Schedule III - Real Estate and Accumulated Depreciation and Amortization Part 5 (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Schedule Of Investments In Real Estate [Line Items]
Total Investment in Real Estate	$ 1,113,651	$ 1,043,106	$ 970,964	$ 782,166
Accumulated Depreciation And Amortization Description [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	1,049,787
Cost Capitalized Subsequent to Initial Investment	63,864
Land	669,351
Building and Improvements	444,300
Total Investment in Real Estate	1,113,651
Accumulated Depreciation and Amortization	165,892
Accumulated Depreciation And Amortization Description [Member] | MASSACHUSETTS [Member] | Gardner Three [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	364
Land	657
Building and Improvements	716
Total Investment in Real Estate	1,373
Accumulated Depreciation and Amortization	$ 536
Date of Initial Leasehold or Acquisition Investment	1985
Accumulated Depreciation And Amortization Description [Member] | MASSACHUSETTS [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	$ 178
Date of Initial Leasehold or Acquisition Investment	1989
Accumulated Depreciation And Amortization Description [Member] | MASSACHUSETTS [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	174
Land	322
Building and Improvements	352
Total Investment in Real Estate	674
Accumulated Depreciation and Amortization	$ 257
Date of Initial Leasehold or Acquisition Investment	1985
Accumulated Depreciation And Amortization Description [Member] | MASSACHUSETTS [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	$ 152
Date of Initial Leasehold or Acquisition Investment	2012
Accumulated Depreciation And Amortization Description [Member] | MASSACHUSETTS [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	$ 78
Date of Initial Leasehold or Acquisition Investment	2017
Accumulated Depreciation And Amortization Description [Member] | MASSACHUSETTS [Member] | Lowell One [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
Accumulated Depreciation And Amortization Description [Member] | MASSACHUSETTS [Member] | Lowell Two [Member]
Schedule Of Investments In Real Estate [Line Items]
Cost Capitalized Subsequent to Initial Investment	$ 633
Land	429
Building and Improvements	204
Total Investment in Real Estate	633
Accumulated Depreciation and Amortization	$ 85
Date of Initial Leasehold or Acquisition Investment	1996
Accumulated Depreciation And Amortization Description [Member] | MASSACHUSETTS [Member] | Lynn One [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
Accumulated Depreciation And Amortization Description [Member] | MASSACHUSETTS [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
Accumulated Depreciation And Amortization Description [Member] | MASSACHUSETTS [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	$ 277
Date of Initial Leasehold or Acquisition Investment	1985
Accumulated Depreciation And Amortization Description [Member] | MASSACHUSETTS [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
Accumulated Depreciation And Amortization Description [Member] | MASSACHUSETTS [Member] | Methuen 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
Accumulated Depreciation And Amortization Description [Member] | MASSACHUSETTS [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	$ 182
Date of Initial Leasehold or Acquisition Investment	1985
Accumulated Depreciation And Amortization Description [Member] | MASSACHUSETTS [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	$ 207
Date of Initial Leasehold or Acquisition Investment	1985
Accumulated Depreciation And Amortization Description [Member] | MASSACHUSETTS [Member] | Methuen Four [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	$ 242
Date of Initial Leasehold or Acquisition Investment	1986
Accumulated Depreciation And Amortization Description [Member] | MASSACHUSETTS [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	101
Land	450
Building and Improvements	342
Total Investment in Real Estate	792
Accumulated Depreciation and Amortization	$ 312
Date of Initial Leasehold or Acquisition Investment	1985
Accumulated Depreciation And Amortization Description [Member] | MASSACHUSETTS [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	$ 153
Date of Initial Leasehold or Acquisition Investment	1985
Accumulated Depreciation And Amortization Description [Member] | MASSACHUSETTS [Member] | Peabody 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
Accumulated Depreciation And Amortization Description [Member] | MASSACHUSETTS [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
Accumulated Depreciation And Amortization Description [Member] | MASSACHUSETTS [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	257
Land	430
Building and Improvements	400
Total Investment in Real Estate	830
Accumulated Depreciation and Amortization	$ 290
Date of Initial Leasehold or Acquisition Investment	1985
Accumulated Depreciation And Amortization Description [Member] | MASSACHUSETTS [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
Accumulated Depreciation And Amortization Description [Member] | MASSACHUSETTS [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	$ 222
Date of Initial Leasehold or Acquisition Investment	1985
Accumulated Depreciation And Amortization Description [Member] | MASSACHUSETTS [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
Accumulated Depreciation And Amortization Description [Member] | MASSACHUSETTS [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	$ 73
Date of Initial Leasehold or Acquisition Investment	1985
Accumulated Depreciation And Amortization Description [Member] | MASSACHUSETTS [Member] | Shrewsbury 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
Accumulated Depreciation And Amortization Description [Member] | MASSACHUSETTS [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	$ 114
