Schedule III - Real Estate and Accumulated Depreciation and Amortization Part 13 (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Schedule Of Investments In Real Estate [Line Items]
Total Investment in Real Estate	$ 970,964	$ 782,166	$ 783,233	$ 595,959
Accumulated Depreciation And Amortization Description [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	914,488
Cost Capitalized Subsequent to Initial Investment	56,476
Land	589,497
Building and Improvements	381,467
Total Investment in Real Estate	970,964
Accumulated Depreciation and Amortization	133,353
NORTH CAROLINA [Member] | Accumulated Depreciation And Amortization Description [Member] | New Bern One [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	350
Cost Capitalized Subsequent to Initial Investment	83
Land	190
Building and Improvements	243
Total Investment in Real Estate	433
Accumulated Depreciation and Amortization	$ 156
Date of Initial Leasehold or Acquisition Investment	2007
NORTH DAKOTA [Member] | Accumulated Depreciation And Amortization Description [Member] | Belfield One [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 1,232
Land	382
Building and Improvements	850
Total Investment in Real Estate	1,232
Accumulated Depreciation and Amortization	$ 745
Date of Initial Leasehold or Acquisition Investment	2007
NEW HAMPSHIRE [Member] | Accumulated Depreciation And Amortization Description [Member] | Allenstown One [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 1,787
Land	467
Building and Improvements	1,320
Total Investment in Real Estate	1,787
Accumulated Depreciation and Amortization	$ 717
Date of Initial Leasehold or Acquisition Investment	2007
NEW HAMPSHIRE [Member] | Accumulated Depreciation And Amortization Description [Member] | Concord One [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 675
Land	675
Total Investment in Real Estate	$ 675
Date of Initial Leasehold or Acquisition Investment	2011
NEW HAMPSHIRE [Member] | Accumulated Depreciation And Amortization Description [Member] | Concord Two [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 900
Land	900
Total Investment in Real Estate	$ 900
Date of Initial Leasehold or Acquisition Investment	2011
NEW HAMPSHIRE [Member] | Accumulated Depreciation And Amortization Description [Member] | Derry One [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 418
Cost Capitalized Subsequent to Initial Investment	16
Land	158
Building and Improvements	276
Total Investment in Real Estate	434
Accumulated Depreciation and Amortization	$ 276
Date of Initial Leasehold or Acquisition Investment	1987
NEW HAMPSHIRE [Member] | Accumulated Depreciation And Amortization Description [Member] | Derry Two [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 950
Land	950
Total Investment in Real Estate	$ 950
Date of Initial Leasehold or Acquisition Investment	2011
NEW HAMPSHIRE [Member] | Accumulated Depreciation And Amortization Description [Member] | Nashua Six [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 1,750
Land	1,750
Total Investment in Real Estate	$ 1,750
Date of Initial Leasehold or Acquisition Investment	2011
NEW HAMPSHIRE [Member] | Accumulated Depreciation And Amortization Description [Member] | Derry 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
NEW HAMPSHIRE [Member] | Accumulated Depreciation And Amortization Description [Member] | Northwood One [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 500
Land	500
Total Investment in Real Estate	$ 500
Date of Initial Leasehold or Acquisition Investment	2011
NEW HAMPSHIRE [Member] | Accumulated Depreciation And Amortization Description [Member] | Derry Two [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 1,200
Land	1,200
Total Investment in Real Estate	$ 1,200
Date of Initial Leasehold or Acquisition Investment	2011
NEW HAMPSHIRE [Member] | Accumulated Depreciation And Amortization Description [Member] | Pelham One [Member]
Schedule Of Investments In Real Estate [Line Items]
Cost Capitalized Subsequent to Initial Investment	$ 730
Land	317
Building and Improvements	413
Total Investment in Real Estate	730
Accumulated Depreciation and Amortization	$ 95
Date of Initial Leasehold or Acquisition Investment	1996
NEW HAMPSHIRE [Member] | Accumulated Depreciation And Amortization Description [Member] | Goffstown One [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 1,737
Land	697
Building and Improvements	1,040
Total Investment in Real Estate	1,737
Accumulated Depreciation and Amortization	$ 392
Date of Initial Leasehold or Acquisition Investment	2012
NEW HAMPSHIRE [Member] | Accumulated Depreciation And Amortization Description [Member] | Plaistow One [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 301
