Schedule III - Real Estate and Accumulated Depreciation and Amortization Part 18 (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
OREGON | 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	1,071
Land	399
Building and Improvements	672
Total Investment in Real Estate	1,071
Accumulated Depreciation and Amortization	$ 174
Date of Initial Leasehold or Acquisition Investment	2015
OREGON | Accumulated Depreciation And Amortization Description [Member] | Salem Two [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 1,350
Land	521
Building and Improvements	829
Total Investment in Real Estate	1,350
Accumulated Depreciation and Amortization	$ 172
Date of Initial Leasehold or Acquisition Investment	2015
OREGON | Accumulated Depreciation And Amortization Description [Member] | Salem Three [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 1,408
Land	524
Building and Improvements	884
Total Investment in Real Estate	1,408
Accumulated Depreciation and Amortization	$ 190
Date of Initial Leasehold or Acquisition Investment	2015
OREGON | Accumulated Depreciation And Amortization Description [Member] | Salem Four [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 4,215
Land	3,182
Building and Improvements	1,033
Total Investment in Real Estate	4,215
Accumulated Depreciation and Amortization	$ 226
Date of Initial Leasehold or Acquisition Investment	2015
OREGON | Accumulated Depreciation And Amortization Description [Member] | Salem Five [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 4,614
Land	3,517
Building and Improvements	1,097
Total Investment in Real Estate	4,614
Accumulated Depreciation and Amortization	$ 225
Date of Initial Leasehold or Acquisition Investment	2015
OREGON | Accumulated Depreciation And Amortization Description [Member] | Springfield One [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 1,398
Land	796
Building and Improvements	602
Total Investment in Real Estate	1,398
Accumulated Depreciation and Amortization	$ 152
Date of Initial Leasehold or Acquisition Investment	2015
OREGON | Accumulated Depreciation And Amortization Description [Member] | Yamhill
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 2,867
Land	394
Building and Improvements	2,473
Total Investment in Real Estate	2,867
Accumulated Depreciation and Amortization	$ 192
Date of Initial Leasehold or Acquisition Investment	2017
PA [Member] | Accumulated Depreciation And Amortization Description [Member] | Allison Park One [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 1,500
Land	850
Building and Improvements	650
Total Investment in Real Estate	1,500
Accumulated Depreciation and Amortization	$ 444
Date of Initial Leasehold or Acquisition Investment	2010
PA [Member] | Accumulated Depreciation And Amortization Description [Member] | Harrisburg One [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 399
Cost Capitalized Subsequent to Initial Investment	213
Land	199
Building and Improvements	413
Total Investment in Real Estate	612
Accumulated Depreciation and Amortization	$ 329
Date of Initial Leasehold or Acquisition Investment	1989
PA [Member] | Accumulated Depreciation And Amortization Description [Member] | Lancaster One [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 642
Cost Capitalized Subsequent to Initial Investment	18
Land	300
Building and Improvements	360
Total Investment in Real Estate	660
Accumulated Depreciation and Amortization	$ 360
Date of Initial Leasehold or Acquisition Investment	1989
PA [Member] | Accumulated Depreciation And Amortization Description [Member] | New Kensington One [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 1,375
Land	675
Building and Improvements	700
Total Investment in Real Estate	1,375
Accumulated Depreciation and Amortization	$ 269
Date of Initial Leasehold or Acquisition Investment	2010
PA [Member] | Accumulated Depreciation And Amortization Description [Member] | Philadelphia One [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 405
Cost Capitalized Subsequent to Initial Investment	175
Land	264
Building and Improvements	316
Total Investment in Real Estate	580
Accumulated Depreciation and Amortization	$ 263
Date of Initial Leasehold or Acquisition Investment	1985
PA [Member] | Accumulated Depreciation And Amortization Description [Member] | Philadelphia Two [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 1,252
Land	814
Building and Improvements	438
Total Investment in Real Estate	1,252
Accumulated Depreciation and Amortization	$ 177
Date of Initial Leasehold or Acquisition Investment	2009
PA [Member] | Accumulated Depreciation And Amortization Description [Member] | Phoenixville [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 385
Cost Capitalized Subsequent to Initial Investment	89
Land	76
Building and Improvements	398
Total Investment in Real Estate	474
Accumulated Depreciation and Amortization	$ 27
Date of Initial Leasehold or Acquisition Investment	1985
PA [Member] | Accumulated Depreciation And Amortization Description [Member] | Pottsville One [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 452
Cost Capitalized Subsequent to Initial Investment	1
Land	148
Building and Improvements	305
Total Investment in Real Estate	453
Accumulated Depreciation and Amortization	$ 304
Date of Initial Leasehold or Acquisition Investment	1990
PA [Member] | Accumulated Depreciation And Amortization Description [Member] | Reading One [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 750
Cost Capitalized Subsequent to Initial Investment	49
Building and Improvements	799
