Amortization Description [Member] | MASSACHUSETTS [Member] | Auburn Five [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 369
Cost Capitalized Subsequent to Initial Investment	249
Land	240
Building and Improvements	378
Total Investment in Real Estate	618
Accumulated Depreciation and Amortization	$ 277
Date of Initial Leasehold or Acquisition Investment	1991
Accumulated Depreciation And Amortization Description [Member] | MASSACHUSETTS [Member] | Barre One [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 536
Cost Capitalized Subsequent to Initial Investment	12
Land	348
Building and Improvements	200
Total Investment in Real Estate	548
Accumulated Depreciation and Amortization	$ 136
Date of Initial Leasehold or Acquisition Investment	1991
Accumulated Depreciation And Amortization Description [Member] | MASSACHUSETTS [Member] | Bedford One [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 1,350
Land	1,350
Total Investment in Real Estate	$ 1,350
Date of Initial Leasehold or Acquisition Investment	2011
Accumulated Depreciation And Amortization Description [Member] | MASSACHUSETTS [Member] | Bellingham One [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 734
Cost Capitalized Subsequent to Initial Investment	73
Land	476
Building and Improvements	331
Total Investment in Real Estate	807
Accumulated Depreciation and Amortization	$ 299
Date of Initial Leasehold or Acquisition Investment	1985
Accumulated Depreciation And Amortization Description [Member] | MASSACHUSETTS [Member] | Bellingham Two [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 3,961
Land	2,042
Building and Improvements	1,919
Total Investment in Real Estate	3,961
Accumulated Depreciation and Amortization	$ 9
Date of Initial Leasehold or Acquisition Investment	2019
Accumulated Depreciation And Amortization Description [Member] | MASSACHUSETTS [Member] | Belmont One [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 390
Cost Capitalized Subsequent to Initial Investment	29
Land	254
Building and Improvements	165
Total Investment in Real Estate	419
Accumulated Depreciation and Amortization	$ 148
Date of Initial Leasehold or Acquisition Investment	1985
Accumulated Depreciation And Amortization Description [Member] | MASSACHUSETTS [Member] | Bradford One [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 650
Land	650
Total Investment in Real Estate	$ 650
Date of Initial Leasehold or Acquisition Investment	2011
Accumulated Depreciation And Amortization Description [Member] | MASSACHUSETTS [Member] | Burlington One [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 600
Land	600
Total Investment in Real Estate	$ 600
Date of Initial Leasehold or Acquisition Investment	2011
Accumulated Depreciation And Amortization Description [Member] | MASSACHUSETTS [Member] | Burlington Two [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 1,250
Land	1,250
Total Investment in Real Estate	$ 1,250
Date of Initial Leasehold or Acquisition Investment	2011
Accumulated Depreciation And Amortization Description [Member] | MASSACHUSETTS [Member] | Dracut One [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 450
Land	450
Total Investment in Real Estate	$ 450
Date of Initial Leasehold or Acquisition Investment	2011
Accumulated Depreciation And Amortization Description [Member] | MASSACHUSETTS [Member] | Falmouth One [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 414
Cost Capitalized Subsequent to Initial Investment	2,371
Land	458
Building and Improvements	2,327
Total Investment in Real Estate	2,785
Accumulated Depreciation and Amortization	$ 204
Date of Initial Leasehold or Acquisition Investment	1988
Accumulated Depreciation And Amortization Description [Member] | MASSACHUSETTS [Member] | Fitchburg One [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 390
Cost Capitalized Subsequent to Initial Investment	33
Land	254
Building and Improvements	169
Total Investment in Real Estate	423
Accumulated Depreciation and Amortization	$ 124
Date of Initial Leasehold or Acquisition Investment	1992
Accumulated Depreciation And Amortization Description [Member] | MASSACHUSETTS [Member] | Foxborough One [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 427
Cost Capitalized Subsequent to Initial Investment	98
Land	325
Building and Improvements	200
Total Investment in Real Estate	525
Accumulated Depreciation and Amortization	$ 159
Date of Initial Leasehold or Acquisition Investment	1990
Accumulated Depreciation And Amortization Description [Member] | MASSACHUSETTS [Member] | Framingham One [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 400
Cost Capitalized Subsequent to Initial Investment	23
Land	260
Building and Improvements	163
Total Investment in Real Estate	423
Accumulated Depreciation and Amortization	$ 116
Date of Initial Leasehold or Acquisition Investment	1991
Accumulated Depreciation And Amortization Description [Member] | MASSACHUSETTS [Member] | Gardner One [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 550
Land	550
Total Investment in Real Estate	$ 550
Date of Initial Leasehold or Acquisition Investment	2011
Accumulated Depreciation And Amortization Description [Member] | MASSACHUSETTS [Member] | Gardner Two [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 787
Land	638
Building and Improvements	149
Total Investment in Real Estate	787
Accumulated Depreciation and Amortization	$ 51
Date of Initial Leasehold or Acquisition Investment	2014