Schedule III - Real Estate and Accumulated Depreciation and Amortization Part 8 (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
MASSACHUSETTS [Member] | Accumulated Depreciation And Amortization Description [Member] | Chelmsford One [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	715
Building and Improvements	715
Total Investment in Real Estate	715
Accumulated Depreciation and Amortization	$ 325
Date of Initial Leasehold or Acquisition Investment	2012
MASSACHUSETTS [Member] | Accumulated Depreciation And Amortization Description [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
MASSACHUSETTS [Member] | Accumulated Depreciation And Amortization Description [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	2,320
Land	458
Building and Improvements	2,276
Total Investment in Real Estate	2,734
Accumulated Depreciation and Amortization	$ 64
Date of Initial Leasehold or Acquisition Investment	1988
MASSACHUSETTS [Member] | Accumulated Depreciation And Amortization Description [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	$ 118
Date of Initial Leasehold or Acquisition Investment	1992
MASSACHUSETTS [Member] | Accumulated Depreciation And Amortization Description [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	$ 151
Date of Initial Leasehold or Acquisition Investment	1990
MASSACHUSETTS [Member] | Accumulated Depreciation And Amortization Description [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	$ 111
Date of Initial Leasehold or Acquisition Investment	1991
MASSACHUSETTS [Member] | Accumulated Depreciation And Amortization Description [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
MASSACHUSETTS [Member] | Accumulated Depreciation And Amortization Description [Member] | Gardner Two [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 1,009
Cost Capitalized Subsequent to Initial Investment	327
Land	657
Building and Improvements	679
Total Investment in Real Estate	1,336
Accumulated Depreciation and Amortization	$ 494
Date of Initial Leasehold or Acquisition Investment	1985
MASSACHUSETTS [Member] | Accumulated Depreciation And Amortization Description [Member] | Gardners One [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	$ 42
Date of Initial Leasehold or Acquisition Investment	2014
MASSACHUSETTS [Member] | Accumulated Depreciation And Amortization Description [Member] | Hingham One [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 353
Cost Capitalized Subsequent to Initial Investment	111
Land	243
Building and Improvements	221
Total Investment in Real Estate	464
Accumulated Depreciation and Amortization	$ 169
Date of Initial Leasehold or Acquisition Investment	1989
MASSACHUSETTS [Member] | Accumulated Depreciation And Amortization Description [Member] | Hyde Park One [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 500
Cost Capitalized Subsequent to Initial Investment	168
Land	322
Building and Improvements	346
Total Investment in Real Estate	668
Accumulated Depreciation and Amortization	$ 235
Date of Initial Leasehold or Acquisition Investment	1985
MASSACHUSETTS [Member] | Accumulated Depreciation And Amortization Description [Member] | Leominster One [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 571
Land	199
Building and Improvements	372
Total Investment in Real Estate	571
Accumulated Depreciation and Amortization	$ 131
Date of Initial Leasehold or Acquisition Investment	2012
MASSACHUSETTS [Member] | Accumulated Depreciation And Amortization Description [Member] | Littleton One [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 1,357
Land	759
Building and Improvements	598
Total Investment in Real Estate	1,357
Accumulated Depreciation and Amortization	$ 45
Date of Initial Leasehold or Acquisition Investment	2017
MASSACHUSETTS [Member] | Accumulated Depreciation And Amortization Description [Member] | Lowell Four [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 361
Cost Capitalized Subsequent to Initial Investment	90
Land	201
Building and Improvements	250
Total Investment in Real Estate	451
Accumulated Depreciation and Amortization	$ 247
Date of Initial Leasehold or Acquisition Investment	1985