Cost Capitalized Subsequent to Initial Investment	100
Land	245
Building and Improvements	156
Total Investment in Real Estate	401
Accumulated Depreciation and Amortization	$ 156
Date of Initial Leasehold or Acquisition Investment	1987
NEW HAMPSHIRE [Member] | Accumulated Depreciation And Amortization Description [Member] | Hooksett One [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 1,562
Land	824
Building and Improvements	738
Total Investment in Real Estate	1,562
Accumulated Depreciation and Amortization	$ 666
Date of Initial Leasehold or Acquisition Investment	2007
NEW HAMPSHIRE [Member] | Accumulated Depreciation And Amortization Description [Member] | Portsmouth One [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 525
Land	525
Total Investment in Real Estate	$ 525
Date of Initial Leasehold or Acquisition Investment	2011
NEW HAMPSHIRE [Member] | Accumulated Depreciation And Amortization Description [Member] | Kingston One [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 1,500
Land	1,500
Total Investment in Real Estate	$ 1,500
Date of Initial Leasehold or Acquisition Investment	2011
NEW HAMPSHIRE [Member] | Accumulated Depreciation And Amortization Description [Member] | Raymond 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
NEW HAMPSHIRE [Member] | Accumulated Depreciation And Amortization Description [Member] | Londonderry One [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 703
Cost Capitalized Subsequent to Initial Investment	30
Land	458
Building and Improvements	275
Total Investment in Real Estate	733
Accumulated Depreciation and Amortization	$ 225
Date of Initial Leasehold or Acquisition Investment	1985
NEW HAMPSHIRE [Member] | Accumulated Depreciation And Amortization Description [Member] | Rochester One [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 700
Land	700
Total Investment in Real Estate	$ 700
Date of Initial Leasehold or Acquisition Investment	2011
NEW HAMPSHIRE [Member] | Accumulated Depreciation And Amortization Description [Member] | Londonderry Two [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 1,100
Land	1,100
Total Investment in Real Estate	$ 1,100
Date of Initial Leasehold or Acquisition Investment	2011
NEW HAMPSHIRE [Member] | Accumulated Depreciation And Amortization Description [Member] | Rochester Two [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 939
Cost Capitalized Subsequent to Initial Investment	12
Land	600
Building and Improvements	351
Total Investment in Real Estate	951
Accumulated Depreciation and Amortization	$ 282
Date of Initial Leasehold or Acquisition Investment	1985
NEW HAMPSHIRE [Member] | Accumulated Depreciation And Amortization Description [Member] | Manchester 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
NEW HAMPSHIRE [Member] | Accumulated Depreciation And Amortization Description [Member] | Rochester Three [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 1,400
Land	1,400
Total Investment in Real Estate	$ 1,400
Date of Initial Leasehold or Acquisition Investment	2011
NEW HAMPSHIRE [Member] | Accumulated Depreciation And Amortization Description [Member] | Nashua One [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 500
Land	500
Total Investment in Real Estate	$ 500
Date of Initial Leasehold or Acquisition Investment	2011
NEW HAMPSHIRE [Member] | Accumulated Depreciation And Amortization Description [Member] | Rochester Four [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 1,600
Land	1,600
Total Investment in Real Estate	$ 1,600
Date of Initial Leasehold or Acquisition Investment	2011
NEW HAMPSHIRE [Member] | Accumulated Depreciation And Amortization Description [Member] | Nashua Two [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
NEW HAMPSHIRE [Member] | Accumulated Depreciation And Amortization Description [Member] | Nashua Three [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 750
Land	750
Total Investment in Real Estate	$ 750
Date of Initial Leasehold or Acquisition Investment	2011
NEW HAMPSHIRE [Member] | Accumulated Depreciation And Amortization Description [Member] | Nashua Four [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 825
Land	825
Total Investment in Real Estate	$ 825
Date of Initial Leasehold or Acquisition Investment	2011
NEW HAMPSHIRE [Member] | Accumulated Depreciation And Amortization Description [Member] | Nashua Five [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 1,132
Land	780
Building and Improvements	352
Total Investment in Real Estate	1,132
Accumulated Depreciation and Amortization	$ 10
Date of Initial Leasehold or Acquisition Investment	2017