Schedule III - Real Estate and Accumulated Depreciation and Amortization Part 17 (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Schedule Of Investments In Real Estate [Line Items]
Total Investment in Real Estate	$ 970,964	$ 782,166	$ 783,233	$ 595,959
Accumulated Depreciation And Amortization Description [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	914,488
Cost Capitalized Subsequent to Initial Investment	56,476
Land	589,497
Building and Improvements	381,467
Total Investment in Real Estate	970,964
Accumulated Depreciation and Amortization	133,353
NEW YORK | Accumulated Depreciation And Amortization Description [Member] | Rego Park One [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	2,783
Land	2,104
Building and Improvements	679
Total Investment in Real Estate	2,783
Accumulated Depreciation and Amortization	$ 175
Date of Initial Leasehold or Acquisition Investment	2013
NEW YORK | Accumulated Depreciation And Amortization Description [Member] | Riverhead One [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 724
Land	432
Building and Improvements	292
Total Investment in Real Estate	724
Accumulated Depreciation and Amortization	$ 229
Date of Initial Leasehold or Acquisition Investment	1998
NEW YORK | Accumulated Depreciation And Amortization Description [Member] | Rochester Two [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 595
Land	305
Building and Improvements	290
Total Investment in Real Estate	595
Accumulated Depreciation and Amortization	$ 133
Date of Initial Leasehold or Acquisition Investment	2008
NEW YORK | Accumulated Depreciation And Amortization Description [Member] | Rokaway Park One
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 1,605
Land	1,605
Total Investment in Real Estate	$ 1,605
Date of Initial Leasehold or Acquisition Investment	2013
NEW YORK | Accumulated Depreciation And Amortization Description [Member] | Rockville Centre One
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 350
Cost Capitalized Subsequent to Initial Investment	66
Land	201
Building and Improvements	215
Total Investment in Real Estate	416
Accumulated Depreciation and Amortization	$ 193
Date of Initial Leasehold or Acquisition Investment	1985
NEW YORK | Accumulated Depreciation And Amortization Description [Member] | Rye One [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 872
Land	872
Total Investment in Real Estate	$ 872
Date of Initial Leasehold or Acquisition Investment	2011
NEW YORK | Accumulated Depreciation And Amortization Description [Member] | Sag Harbor One [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 704
Cost Capitalized Subsequent to Initial Investment	35
Land	458
Building and Improvements	281
Total Investment in Real Estate	739
Accumulated Depreciation and Amortization	$ 231
Date of Initial Leasehold or Acquisition Investment	1985
NEW YORK | Accumulated Depreciation And Amortization Description [Member] | Sayville One [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 344
Cost Capitalized Subsequent to Initial Investment	246
Land	300
Building and Improvements	290
Total Investment in Real Estate	590
Accumulated Depreciation and Amortization	$ 108
Date of Initial Leasehold or Acquisition Investment	1998
NEW YORK | Accumulated Depreciation And Amortization Description [Member] | Scarsdale One [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 1,301
Land	1,301
Total Investment in Real Estate	$ 1,301
Date of Initial Leasehold or Acquisition Investment	2011
NEW YORK | Accumulated Depreciation And Amortization Description [Member] | Shrub Oak One [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 1,061
Cost Capitalized Subsequent to Initial Investment	494
Land	691
Building and Improvements	864
Total Investment in Real Estate	1,555
Accumulated Depreciation and Amortization	$ 547
Date of Initial Leasehold or Acquisition Investment	1985
NEW YORK | Accumulated Depreciation And Amortization Description [Member] | Sleepy Hollow One [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 281
Cost Capitalized Subsequent to Initial Investment	306
Land	130
Building and Improvements	457
Total Investment in Real Estate	587
Accumulated Depreciation and Amortization	$ 370
Date of Initial Leasehold or Acquisition Investment	1969
NEW YORK | Accumulated Depreciation And Amortization Description [Member] | Staten Island One [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 357
Cost Capitalized Subsequent to Initial Investment	35
Land	230
Building and Improvements	162
Total Investment in Real Estate	392
Accumulated Depreciation and Amortization	$ 137
Date of Initial Leasehold or Acquisition Investment	1985
NEW YORK | Accumulated Depreciation And Amortization Description [Member] | Spring Valley One [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 749
Land	749
Total Investment in Real Estate	$ 749
Date of Initial Leasehold or Acquisition Investment	2011
NEW YORK | Accumulated Depreciation And Amortization Description [Member] | Staten Island Two [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 390
Cost Capitalized Subsequent to Initial Investment	89
Land	254
Building and Improvements	225
Total Investment in Real Estate	479
Accumulated Depreciation and Amortization	$ 197
