Schedule III - Real Estate and Accumulated Depreciation and Amortization Part 9 (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] | Lowell Two [Member]
Schedule Of Investments In Real Estate [Line Items]
Cost Capitalized Subsequent to Initial Investment	631
Land	429
Building and Improvements	202
Total Investment in Real Estate	631
Accumulated Depreciation and Amortization	$ 69
Date of Initial Leasehold or Acquisition Investment	1996
MASSACHUSETTS [Member] | Accumulated Depreciation And Amortization Description [Member] | Shrewsbury 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 [Member] | 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 [Member] | Accumulated Depreciation And Amortization Description [Member] | Shrewsbury Two [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] | 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 [Member] | Accumulated Depreciation And Amortization Description [Member] | Sterling One [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 476
Cost Capitalized Subsequent to Initial Investment	2
Land	309
Building and Improvements	169
Total Investment in Real Estate	478
Accumulated Depreciation and Amortization	$ 107
Date of Initial Leasehold or Acquisition Investment	1991
MASSACHUSETTS [Member] | 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 [Member] | Accumulated Depreciation And Amortization Description [Member] | Sutton One [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 714
Cost Capitalized Subsequent to Initial Investment	57
Land	464
Building and Improvements	307
Total Investment in Real Estate	771
Accumulated Depreciation and Amortization	$ 214
Date of Initial Leasehold or Acquisition Investment	1993
MASSACHUSETTS [Member] | 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	$ 258
Date of Initial Leasehold or Acquisition Investment	1985
MASSACHUSETTS [Member] | Accumulated Depreciation And Amortization Description [Member] | Tewksbury One [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 125
Cost Capitalized Subsequent to Initial Investment	545
Land	75
Building and Improvements	595
Total Investment in Real Estate	670
Accumulated Depreciation and Amortization	$ 267
Date of Initial Leasehold or Acquisition Investment	1986
MASSACHUSETTS [Member] | 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 [Member] | Accumulated Depreciation And Amortization Description [Member] | Tewksbury 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
MASSACHUSETTS [Member] | 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	$ 234
Date of Initial Leasehold or Acquisition Investment	1986
MASSACHUSETTS [Member] | Accumulated Depreciation And Amortization Description [Member] | Upton One [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 429
Cost Capitalized Subsequent to Initial Investment	114
Land	279
Building and Improvements	264
Total Investment in Real Estate	543
Accumulated Depreciation and Amortization	$ 154
Date of Initial Leasehold or Acquisition Investment	1991
MASSACHUSETTS [Member] | 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	$ 176
Date of Initial Leasehold or Acquisition Investment	1985
MASSACHUSETTS [Member] | Accumulated Depreciation And Amortization Description [Member] | Wakefield One [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
MASSACHUSETTS [Member] | 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	$ 490
Cost Capitalized Subsequent to Initial Investment	98
Land	319
Building and Improvements	269
Total Investment in Real Estate	588
Accumulated Depreciation and Amortization	$ 192
Date of Initial Leasehold or Acquisition Investment	1985
MASSACHUSETTS [Member] | Accumulated Depreciation And Amortization Description [Member] | Walpole One [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 450
Cost Capitalized Subsequent to Initial Investment	92
Land	293
Building and Improvements	249
Total Investment in Real Estate	542
Accumulated Depreciation and Amortization	$ 174
Date of Initial Leasehold or Acquisition Investment	1985
MASSACHUSETTS [Member] | 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 [Member] | Accumulated Depreciation And Amortization Description [Member] | Watertown One [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 358
Cost Capitalized Subsequent to Initial Investment	209
Land	321
Building and Improvements	246
Total Investment in Real Estate	567
Accumulated Depreciation and Amortization	$ 171
Date of Initial Leasehold or Acquisition Investment	1985
MASSACHUSETTS [Member] | 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	98
Land	450
Building and Improvements	339
Total Investment in Real Estate	789
Accumulated Depreciation and Amortization	$ 300
Date of Initial Leasehold or Acquisition Investment	1985
MASSACHUSETTS [Member] | Accumulated Depreciation And Amortization Description [Member] | Webster One [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 1,012
Cost Capitalized Subsequent to Initial Investment	711
Land	659
Building and Improvements	1,064
Total Investment in Real Estate	1,723
Accumulated Depreciation and Amortization	$ 622
Date of Initial Leasehold or Acquisition Investment	1985
MASSACHUSETTS [Member] | Accumulated Depreciation And Amortization Description [Member] | Peabody 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	18
Land	252
Building and Improvements	166
Total Investment in Real Estate	418
Accumulated Depreciation and Amortization	$ 166
Date of Initial Leasehold or Acquisition Investment	1986
MASSACHUSETTS [Member] | Accumulated Depreciation And Amortization Description [Member] | West Roxbury One [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 490
Cost Capitalized Subsequent to Initial Investment	105
Land	319
Building and Improvements	276
Total Investment in Real Estate	595
Accumulated Depreciation and Amortization	$ 212
Date of Initial Leasehold or Acquisition Investment	1985
MASSACHUSETTS [Member] | Accumulated Depreciation And Amortization Description [Member] | Peabody 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
MASSACHUSETTS [Member] | Accumulated Depreciation And Amortization Description [Member] | Westborough 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] | Peabody Three [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] | Randolph One [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 573
Cost Capitalized Subsequent to Initial Investment	238
Land	430
Building and Improvements	381
Total Investment in Real Estate	811
Accumulated Depreciation and Amortization	$ 267
Date of Initial Leasehold or Acquisition Investment	1985
MASSACHUSETTS [Member] | Accumulated Depreciation And Amortization Description [Member] | Revere One [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 1,300
Land	1,300
Total Investment in Real Estate	$ 1,300
Date of Initial Leasehold or Acquisition Investment	2011
MASSACHUSETTS [Member] | Accumulated Depreciation And Amortization Description [Member] | Wilmington 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] | Rockland One [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 579
Cost Capitalized Subsequent to Initial Investment	45
Land	377
Building and Improvements	247
Total Investment in Real Estate	624
Accumulated Depreciation and Amortization	$ 214
Date of Initial Leasehold or Acquisition Investment	1985
MASSACHUSETTS [Member] | Accumulated Depreciation And Amortization Description [Member] | Wilmington Two [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 1,300
Land	1,300
Total Investment in Real Estate	$ 1,300
Date of Initial Leasehold or Acquisition Investment	2011
MASSACHUSETTS [Member] | Accumulated Depreciation And Amortization Description [Member] | Salem 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] | Seekonk One [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 1,073
Cost Capitalized Subsequent to Initial Investment	(373)
Land	576
Building and Improvements	124
Total Investment in Real Estate	700
Accumulated Depreciation and Amortization	$ 59
Date of Initial Leasehold or Acquisition Investment	1985