Schedule III - Real Estate and Accumulated Depreciation and Amortization Part 10 (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
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	623
Land	429
Building and Improvements	194
Total Investment in Real Estate	623
Accumulated Depreciation and Amortization	$ 52
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	$ 100
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	58
Land	464
Building and Improvements	308
Total Investment in Real Estate	772
Accumulated Depreciation and Amortization	$ 205
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	$ 239
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	508
Land	75
Building and Improvements	558
Total Investment in Real Estate	633
Accumulated Depreciation and Amortization	$ 218
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	$ 226
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	$ 139
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	$ 171
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	$ 177
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	$ 160
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	210
Land	321
Building and Improvements	247
Total Investment in Real Estate	568
Accumulated Depreciation and Amortization	$ 156
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	103
Land	450
Building and Improvements	344
Total Investment in Real Estate	794
Accumulated Depreciation and Amortization	$ 287
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	641
Land	659
Building and Improvements	994
Total Investment in Real Estate	1,653
Accumulated Depreciation and Amortization	$ 544
Date of Initial Leasehold or Acquisition Investment	1985
MASSACHUSETTS [Member] | 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	$ 393
Cost Capitalized Subsequent to Initial Investment	33
Land	256
Building and Improvements	170
Total Investment in Real Estate	426
Accumulated Depreciation and Amortization	$ 142
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	87
Land	319
Building and Improvements	258
Total Investment in Real Estate	577
Accumulated Depreciation and Amortization	$ 196
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	$ 574
Cost Capitalized Subsequent to Initial Investment	209
Land	430
Building and Improvements	353
Total Investment in Real Estate	783
Accumulated Depreciation and Amortization	$ 247
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	$ 206
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	$ 45
Date of Initial Leasehold or Acquisition Investment	1985