Date of Initial Leasehold or Acquisition Investment	1985
NEW YORK | Accumulated Depreciation And Amortization Description [Member] | St Albans One [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 330
Cost Capitalized Subsequent to Initial Investment	106
Land	215
Building and Improvements	221
Total Investment in Real Estate	436
Accumulated Depreciation and Amortization	$ 174
Date of Initial Leasehold or Acquisition Investment	1985
NEW YORK | Accumulated Depreciation And Amortization Description [Member] | Staten Island Three [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 301
Cost Capitalized Subsequent to Initial Investment	323
Land	196
Building and Improvements	428
Total Investment in Real Estate	624
Accumulated Depreciation and Amortization	$ 235
Date of Initial Leasehold or Acquisition Investment	1985
NEW YORK | Accumulated Depreciation And Amortization Description [Member] | Staten Island Four [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	290
Land	228
Building and Improvements	412
Total Investment in Real Estate	640
Accumulated Depreciation and Amortization	$ 215
Date of Initial Leasehold or Acquisition Investment	1985
NEW YORK | Accumulated Depreciation And Amortization Description [Member] | Stony Brook One [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 176
Cost Capitalized Subsequent to Initial Investment	281
Land	105
Building and Improvements	352
Total Investment in Real Estate	457
Accumulated Depreciation and Amortization	$ 186
Date of Initial Leasehold or Acquisition Investment	1978
NEW YORK | Accumulated Depreciation And Amortization Description [Member] | Tarrytown One [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 956
Land	956
Total Investment in Real Estate	$ 956
Date of Initial Leasehold or Acquisition Investment	2011
NEW YORK | Accumulated Depreciation And Amortization Description [Member] | Tuchahoe One [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 1,650
Land	1,650
Total Investment in Real Estate	$ 1,650
Date of Initial Leasehold or Acquisition Investment	2011
NEW YORK | Accumulated Depreciation And Amortization Description [Member] | Wantagh One [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 640
Land	370
Building and Improvements	270
Total Investment in Real Estate	640
Accumulated Depreciation and Amortization	$ 209
Date of Initial Leasehold or Acquisition Investment	1998
NEW YORK | Accumulated Depreciation And Amortization Description [Member] | Wappingers Falls One [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 452
Building and Improvements	452
Total Investment in Real Estate	452
Accumulated Depreciation and Amortization	$ 282
Date of Initial Leasehold or Acquisition Investment	2011
NEW YORK | Accumulated Depreciation And Amortization Description [Member] | Wappingers Falls Two [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 1,488
Land	1,488
Total Investment in Real Estate	$ 1,488
Date of Initial Leasehold or Acquisition Investment	2011
NEW YORK | Accumulated Depreciation And Amortization Description [Member] | Warsaw One [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 990
Land	690
Building and Improvements	300
Total Investment in Real Estate	990
Accumulated Depreciation and Amortization	$ 142
Date of Initial Leasehold or Acquisition Investment	2006
NEW YORK | Accumulated Depreciation And Amortization Description [Member] | Warwick One [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 1,049
Land	1,049
Total Investment in Real Estate	$ 1,049
Date of Initial Leasehold or Acquisition Investment	2011
NEW YORK | Accumulated Depreciation And Amortization Description [Member] | West Nyack One [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 936
Land	936
Total Investment in Real Estate	$ 936
Date of Initial Leasehold or Acquisition Investment	2011
NEW YORK | Accumulated Depreciation And Amortization Description [Member] | West Taghkanic One [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 203
Cost Capitalized Subsequent to Initial Investment	486
Land	122
Building and Improvements	567
Total Investment in Real Estate	689
Accumulated Depreciation and Amortization	$ 342
Date of Initial Leasehold or Acquisition Investment	1986
NEW YORK | Accumulated Depreciation And Amortization Description [Member] | White Plains One [Member]
Schedule Of Investments In Real Estate [Line Items]
Cost Capitalized Subsequent to Initial Investment	$ 569
Land	303
Building and Improvements	266
Total Investment in Real Estate	569
Accumulated Depreciation and Amortization	$ 193
Date of Initial Leasehold or Acquisition Investment	1972
NEW YORK | Accumulated Depreciation And Amortization Description [Member] | White Plains Two [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 1,458
Land	1,458
Total Investment in Real Estate	$ 1,458
Date of Initial Leasehold or Acquisition Investment	2011
NEW YORK | Accumulated Depreciation And Amortization Description [Member] | Yaphank One [Member]
Schedule Of Investments In Real Estate [Line Items]
Cost Capitalized Subsequent to Initial Investment	$ 798
Land	375
Building and Improvements	423
Total Investment in Real Estate	798
Accumulated Depreciation and Amortization	$ 143
Date of Initial Leasehold or Acquisition Investment	1993
