Schedule III - Real Estate and Accumulated Depreciation and Amortization Part 19 (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
Accumulated Depreciation And Amortization Description [Member] | Miscellaneous [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	32,937
Cost Capitalized Subsequent to Initial Investment	11,470
Land	14,749
Building and Improvements	29,658
Total Investment in Real Estate	44,407
Accumulated Depreciation and Amortization	19,388
VIRGINIA | Accumulated Depreciation And Amortization Description [Member] | Glen Allen One [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	1,037
Land	412
Building and Improvements	625
Total Investment in Real Estate	1,037
Accumulated Depreciation and Amortization	$ 294
Date of Initial Leasehold or Acquisition Investment	2005
VIRGINIA | Accumulated Depreciation And Amortization Description [Member] | Glen Allen Two [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 1,077
Land	322
Building and Improvements	755
Total Investment in Real Estate	1,077
Accumulated Depreciation and Amortization	$ 355
Date of Initial Leasehold or Acquisition Investment	2005
VIRGINIA | Accumulated Depreciation And Amortization Description [Member] | King William One [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 1,688
Land	1,068
Building and Improvements	620
Total Investment in Real Estate	1,688
Accumulated Depreciation and Amortization	$ 292
Date of Initial Leasehold or Acquisition Investment	2005
VIRGINIA | Accumulated Depreciation And Amortization Description [Member] | Mechanicsville One [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 903
Land	273
Building and Improvements	630
Total Investment in Real Estate	903
Accumulated Depreciation and Amortization	$ 296
Date of Initial Leasehold or Acquisition Investment	2005
VIRGINIA | Accumulated Depreciation And Amortization Description [Member] | Mechanicsville Two [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 957
Land	324
Building and Improvements	633
Total Investment in Real Estate	957
Accumulated Depreciation and Amortization	$ 323
Date of Initial Leasehold or Acquisition Investment	2005
VIRGINIA | Accumulated Depreciation And Amortization Description [Member] | Mechanicsville Three [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 1,043
Land	223
Building and Improvements	820
Total Investment in Real Estate	1,043
Accumulated Depreciation and Amortization	$ 386
Date of Initial Leasehold or Acquisition Investment	2005
VIRGINIA | Accumulated Depreciation And Amortization Description [Member] | Mechanicsville Four [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 1,125
Land	505
Building and Improvements	620
Total Investment in Real Estate	1,125
Accumulated Depreciation and Amortization	$ 292
Date of Initial Leasehold or Acquisition Investment	2005
VIRGINIA | Accumulated Depreciation And Amortization Description [Member] | Mechanicsville Five [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 1,476
Land	876
Building and Improvements	600
Total Investment in Real Estate	1,476
Accumulated Depreciation and Amortization	$ 282
Date of Initial Leasehold or Acquisition Investment	2005
VIRGINIA | Accumulated Depreciation And Amortization Description [Member] | Mechanicsville Six [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 1,677
Land	1,157
Building and Improvements	520
Total Investment in Real Estate	1,677
Accumulated Depreciation and Amortization	$ 245
Date of Initial Leasehold or Acquisition Investment	2005
VIRGINIA | Accumulated Depreciation And Amortization Description [Member] | Montpelier One [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 2,481
Cost Capitalized Subsequent to Initial Investment	(114)
Land	1,612
Building and Improvements	755
Total Investment in Real Estate	2,367
Accumulated Depreciation and Amortization	$ 355
Date of Initial Leasehold or Acquisition Investment	2005
VIRGINIA | Accumulated Depreciation And Amortization Description [Member] | Norfolk One [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 535
Cost Capitalized Subsequent to Initial Investment	6
Land	311
Building and Improvements	230
Total Investment in Real Estate	541
Accumulated Depreciation and Amortization	$ 230
Date of Initial Leasehold or Acquisition Investment	1990
VIRGINIA | Accumulated Depreciation And Amortization Description [Member] | Petersburg One [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 1,441
Land	816
Building and Improvements	625
Total Investment in Real Estate	1,441
Accumulated Depreciation and Amortization	$ 294
Date of Initial Leasehold or Acquisition Investment	2005
VIRGINIA | Accumulated Depreciation And Amortization Description [Member] | Portsmouth One [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 562
Cost Capitalized Subsequent to Initial Investment	34
Land	222
Building and Improvements	374
