Schedule III - Real Estate and Accumulated Depreciation and Amortization Part 7 (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Schedule Of Investments In Real Estate [Line Items]
Total Investment in Real Estate	$ 1,043,106	$ 970,964	$ 782,166	$ 783,233
FLORIDA [Member] | Accumulated Depreciation And Amortization Description [Member] | Nassau Two [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	$ 71
Date of Initial Leasehold or Acquisition Investment	2017
FLORIDA [Member] | Accumulated Depreciation And Amortization Description [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	$ 378
Date of Initial Leasehold or Acquisition Investment	2000
MASSACHUSETTS [Member] | Accumulated Depreciation And Amortization Description [Member] | Auburn Two [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
MASSACHUSETTS [Member] | Accumulated Depreciation And Amortization Description [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	$ 127
Date of Initial Leasehold or Acquisition Investment	1991
MASSACHUSETTS [Member] | Accumulated Depreciation And Amortization Description [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
MASSACHUSETTS [Member] | Accumulated Depreciation And Amortization Description [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	$ 289
Date of Initial Leasehold or Acquisition Investment	1985
MASSACHUSETTS [Member] | Accumulated Depreciation And Amortization Description [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	$ 143
Date of Initial Leasehold or Acquisition Investment	1985
MASSACHUSETTS [Member] | Accumulated Depreciation And Amortization Description [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
MASSACHUSETTS [Member] | Accumulated Depreciation And Amortization Description [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
MASSACHUSETTS [Member] | Accumulated Depreciation And Amortization Description [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
MASSACHUSETTS [Member] | Accumulated Depreciation And Amortization Description [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	$ 178
Date of Initial Leasehold or Acquisition Investment	1985
MASSACHUSETTS [Member] | Accumulated Depreciation And Amortization Description [Member] | Auburn One [Member]
Schedule Of Investments In Real Estate [Line Items]
Cost Capitalized Subsequent to Initial Investment	$ 535
Land	388
Building and Improvements	147
Total Investment in Real Estate	535
Accumulated Depreciation and Amortization	$ 51
Date of Initial Leasehold or Acquisition Investment	1996
MASSACHUSETTS [Member] | Accumulated Depreciation And Amortization Description [Member] | Auburn Three [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
MASSACHUSETTS [Member] | Accumulated Depreciation And Amortization Description [Member] | Auburn Four [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 370
Cost Capitalized Subsequent to Initial Investment	305
Land	240
Building and Improvements	435
Total Investment in Real Estate	675
Accumulated Depreciation and Amortization	$ 245
Date of Initial Leasehold or Acquisition Investment	1991
MASSACHUSETTS [Member] | Accumulated Depreciation And Amortization Description [Member] | Auburn Five [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
MASSACHUSETTS [Member] | Accumulated Depreciation And Amortization Description [Member] | Auburn Six [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 800
Building and Improvements	800
Total Investment in Real Estate	800
Accumulated Depreciation and Amortization	$ 605
Date of Initial Leasehold or Acquisition Investment	2011
GEORGIA [Member] | Accumulated Depreciation And Amortization Description [Member] | Houston [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	$ 37
Date of Initial Leasehold or Acquisition Investment	2017
GEORGIA [Member] | Accumulated Depreciation And Amortization Description [Member] | Richmond [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	$ 236
Date of Initial Leasehold or Acquisition Investment	2017
HAWAII | Accumulated Depreciation And Amortization Description [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	$ 325
Date of Initial Leasehold or Acquisition Investment	2007
HAWAII | Accumulated Depreciation And Amortization Description [Member] | Honolulu One [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	$ 82
Date of Initial Leasehold or Acquisition Investment	2007
HAWAII | Accumulated Depreciation And Amortization Description [Member] | Honolulu Two [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	$ 184
Date of Initial Leasehold or Acquisition Investment	2007
HAWAII | Accumulated Depreciation And Amortization Description [Member] | Honolulu Three [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	$ 310
Date of Initial Leasehold or Acquisition Investment	2007
HAWAII | Accumulated Depreciation And Amortization Description [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	$ 562
Date of Initial Leasehold or Acquisition Investment	2007
HAWAII | Accumulated Depreciation And Amortization Description [Member] | Kaneohe One [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	$ 329
Date of Initial Leasehold or Acquisition Investment	2007
HAWAII | Accumulated Depreciation And Amortization Description [Member] | Kaneohe Two [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	165
Land	1,473
Building and Improvements	669
Total Investment in Real Estate	2,142
Accumulated Depreciation and Amortization	$ 360
Date of Initial Leasehold or Acquisition Investment	2007
HAWAII | Accumulated Depreciation And Amortization Description [Member] | Waianae One [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	$ 469
Date of Initial Leasehold or Acquisition Investment	2007
HAWAII | Accumulated Depreciation And Amortization Description [Member] | Waianae Two [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	$ 609
Date of Initial Leasehold or Acquisition Investment	2007
HAWAII | Accumulated Depreciation And Amortization Description [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	$ 789
Date of Initial Leasehold or Acquisition Investment	2007
ILLINOIS [Member] | Accumulated Depreciation And Amortization Description [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	$ 28
Date of Initial Leasehold or Acquisition Investment	2018
LOUISIANA [Member] | Accumulated Depreciation And Amortization Description [Member] | Bossier [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	$ 71
Date of Initial Leasehold or Acquisition Investment	2017
LOUISIANA [Member] | Accumulated Depreciation And Amortization Description [Member] | Lake Charles One [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 1,069
Land	620
Building and Improvements	449
Total Investment in Real Estate	1,069
Accumulated Depreciation and Amortization	$ 18
Date of Initial Leasehold or Acquisition Investment	2018
LOUISIANA [Member] | Accumulated Depreciation And Amortization Description [Member] | Lake Charles Two [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 1,468
Land	1,002
Building and Improvements	466
Total Investment in Real Estate	1,468
Accumulated Depreciation and Amortization	$ 17
Date of Initial Leasehold or Acquisition Investment	2018
LOUISIANA [Member] | Accumulated Depreciation And Amortization Description [Member] | Sulphur [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 777
Land	375
Building and Improvements	402
Total Investment in Real Estate	777
Accumulated Depreciation and Amortization	$ 18
Date of Initial Leasehold or Acquisition Investment	2018