Date of Initial Leasehold or Acquisition Investment	1991
Accumulated Depreciation And Amortization Description [Member] | MASSACHUSETTS [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	$ 224
Date of Initial Leasehold or Acquisition Investment	1993
Accumulated Depreciation And Amortization Description [Member] | MASSACHUSETTS [Member] | Tewksbury One [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
Accumulated Depreciation And Amortization Description [Member] | MASSACHUSETTS [Member] | Tewksbury Two [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	598
Land	75
Building and Improvements	648
Total Investment in Real Estate	723
Accumulated Depreciation and Amortization	$ 321
Date of Initial Leasehold or Acquisition Investment	1986
Accumulated Depreciation And Amortization Description [Member] | MASSACHUSETTS [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	$ 169
Date of Initial Leasehold or Acquisition Investment	1991
Accumulated Depreciation And Amortization Description [Member] | MASSACHUSETTS [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
Accumulated Depreciation And Amortization Description [Member] | MASSACHUSETTS [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	$ 189
Date of Initial Leasehold or Acquisition Investment	1985
Accumulated Depreciation And Amortization Description [Member] | MASSACHUSETTS [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	$ 185
Date of Initial Leasehold or Acquisition Investment	1985
Accumulated Depreciation And Amortization Description [Member] | MASSACHUSETTS [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	832
Land	659
Building and Improvements	1,185
Total Investment in Real Estate	1,844
Accumulated Depreciation and Amortization	$ 711
Date of Initial Leasehold or Acquisition Investment	1985
Accumulated Depreciation And Amortization Description [Member] | MASSACHUSETTS [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	110
Land	319
Building and Improvements	281
Total Investment in Real Estate	600
Accumulated Depreciation and Amortization	$ 228
Date of Initial Leasehold or Acquisition Investment	1985
Accumulated Depreciation And Amortization Description [Member] | MASSACHUSETTS [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
Accumulated Depreciation And Amortization Description [Member] | MASSACHUSETTS [Member] | Wilmington 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
Accumulated Depreciation And Amortization Description [Member] | MASSACHUSETTS [Member] | Wilmington Two [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
Accumulated Depreciation And Amortization Description [Member] | MASSACHUSETTS [Member] | Woburn One [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 508
Cost Capitalized Subsequent to Initial Investment	394
Land	508
Building and Improvements	394
Total Investment in Real Estate	902
Accumulated Depreciation and Amortization	$ 313
Date of Initial Leasehold or Acquisition Investment	1985
Accumulated Depreciation And Amortization Description [Member] | MASSACHUSETTS [Member] | Worcester 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
Accumulated Depreciation And Amortization Description [Member] | MASSACHUSETTS [Member] | Worcester Two [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
Accumulated Depreciation And Amortization Description [Member] | MASSACHUSETTS [Member] | Worcester Three [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 498
Cost Capitalized Subsequent to Initial Investment	465
Land	322
Building and Improvements	641
Total Investment in Real Estate	963
Accumulated Depreciation and Amortization	$ 373
Date of Initial Leasehold or Acquisition Investment	1985
Accumulated Depreciation And Amortization Description [Member] | MASSACHUSETTS [Member] | Worcester Four [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 548
Cost Capitalized Subsequent to Initial Investment	10
Land	356
Building and Improvements	202
Total Investment in Real Estate	558
Accumulated Depreciation and Amortization	$ 138
Date of Initial Leasehold or Acquisition Investment	1991
Accumulated Depreciation And Amortization Description [Member] | MASSACHUSETTS [Member] | Worcester Five [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 978
Cost Capitalized Subsequent to Initial Investment	8
Land	636
Building and Improvements	350
Total Investment in Real Estate	986
Accumulated Depreciation and Amortization	$ 236
Date of Initial Leasehold or Acquisition Investment	1991
Accumulated Depreciation And Amortization Description [Member] | MASSACHUSETTS [Member] | Worcester Six [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 196
Cost Capitalized Subsequent to Initial Investment	788
Building and Improvements	984
Total Investment in Real Estate	984
Accumulated Depreciation and Amortization	$ 89
Date of Initial Leasehold or Acquisition Investment	2017
Accumulated Depreciation And Amortization Description [Member] | MARYLAND [Member] | Accokeek One [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 692
Land	692
Total Investment in Real Estate	$ 692
Date of Initial Leasehold or Acquisition Investment	2010
Accumulated Depreciation And Amortization Description [Member] | MARYLAND [Member] | Baltimore One [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 2,259
Land	722
Building and Improvements	1,537
Total Investment in Real Estate	2,259
Accumulated Depreciation and Amortization	$ 869
Date of Initial Leasehold or Acquisition Investment	2007
Accumulated Depreciation And Amortization Description [Member] | MARYLAND [Member] | Baltimore Two [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 802
Building and Improvements	802
Total Investment in Real Estate	802
Accumulated Depreciation and Amortization	$ 513
Date of Initial Leasehold or Acquisition Investment	2007