Total Investment in Real Estate	596
Accumulated Depreciation and Amortization	$ 363
Date of Initial Leasehold or Acquisition Investment	1990
VIRGINIA | Accumulated Depreciation And Amortization Description [Member] | Richmond One [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 1,132
Cost Capitalized Subsequent to Initial Investment	(41)
Land	506
Building and Improvements	585
Total Investment in Real Estate	1,091
Accumulated Depreciation and Amortization	$ 275
Date of Initial Leasehold or Acquisition Investment	2005
VIRGINIA | Accumulated Depreciation And Amortization Description [Member] | Ruther Glen One [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 466
Land	31
Building and Improvements	435
Total Investment in Real Estate	466
Accumulated Depreciation and Amortization	$ 205
Date of Initial Leasehold or Acquisition Investment	2005
VIRGINIA | Accumulated Depreciation And Amortization Description [Member] | Sandston One [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 722
Land	102
Building and Improvements	620
Total Investment in Real Estate	722
Accumulated Depreciation and Amortization	$ 292
Date of Initial Leasehold or Acquisition Investment	2005
VIRGINIA | Accumulated Depreciation And Amortization Description [Member] | Spotsylvania One [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 1,290
Land	490
Building and Improvements	800
Total Investment in Real Estate	1,290
Accumulated Depreciation and Amortization	$ 376
Date of Initial Leasehold or Acquisition Investment	2005
VIRGINIA | 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	$ 4,257
Land	2,969
Building and Improvements	1,288
Total Investment in Real Estate	4,257
Accumulated Depreciation and Amortization	$ 250
Date of Initial Leasehold or Acquisition Investment	2013
WASHINGTON | Accumulated Depreciation And Amortization Description [Member] | Auburn One [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 3,022
Land	1,965
Building and Improvements	1,057
Total Investment in Real Estate	3,022
Accumulated Depreciation and Amortization	$ 99
Date of Initial Leasehold or Acquisition Investment	2015
WASHINGTON | Accumulated Depreciation And Amortization Description [Member] | Bellevue One [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 1,725
Land	886
Building and Improvements	839
Total Investment in Real Estate	1,725
Accumulated Depreciation and Amortization	$ 79
Date of Initial Leasehold or Acquisition Investment	2015
WASHINGTON | Accumulated Depreciation And Amortization Description [Member] | Chehalis One [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 1,176
Land	313
Building and Improvements	863
Total Investment in Real Estate	1,176
Accumulated Depreciation and Amortization	$ 89
Date of Initial Leasehold or Acquisition Investment	2015
WASHINGTON | Accumulated Depreciation And Amortization Description [Member] | Colfax One Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 4,800
Land	3,611
Building and Improvements	1,189
Total Investment in Real Estate	4,800
Accumulated Depreciation and Amortization	$ 112
Date of Initial Leasehold or Acquisition Investment	2015
WASHINGTON | Accumulated Depreciation And Amortization Description [Member] | Federal Way One [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 4,218
Land	2,973
Building and Improvements	1,245
Total Investment in Real Estate	4,218
Accumulated Depreciation and Amortization	$ 126
Date of Initial Leasehold or Acquisition Investment	2015
WASHINGTON | Accumulated Depreciation And Amortization Description [Member] | Fife One [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 1,181
Land	414
Building and Improvements	767
Total Investment in Real Estate	1,181
Accumulated Depreciation and Amortization	$ 78
Date of Initial Leasehold or Acquisition Investment	2015
WASHINGTON | Accumulated Depreciation And Amortization Description [Member] | Kent One [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 2,900
Land	2,066
Building and Improvements	834
Total Investment in Real Estate	2,900
Accumulated Depreciation and Amortization	$ 85
Date of Initial Leasehold or Acquisition Investment	2015
WASHINGTON | Accumulated Depreciation And Amortization Description [Member] | Monroe One [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 2,792
Land	1,556
Building and Improvements	1,236
Total Investment in Real Estate	2,792
Accumulated Depreciation and Amortization	$ 119
Date of Initial Leasehold or Acquisition Investment	2015
WASHINGTON | Accumulated Depreciation And Amortization Description [Member] | PortOrchard One [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 2,019
Land	161
Building and Improvements	1,858
Total Investment in Real Estate	2,019
Accumulated Depreciation and Amortization	$ 149
Date of Initial Leasehold or Acquisition Investment	2015
