Schedule III - Real Estate and Accumulated Depreciation and Amortization Part 9 (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Schedule Of Investments In Real Estate [Line Items]
Total Investment in Real Estate	$ 782,166	$ 783,233	$ 595,959	$ 570,275
Accumulated Depreciation And Amortization Description [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	720,099
Cost Capitalized Subsequent to Initial Investment	62,067
Land	474,232
Building and Improvements	307,934
Total Investment in Real Estate	782,166
Accumulated Depreciation and Amortization	120,576
MASSACHUSETTS | 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	$ 231
Date of Initial Leasehold or Acquisition Investment	2012
MASSACHUSETTS | Accumulated Depreciation And Amortization Description [Member] | Danvers One [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 400
Land	400
Total Investment in Real Estate	$ 400
Date of Initial Leasehold or Acquisition Investment	2011
MASSACHUSETTS | 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 | 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	$ 519
Cost Capitalized Subsequent to Initial Investment	127
Land	458
Building and Improvements	188
Total Investment in Real Estate	646
Accumulated Depreciation and Amortization	$ 121
Date of Initial Leasehold or Acquisition Investment	1988
MASSACHUSETTS | 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	$ 107
Date of Initial Leasehold or Acquisition Investment	1992
MASSACHUSETTS | 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	$ 135
Date of Initial Leasehold or Acquisition Investment	1990
MASSACHUSETTS | 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	$ 100
Date of Initial Leasehold or Acquisition Investment	1991
MASSACHUSETTS | 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 | 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	$ 1,008
Cost Capitalized Subsequent to Initial Investment	282
Land	657
Building and Improvements	633
Total Investment in Real Estate	1,290
Accumulated Depreciation and Amortization	$ 416
Date of Initial Leasehold or Acquisition Investment	1985
MASSACHUSETTS | 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	$ 23
Date of Initial Leasehold or Acquisition Investment	2014
MASSACHUSETTS | 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	$ 152
Date of Initial Leasehold or Acquisition Investment	1989
MASSACHUSETTS | 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	$ 499
Cost Capitalized Subsequent to Initial Investment	157
Land	322
Building and Improvements	334
Total Investment in Real Estate	656
Accumulated Depreciation and Amortization	$ 194
Date of Initial Leasehold or Acquisition Investment	1985
MASSACHUSETTS | 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	$ 89
Date of Initial Leasehold or Acquisition Investment	2012
MASSACHUSETTS | Accumulated Depreciation And Amortization Description [Member] | Lowell One [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 375
Cost Capitalized Subsequent to Initial Investment	9
Land	250
Building and Improvements	134
Total Investment in Real Estate	384
Accumulated Depreciation and Amortization	$ 134
Date of Initial Leasehold or Acquisition Investment	1986
MASSACHUSETTS | Accumulated Depreciation And Amortization Description [Member] | Lowell Two [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	$ 246
Date of Initial Leasehold or Acquisition Investment	1985
MASSACHUSETTS | Accumulated Depreciation And Amortization Description [Member] | Lowell Three [Member]
Schedule Of Investments In Real Estate [Line Items]
Cost Capitalized Subsequent to Initial Investment	$ 619
Land	429
Building and Improvements	190
Total Investment in Real Estate	619
Accumulated Depreciation and Amortization	$ 37
Date of Initial Leasehold or Acquisition Investment	1996
MASSACHUSETTS | Accumulated Depreciation And Amortization Description [Member] | Lynn One [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 400
Land	400
Total Investment in Real Estate	$ 400
Date of Initial Leasehold or Acquisition Investment	2011
MASSACHUSETTS | Accumulated Depreciation And Amortization Description [Member] | Lynn Two [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 850
Land	850
Total Investment in Real Estate	$ 850
Date of Initial Leasehold or Acquisition Investment	2011
MASSACHUSETTS | Accumulated Depreciation And Amortization Description [Member] | Marlborough 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 | Accumulated Depreciation And Amortization Description [Member] | Maynard One [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 736
Cost Capitalized Subsequent to Initial Investment	98
Land	479
Building and Improvements	355
Total Investment in Real Estate	834
Accumulated Depreciation and Amortization	$ 219
Date of Initial Leasehold or Acquisition Investment	1985
MASSACHUSETTS | Accumulated Depreciation And Amortization Description [Member] | Melrose 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 | Accumulated Depreciation And Amortization Description [Member] | Methuen One [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 300
Cost Capitalized Subsequent to Initial Investment	134
Land	150
Building and Improvements	284
Total Investment in Real Estate	434
Accumulated Depreciation and Amortization	$ 217
Date of Initial Leasehold or Acquisition Investment	1986
MASSACHUSETTS | Accumulated Depreciation And Amortization Description [Member] | Methuen Two [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 380
Cost Capitalized Subsequent to Initial Investment	64
Land	246
Building and Improvements	198
Total Investment in Real Estate	444
Accumulated Depreciation and Amortization	$ 166
Date of Initial Leasehold or Acquisition Investment	1985
MASSACHUSETTS | Accumulated Depreciation And Amortization Description [Member] | Methuen Three [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 491
Cost Capitalized Subsequent to Initial Investment	97
Land	319
Building and Improvements	269
Total Investment in Real Estate	588
Accumulated Depreciation and Amortization	$ 162
Date of Initial Leasehold or Acquisition Investment	1985
MASSACHUSETTS | Accumulated Depreciation And Amortization Description [Member] | Methuen Four [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 | Accumulated Depreciation And Amortization Description [Member] | Newton One [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 691
Cost Capitalized Subsequent to Initial Investment	124
Land	450
Building and Improvements	365
Total Investment in Real Estate	815
Accumulated Depreciation and Amortization	$ 268
Date of Initial Leasehold or Acquisition Investment	1985
MASSACHUSETTS | Accumulated Depreciation And Amortization Description [Member] | North Andover One [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 394
Cost Capitalized Subsequent to Initial Investment	32
Land	256
Building and Improvements	170
Total Investment in Real Estate	426
Accumulated Depreciation and Amortization	$ 137
Date of Initial Leasehold or Acquisition Investment	1985