Schedule III - Real Estate and Accumulated Depreciation and Amortization Part 12 (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
MARYLAND [Member] | Accumulated Depreciation And Amortization Description [Member] | Beltsville One [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	731
Land	731
Total Investment in Real Estate	$ 731
Date of Initial Leasehold or Acquisition Investment	2009
MARYLAND [Member] | Accumulated Depreciation And Amortization Description [Member] | Greenbelt One [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 1,153
Land	1,153
Total Investment in Real Estate	$ 1,153
Date of Initial Leasehold or Acquisition Investment	2009
MARYLAND [Member] | Accumulated Depreciation And Amortization Description [Member] | Beltsville Two [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 1,050
Land	1,050
Total Investment in Real Estate	$ 1,050
Date of Initial Leasehold or Acquisition Investment	2009
MARYLAND [Member] | Accumulated Depreciation And Amortization Description [Member] | Hyattsville One [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 491
Land	491
Total Investment in Real Estate	$ 491
Date of Initial Leasehold or Acquisition Investment	2009
MARYLAND [Member] | Accumulated Depreciation And Amortization Description [Member] | Beltsville Three [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 1,130
Land	1,130
Total Investment in Real Estate	$ 1,130
Date of Initial Leasehold or Acquisition Investment	2009
MARYLAND [Member] | Accumulated Depreciation And Amortization Description [Member] | Hyattsville Two [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 594
Land	594
Total Investment in Real Estate	$ 594
Date of Initial Leasehold or Acquisition Investment	2009
MARYLAND [Member] | Accumulated Depreciation And Amortization Description [Member] | Landover Hills One [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 457
Land	457
Total Investment in Real Estate	$ 457
Date of Initial Leasehold or Acquisition Investment	2009
MARYLAND [Member] | Accumulated Depreciation And Amortization Description [Member] | Bladensburg One [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 571
Land	571
Total Investment in Real Estate	$ 571
Date of Initial Leasehold or Acquisition Investment	2009
MARYLAND [Member] | Accumulated Depreciation And Amortization Description [Member] | Landover Hills Two [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 1,358
Land	1,358
Total Investment in Real Estate	$ 1,358
Date of Initial Leasehold or Acquisition Investment	2009
MARYLAND [Member] | Accumulated Depreciation And Amortization Description [Member] | Bowie One [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 1,084
Land	1,084
Total Investment in Real Estate	$ 1,084
Date of Initial Leasehold or Acquisition Investment	2009
MARYLAND [Member] | Accumulated Depreciation And Amortization Description [Member] | Landover One [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 662
Land	662
Total Investment in Real Estate	$ 662
Date of Initial Leasehold or Acquisition Investment	2009
MARYLAND [Member] | Accumulated Depreciation And Amortization Description [Member] | Capitol Heights One [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 628
Land	628
Total Investment in Real Estate	$ 628
Date of Initial Leasehold or Acquisition Investment	2009
MARYLAND [Member] | Accumulated Depreciation And Amortization Description [Member] | Clinton One [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 651
Land	651
Total Investment in Real Estate	$ 651
Date of Initial Leasehold or Acquisition Investment	2009
MARYLAND [Member] | Accumulated Depreciation And Amortization Description [Member] | Lanham One [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 822
Land	822
Total Investment in Real Estate	$ 822
Date of Initial Leasehold or Acquisition Investment	2009
MARYLAND [Member] | Accumulated Depreciation And Amortization Description [Member] | College Park One [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 445
Land	445
Total Investment in Real Estate	$ 445
Date of Initial Leasehold or Acquisition Investment	2009
MARYLAND [Member] | Accumulated Depreciation And Amortization Description [Member] | Laurel One [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 696
Land	696
Total Investment in Real Estate	$ 696
Date of Initial Leasehold or Acquisition Investment	2009
MARYLAND [Member] | Accumulated Depreciation And Amortization Description [Member] | College Park Two [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 536
Land	536
Total Investment in Real Estate	$ 536
Date of Initial Leasehold or Acquisition Investment	2009
MARYLAND [Member] | Accumulated Depreciation And Amortization Description [Member] | Laurel Two [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 1,210
Land	1,210
Total Investment in Real Estate	$ 1,210
Date of Initial Leasehold or Acquisition Investment	2009
MARYLAND [Member] | Accumulated Depreciation And Amortization Description [Member] | District Heights One [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 479
Land	479
Total Investment in Real Estate	$ 479
Date of Initial Leasehold or Acquisition Investment	2009