Total Investment in Real Estate	799
Accumulated Depreciation and Amortization	$ 799
Date of Initial Leasehold or Acquisition Investment	1989
RHODE ISLAND | Accumulated Depreciation And Amortization Description [Member] | Barrington 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	180
Land	319
Building and Improvements	351
Total Investment in Real Estate	670
Accumulated Depreciation and Amortization	$ 266
Date of Initial Leasehold or Acquisition Investment	1985
RHODE ISLAND | Accumulated Depreciation And Amortization Description [Member] | East Providence One [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 2,298
Cost Capitalized Subsequent to Initial Investment	(1,855)
Land	14
Building and Improvements	429
Total Investment in Real Estate	443
Accumulated Depreciation and Amortization	$ 74
Date of Initial Leasehold or Acquisition Investment	1985
RHODE ISLAND | Accumulated Depreciation And Amortization Description [Member] | N. Providence [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 543
Cost Capitalized Subsequent to Initial Investment	158
Land	353
Building and Improvements	348
Total Investment in Real Estate	701
Accumulated Depreciation and Amortization	$ 255
Date of Initial Leasehold or Acquisition Investment	1985
SOUTH CAROLINA | Accumulated Depreciation And Amortization Description [Member] | Columbia One [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 1,995
Land	1,130
Building and Improvements	865
Total Investment in Real Estate	1,995
Accumulated Depreciation and Amortization	$ 21
Date of Initial Leasehold or Acquisition Investment	2018
SOUTH CAROLINA | Accumulated Depreciation And Amortization Description [Member] | Columbia Two [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 2,109
Land	1,120
Building and Improvements	989
Total Investment in Real Estate	2,109
Accumulated Depreciation and Amortization	$ 23
Date of Initial Leasehold or Acquisition Investment	2018
SOUTH CAROLINA | Accumulated Depreciation And Amortization Description [Member] | Columbia Three [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 2,531
Land	1,612
Building and Improvements	919
Total Investment in Real Estate	2,531
Accumulated Depreciation and Amortization	$ 21
Date of Initial Leasehold or Acquisition Investment	2018
SOUTH CAROLINA | Accumulated Depreciation And Amortization Description [Member] | Johns Island
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 2,561
Land	1,885
Building and Improvements	676
Total Investment in Real Estate	2,561
Accumulated Depreciation and Amortization	$ 6
Date of Initial Leasehold or Acquisition Investment	2018
SOUTH CAROLINA | Accumulated Depreciation And Amortization Description [Member] | Kershaw One [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 2,082
Land	1,166
Building and Improvements	916
Total Investment in Real Estate	2,082
Accumulated Depreciation and Amortization	$ 68
Date of Initial Leasehold or Acquisition Investment	2017
SOUTH CAROLINA | Accumulated Depreciation And Amortization Description [Member] | Kershaw Two [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 2,177
Land	974
Building and Improvements	1,203
Total Investment in Real Estate	2,177
Accumulated Depreciation and Amortization	$ 84
Date of Initial Leasehold or Acquisition Investment	2017
SOUTH CAROLINA | Accumulated Depreciation And Amortization Description [Member] | Lexington One [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 412
Land	145
Building and Improvements	267
Total Investment in Real Estate	412
Accumulated Depreciation and Amortization	$ 19
Date of Initial Leasehold or Acquisition Investment	2017
SOUTH CAROLINA | Accumulated Depreciation And Amortization Description [Member] | Lexington Two [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 633
Land	309
Building and Improvements	324
Total Investment in Real Estate	633
Accumulated Depreciation and Amortization	$ 24
Date of Initial Leasehold or Acquisition Investment	2017
SOUTH CAROLINA | Accumulated Depreciation And Amortization Description [Member] | Lexington Three [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 412
Land	145
Building and Improvements	267
Total Investment in Real Estate	412
Accumulated Depreciation and Amortization	$ 19
Date of Initial Leasehold or Acquisition Investment	2017
SOUTH CAROLINA | Accumulated Depreciation And Amortization Description [Member] | Lexington Four [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 633
Land	309
Building and Improvements	324
Total Investment in Real Estate	633
Accumulated Depreciation and Amortization	$ 24
Date of Initial Leasehold or Acquisition Investment	2017
SOUTH CAROLINA | Accumulated Depreciation And Amortization Description [Member] | Lexington Five [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 694
Land	172
Building and Improvements	522
Total Investment in Real Estate	694
Accumulated Depreciation and Amortization	$ 41
Date of Initial Leasehold or Acquisition Investment	2017
SOUTH CAROLINA | Accumulated Depreciation And Amortization Description [Member] | Lexington Six [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 720
Land	219
Building and Improvements	501
Total Investment in Real Estate	720
Accumulated Depreciation and Amortization	$ 35
Date of Initial Leasehold or Acquisition Investment	2017
SOUTH CAROLINA | Accumulated Depreciation And Amortization Description [Member] | Lexington Seven [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 816
Land	336
Building and Improvements	480
Total Investment in Real Estate	816
Accumulated Depreciation and Amortization	$ 27