WASHINGTON | Accumulated Depreciation And Amortization Description [Member] | Puyallup One [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 831
Land	172
Building and Improvements	659
Total Investment in Real Estate	831
Accumulated Depreciation and Amortization	$ 72
Date of Initial Leasehold or Acquisition Investment	2015
WASHINGTON | Accumulated Depreciation And Amortization Description [Member] | Puyallup Two [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 2,035
Land	465
Building and Improvements	1,570
Total Investment in Real Estate	2,035
Accumulated Depreciation and Amortization	$ 145
Date of Initial Leasehold or Acquisition Investment	2015
WASHINGTON | Accumulated Depreciation And Amortization Description [Member] | Puyallup Three [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 4,050
Land	2,394
Building and Improvements	1,656
Total Investment in Real Estate	4,050
Accumulated Depreciation and Amortization	$ 190
Date of Initial Leasehold or Acquisition Investment	2015
WASHINGTON | Accumulated Depreciation And Amortization Description [Member] | Renton One [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 1,485
Land	952
Building and Improvements	533
Total Investment in Real Estate	1,485
Accumulated Depreciation and Amortization	$ 68
Date of Initial Leasehold or Acquisition Investment	2015
WASHINGTON | Accumulated Depreciation And Amortization Description [Member] | Seattle One [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 346
Land	346
Total Investment in Real Estate	$ 346
Date of Initial Leasehold or Acquisition Investment	2015
WASHINGTON | Accumulated Depreciation And Amortization Description [Member] | Seattle Two [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 717
Land	193
Building and Improvements	524
Total Investment in Real Estate	717
Accumulated Depreciation and Amortization	$ 47
Date of Initial Leasehold or Acquisition Investment	2015
WASHINGTON | Accumulated Depreciation And Amortization Description [Member] | Seattle Three [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 1,884
Land	1,223
Building and Improvements	661
Total Investment in Real Estate	1,884
Accumulated Depreciation and Amortization	$ 60
Date of Initial Leasehold or Acquisition Investment	2015
WASHINGTON | Accumulated Depreciation And Amortization Description [Member] | Silverdale One [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 2,178
Land	1,217
Building and Improvements	961
Total Investment in Real Estate	2,178
Accumulated Depreciation and Amortization	$ 97
Date of Initial Leasehold or Acquisition Investment	2015
WASHINGTON | Accumulated Depreciation And Amortization Description [Member] | Snohomish One [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 955
Land	955
Total Investment in Real Estate	$ 955
Date of Initial Leasehold or Acquisition Investment	2015
WASHINGTON | Accumulated Depreciation And Amortization Description [Member] | South Bend One [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 760
Land	121
Building and Improvements	639
Total Investment in Real Estate	760
Accumulated Depreciation and Amortization	$ 56
Date of Initial Leasehold or Acquisition Investment	2015
WASHINGTON | Accumulated Depreciation And Amortization Description [Member] | Spokane One [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 346
Land	346
Total Investment in Real Estate	$ 346
Date of Initial Leasehold or Acquisition Investment	2015
WASHINGTON | Accumulated Depreciation And Amortization Description [Member] | Tacoma One [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 518
Land	518
Total Investment in Real Estate	$ 518
Date of Initial Leasehold or Acquisition Investment	2015
WASHINGTON | Accumulated Depreciation And Amortization Description [Member] | Tacoma Two [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 671
Land	671
Total Investment in Real Estate	$ 671
Date of Initial Leasehold or Acquisition Investment	2015
WASHINGTON | Accumulated Depreciation And Amortization Description [Member] | Tenino One [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 937
Land	219
Building and Improvements	718
Total Investment in Real Estate	937
Accumulated Depreciation and Amortization	$ 64
Date of Initial Leasehold or Acquisition Investment	2015
WASHINGTON | Accumulated Depreciation And Amortization Description [Member] | Vancouver One [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 1,214
Land	163
Building and Improvements	1,051
Total Investment in Real Estate	1,214
Accumulated Depreciation and Amortization	$ 84
Date of Initial Leasehold or Acquisition Investment	2015
WASHINGTON | Accumulated Depreciation And Amortization Description [Member] | Wilbur One [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 629
Land	153
Building and Improvements	476
Total Investment in Real Estate	629
Accumulated Depreciation and Amortization	$ 47
Date of Initial Leasehold or Acquisition Investment	2015