MARYLAND [Member] | Accumulated Depreciation And Amortization Description [Member] | Laurel Three [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 1,267
Land	1,267
Total Investment in Real Estate	$ 1,267
Date of Initial Leasehold or Acquisition Investment	2009
MARYLAND [Member] | Accumulated Depreciation And Amortization Description [Member] | Laurel Four [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 1,415
Land	1,415
Total Investment in Real Estate	$ 1,415
Date of Initial Leasehold or Acquisition Investment	2009
MARYLAND [Member] | Accumulated Depreciation And Amortization Description [Member] | Ellicott City One [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 895
Building and Improvements	895
Total Investment in Real Estate	895
Accumulated Depreciation and Amortization	$ 555
Date of Initial Leasehold or Acquisition Investment	2007
MARYLAND [Member] | Accumulated Depreciation And Amortization Description [Member] | Laurel Five [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 1,530
Land	1,530
Total Investment in Real Estate	$ 1,530
Date of Initial Leasehold or Acquisition Investment	2009
MARYLAND [Member] | Accumulated Depreciation And Amortization Description [Member] | Forestville One [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 1,039
Land	1,039
Total Investment in Real Estate	$ 1,039
Date of Initial Leasehold or Acquisition Investment	2009
MARYLAND [Member] | Accumulated Depreciation And Amortization Description [Member] | Laurel Six [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 2,523
Land	2,523
Total Investment in Real Estate	$ 2,523
Date of Initial Leasehold or Acquisition Investment	2009
MARYLAND [Member] | Accumulated Depreciation And Amortization Description [Member] | Fort Washington One [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 422
Land	422
Total Investment in Real Estate	$ 422
Date of Initial Leasehold or Acquisition Investment	2009
MARYLAND [Member] | Accumulated Depreciation And Amortization Description [Member] | Oxon Hill One [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 1,256
Land	1,256
Total Investment in Real Estate	$ 1,256
Date of Initial Leasehold or Acquisition Investment	2009
MARYLAND [Member] | Accumulated Depreciation And Amortization Description [Member] | Riverdale One [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 582
Land	582
Total Investment in Real Estate	$ 582
Date of Initial Leasehold or Acquisition Investment	2009
MARYLAND [Member] | Accumulated Depreciation And Amortization Description [Member] | Seat Pleasant One [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 468
Land	468
Total Investment in Real Estate	$ 468
Date of Initial Leasehold or Acquisition Investment	2009
MARYLAND [Member] | Accumulated Depreciation And Amortization Description [Member] | Suitland One [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 673
Land	673
Total Investment in Real Estate	$ 673
Date of Initial Leasehold or Acquisition Investment	2009
MARYLAND [Member] | Accumulated Depreciation And Amortization Description [Member] | Landover Two [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 753
Land	753
Total Investment in Real Estate	$ 753
Date of Initial Leasehold or Acquisition Investment	2009
MARYLAND [Member] | Accumulated Depreciation And Amortization Description [Member] | Upper Marlboro One [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 845
Land	845
Total Investment in Real Estate	$ 845
Date of Initial Leasehold or Acquisition Investment	2009
MAINE [Member] | Accumulated Depreciation And Amortization Description [Member] | Biddeford One [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 618
Cost Capitalized Subsequent to Initial Investment	8
Land	235
Building and Improvements	391
Total Investment in Real Estate	626
Accumulated Depreciation and Amortization	$ 391
Date of Initial Leasehold or Acquisition Investment	1985
MAINE [Member] | Accumulated Depreciation And Amortization Description [Member] | Lewiston One [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 342
Cost Capitalized Subsequent to Initial Investment	188
Land	222
Building and Improvements	308
Total Investment in Real Estate	530
Accumulated Depreciation and Amortization	$ 229
Date of Initial Leasehold or Acquisition Investment	1985
NORTH CAROLINA [Member] | Accumulated Depreciation And Amortization Description [Member] | Kernersville One [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 449
Land	338
Building and Improvements	111
Total Investment in Real Estate	449
Accumulated Depreciation and Amortization	$ 105
Date of Initial Leasehold or Acquisition Investment	2007
NORTH CAROLINA [Member] | Accumulated Depreciation And Amortization Description [Member] | Davidson [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 1,776
Land	301
Building and Improvements	1,475
Total Investment in Real Estate	1,776
Accumulated Depreciation and Amortization	$ 79
Date of Initial Leasehold or Acquisition Investment	2017
NORTH CAROLINA [Member] | Accumulated Depreciation And Amortization Description [Member] | Fayetteville Three [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 986
Land	509
Building and Improvements	477
Total Investment in Real Estate	986
Accumulated Depreciation and Amortization	$ 19
Date of Initial Leasehold or Acquisition Investment	2018