Date of Initial Leasehold or Acquisition Investment	2017
SOUTH CAROLINA | Accumulated Depreciation And Amortization Description [Member] | Lexington Eight [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 973
Land	582
Building and Improvements	391
Total Investment in Real Estate	973
Accumulated Depreciation and Amortization	$ 30
Date of Initial Leasehold or Acquisition Investment	2017
SOUTH CAROLINA | Accumulated Depreciation And Amortization Description [Member] | Lexington Nine [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 1,036
Land	434
Building and Improvements	602
Total Investment in Real Estate	1,036
Accumulated Depreciation and Amortization	$ 42
Date of Initial Leasehold or Acquisition Investment	2017
SOUTH CAROLINA | Accumulated Depreciation And Amortization Description [Member] | Lexington Ten [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 1,056
Land	432
Building and Improvements	624
Total Investment in Real Estate	1,056
Accumulated Depreciation and Amortization	$ 47
Date of Initial Leasehold or Acquisition Investment	2017
SOUTH CAROLINA | Accumulated Depreciation And Amortization Description [Member] | Lexington Eleven [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 1,116
Land	50
Building and Improvements	1,066
Total Investment in Real Estate	1,116
Accumulated Depreciation and Amortization	$ 77
Date of Initial Leasehold or Acquisition Investment	2017
SOUTH CAROLINA | Accumulated Depreciation And Amortization Description [Member] | Lexington Twelve [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 1,436
Land	472
Building and Improvements	964
Total Investment in Real Estate	1,436
Accumulated Depreciation and Amortization	$ 70
Date of Initial Leasehold or Acquisition Investment	2017
SOUTH CAROLINA | Accumulated Depreciation And Amortization Description [Member] | Lexington Thirteen [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 1,624
Land	999
Building and Improvements	625
Total Investment in Real Estate	1,624
Accumulated Depreciation and Amortization	$ 45
Date of Initial Leasehold or Acquisition Investment	2017
SOUTH CAROLINA | Accumulated Depreciation And Amortization Description [Member] | Lexington Fourteen [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 1,644
Land	1,283
Building and Improvements	361
Total Investment in Real Estate	1,644
Accumulated Depreciation and Amortization	$ 27
Date of Initial Leasehold or Acquisition Investment	2017
SOUTH CAROLINA | Accumulated Depreciation And Amortization Description [Member] | Lexington Fifteen [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 1,682
Land	1,135
Building and Improvements	547
Total Investment in Real Estate	1,682
Accumulated Depreciation and Amortization	$ 43
Date of Initial Leasehold or Acquisition Investment	2017
SOUTH CAROLINA | Accumulated Depreciation And Amortization Description [Member] | Lexington Sixteen [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 1,712
Land	1,410
Building and Improvements	302
Total Investment in Real Estate	1,712
Accumulated Depreciation and Amortization	$ 19
Date of Initial Leasehold or Acquisition Investment	2017
SOUTH CAROLINA | Accumulated Depreciation And Amortization Description [Member] | Lexington Seventeen [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 1,729
Land	1,268
Building and Improvements	461
Total Investment in Real Estate	1,729
Accumulated Depreciation and Amortization	$ 39
Date of Initial Leasehold or Acquisition Investment	2017
SOUTH CAROLINA | Accumulated Depreciation And Amortization Description [Member] | Lexington Eighteen [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 1,738
Land	1,189
Building and Improvements	549
Total Investment in Real Estate	1,738
Accumulated Depreciation and Amortization	$ 31
Date of Initial Leasehold or Acquisition Investment	2017
SOUTH CAROLINA | Accumulated Depreciation And Amortization Description [Member] | Lexington Nineteen [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 1,901
Land	1,021
Building and Improvements	880
Total Investment in Real Estate	1,901
Accumulated Depreciation and Amortization	$ 73
Date of Initial Leasehold or Acquisition Investment	2017
SOUTH CAROLINA | Accumulated Depreciation And Amortization Description [Member] | Lexington Twenty [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 2,046
Land	746
Building and Improvements	1,300
Total Investment in Real Estate	2,046
Accumulated Depreciation and Amortization	$ 90
Date of Initial Leasehold or Acquisition Investment	2017
SOUTH CAROLINA | Accumulated Depreciation And Amortization Description [Member] | Lexington Twenty One [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 2,179
Land	1,476
Building and Improvements	703
Total Investment in Real Estate	2,179
Accumulated Depreciation and Amortization	$ 50
Date of Initial Leasehold or Acquisition Investment	2017
SOUTH CAROLINA | Accumulated Depreciation And Amortization Description [Member] | Lexington Twenty Two [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 2,230
Land	934
Building and Improvements	1,296
Total Investment in Real Estate	2,230
Accumulated Depreciation and Amortization	$ 92
Date of Initial Leasehold or Acquisition Investment	2017
SOUTH CAROLINA | Accumulated Depreciation And Amortization Description [Member] | Lexington Twenty Three [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 2,603
Land	1,869
Building and Improvements	734
Total Investment in Real Estate	2,603
Accumulated Depreciation and Amortization	$ 19
Date of Initial Leasehold or Acquisition Investment	2018