Schedule III - Real Estate and Accumulated Depreciation and Amortization Part 6 (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] | MARYLAND [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
Building and Improvements	0
Total Investment in Real Estate	1,130
Accumulated Depreciation and Amortization	$ 0
Date of Initial Leasehold or Acquisition Investment	2009
Accumulated Depreciation And Amortization Description [Member] | MARYLAND [Member] | Beltsville Three [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
Accumulated Depreciation And Amortization Description [Member] | MARYLAND [Member] | Beltsville Five [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 525
Land	525
Total Investment in Real Estate	$ 525
Date of Initial Leasehold or Acquisition Investment	2009
Accumulated Depreciation And Amortization Description [Member] | MARYLAND [Member] | Beltsville Six [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
Accumulated Depreciation And Amortization Description [Member] | MARYLAND [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
Accumulated Depreciation And Amortization Description [Member] | MARYLAND [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
Accumulated Depreciation And Amortization Description [Member] | MARYLAND [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
Accumulated Depreciation And Amortization Description [Member] | MARYLAND [Member] | Capitol Heights 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
Accumulated Depreciation And Amortization Description [Member] | MARYLAND [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
Accumulated Depreciation And Amortization Description [Member] | MARYLAND [Member] | College Park One [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
Accumulated Depreciation And Amortization Description [Member] | MARYLAND [Member] | College Park Two [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
Accumulated Depreciation And Amortization Description [Member] | MARYLAND [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
Accumulated Depreciation And Amortization Description [Member] | MARYLAND [Member] | District Heights Two [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
Accumulated Depreciation And Amortization Description [Member] | MARYLAND [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	$ 602
Date of Initial Leasehold or Acquisition Investment	2007
Accumulated Depreciation And Amortization Description [Member] | MARYLAND [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
Accumulated Depreciation And Amortization Description [Member] | MARYLAND [Member] | Greater Landover One [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
Accumulated Depreciation And Amortization Description [Member] | MARYLAND [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
Accumulated Depreciation And Amortization Description [Member] | MARYLAND [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
Accumulated Depreciation And Amortization Description [Member] | MARYLAND [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
Accumulated Depreciation And Amortization Description [Member] | MARYLAND [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
Accumulated Depreciation And Amortization Description [Member] | MARYLAND [Member] | Landover Hills One [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
Accumulated Depreciation And Amortization Description [Member] | MARYLAND [Member] | Landover Hills Two [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
Accumulated Depreciation And Amortization Description [Member] | MARYLAND [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
Accumulated Depreciation And Amortization Description [Member] | MARYLAND [Member] | Laurel One [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
Accumulated Depreciation And Amortization Description [Member] | MARYLAND [Member] | Laurel Two [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
Accumulated Depreciation And Amortization Description [Member] | MARYLAND [Member] | Laurel Three [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
Accumulated Depreciation And Amortization Description [Member] | MARYLAND [Member] | Laurel Four [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
Accumulated Depreciation And Amortization Description [Member] | MARYLAND [Member] | Laurel Five [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
Accumulated Depreciation And Amortization Description [Member] | MARYLAND [Member] | Laurel Six [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
Accumulated Depreciation And Amortization Description [Member] | MARYLAND [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
Accumulated Depreciation And Amortization Description [Member] | MARYLAND [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
Accumulated Depreciation And Amortization Description [Member] | MARYLAND [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
Accumulated Depreciation And Amortization Description [Member] | MARYLAND [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
Accumulated Depreciation And Amortization Description [Member] | MAINE [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
Accumulated Depreciation And Amortization Description [Member] | MAINE [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	$ 244
Date of Initial Leasehold or Acquisition Investment	1985
Accumulated Depreciation And Amortization Description [Member] | MINNESOTA [Member] | Maple Grove One [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 4,233
Land	955
Building and Improvements	3,278
Total Investment in Real Estate	4,233
Accumulated Depreciation and Amortization	$ 73
Date of Initial Leasehold or Acquisition Investment	2019
Accumulated Depreciation And Amortization Description [Member] | NORTH CAROLINA [Member] | Fayetteville One [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	$ 44
Date of Initial Leasehold or Acquisition Investment	2018
Accumulated Depreciation And Amortization Description [Member] | NORTH CAROLINA [Member] | Kannapolis One [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 3,791
Land	616
Building and Improvements	3,175
Total Investment in Real Estate	3,791
Accumulated Depreciation and Amortization	$ 16
Date of Initial Leasehold or Acquisition Investment	2019
Accumulated Depreciation And Amortization Description [Member] | NORTH CAROLINA [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
Accumulated Depreciation And Amortization Description [Member] | NORTH CAROLINA [Member] | Lexington One [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	$ 141
Date of Initial Leasehold or Acquisition Investment	2017
Accumulated Depreciation And Amortization Description [Member] | NORTH CAROLINA [Member] | New Bern One [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 350
Cost Capitalized Subsequent to Initial Investment	83
Land	190
Building and Improvements	243
Total Investment in Real Estate	433
Accumulated Depreciation and Amortization	$ 179
Date of Initial Leasehold or Acquisition Investment	2007
Accumulated Depreciation And Amortization Description [Member] | NORTH CAROLINA [Member] | Raleigh One [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 1,601
Land	1,149
Building and Improvements	452
Total Investment in Real Estate	1,601
Accumulated Depreciation and Amortization	$ 16
Date of Initial Leasehold or Acquisition Investment	2019
Accumulated Depreciation And Amortization Description [Member] | NORTH CAROLINA [Member] | Rockingham One [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 3,035
Land	233
Building and Improvements	2,802
Total Investment in Real Estate	3,035
Accumulated Depreciation and Amortization	$ 39
Date of Initial Leasehold or Acquisition Investment	2019
Accumulated Depreciation And Amortization Description [Member] | NORTH DAKOTA [Member] | Belfield One [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 1,232
Land	382
Building and Improvements	850
Total Investment in Real Estate	1,232
Accumulated Depreciation and Amortization	$ 760
Date of Initial Leasehold or Acquisition Investment	2007
Accumulated Depreciation And Amortization Description [Member] | NEW HAMPSHIRE [Member] | Allenstown One [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 1,787
Land	467
Building and Improvements	1,320
Total Investment in Real Estate	1,787
Accumulated Depreciation and Amortization	$ 802
Date of Initial Leasehold or Acquisition Investment	2007
Accumulated Depreciation And Amortization Description [Member] | NEW HAMPSHIRE [Member] | Concord One [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 675
Land	675
Total Investment in Real Estate	$ 675
Date of Initial Leasehold or Acquisition Investment	2011
Accumulated Depreciation And Amortization Description [Member] | NEW HAMPSHIRE [Member] | Concord Two [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] | NEW HAMPSHIRE [Member] | Derry Two [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 418
Cost Capitalized Subsequent to Initial Investment	16
Land	158
Building and Improvements	276
Total Investment in Real Estate	434
Accumulated Depreciation and Amortization	$ 276
Date of Initial Leasehold or Acquisition Investment	1987