NEW YORK | Accumulated Depreciation And Amortization Description [Member] | Yonkers One [Member]
Schedule Of Investments In Real Estate [Line Items]
Cost Capitalized Subsequent to Initial Investment	$ 590
Building and Improvements	590
Total Investment in Real Estate	590
Accumulated Depreciation and Amortization	$ 298
Date of Initial Leasehold or Acquisition Investment	1970
NEW YORK | Accumulated Depreciation And Amortization Description [Member] | Yorktown Heights Two [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 2,365
Land	2,365
Total Investment in Real Estate	$ 2,365
Date of Initial Leasehold or Acquisition Investment	2011
NEW YORK | Accumulated Depreciation And Amortization Description [Member] | Yonkers Two [Member]
Schedule Of Investments In Real Estate [Line Items]
Cost Capitalized Subsequent to Initial Investment	$ 944
Land	684
Building and Improvements	260
Total Investment in Real Estate	944
Accumulated Depreciation and Amortization	$ 98
Date of Initial Leasehold or Acquisition Investment	1990
NEW YORK | Accumulated Depreciation And Amortization Description [Member] | Yonkers Three [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 1,021
Cost Capitalized Subsequent to Initial Investment	63
Land	665
Building and Improvements	419
Total Investment in Real Estate	1,084
Accumulated Depreciation and Amortization	$ 347
Date of Initial Leasehold or Acquisition Investment	1985
NEW YORK | Accumulated Depreciation And Amortization Description [Member] | Yonkers Four [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 291
Cost Capitalized Subsequent to Initial Investment	1,050
Land	216
Building and Improvements	1,125
Total Investment in Real Estate	1,341
Accumulated Depreciation and Amortization	$ 453
Date of Initial Leasehold or Acquisition Investment	1972
NEW YORK | Accumulated Depreciation And Amortization Description [Member] | Yonkers Five [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 1,907
Land	1,907
Total Investment in Real Estate	$ 1,907
Date of Initial Leasehold or Acquisition Investment	2011
NEW YORK | Accumulated Depreciation And Amortization Description [Member] | Yorktown Heights One [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 1,700
Building and Improvements	1,700
Total Investment in Real Estate	1,700
Accumulated Depreciation and Amortization	$ 161
Date of Initial Leasehold or Acquisition Investment	2013
OHIO | Accumulated Depreciation And Amortization Description [Member] | Akron One [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 1,530
Land	385
Building and Improvements	1,145
Total Investment in Real Estate	1,530
Accumulated Depreciation and Amortization	$ 43
Date of Initial Leasehold or Acquisition Investment	2017
OHIO | Accumulated Depreciation And Amortization Description [Member] | Loveland One [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 1,045
Land	362
Building and Improvements	683
Total Investment in Real Estate	1,045
Accumulated Depreciation and Amortization	$ 29
Date of Initial Leasehold or Acquisition Investment	2017
OHIO | Accumulated Depreciation And Amortization Description [Member] | Crestline One [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 1,202
Land	285
Building and Improvements	917
Total Investment in Real Estate	1,202
Accumulated Depreciation and Amortization	$ 422
Date of Initial Leasehold or Acquisition Investment	2008
OHIO | Accumulated Depreciation And Amortization Description [Member] | Mansfield One [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 922
Land	332
Building and Improvements	590
Total Investment in Real Estate	922
Accumulated Depreciation and Amortization	$ 255
Date of Initial Leasehold or Acquisition Investment	2008
OHIO | Accumulated Depreciation And Amortization Description [Member] | Mansfield Two [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 1,950
Land	700
Building and Improvements	1,250
Total Investment in Real Estate	1,950
Accumulated Depreciation and Amortization	$ 528
Date of Initial Leasehold or Acquisition Investment	2009
OHIO | Accumulated Depreciation And Amortization Description [Member] | Monroeville One [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 2,580
Land	485
Building and Improvements	2,095
Total Investment in Real Estate	2,580
Accumulated Depreciation and Amortization	$ 867
Date of Initial Leasehold or Acquisition Investment	2009
OREGON | Accumulated Depreciation And Amortization Description [Member] | McMinnville One [Member]
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	$ 77
Date of Initial Leasehold or Acquisition Investment	2017
OREGON | Accumulated Depreciation And Amortization Description [Member] | Banks One [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 498
Land	498
Total Investment in Real Estate	$ 498
Date of Initial Leasehold or Acquisition Investment	2015
OREGON | Accumulated Depreciation And Amortization Description [Member] | Estacada One [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 646
Land	84
Building and Improvements	562
Total Investment in Real Estate	646
Accumulated Depreciation and Amortization	$ 79
Date of Initial Leasehold or Acquisition Investment	2015
OREGON | Accumulated Depreciation And Amortization Description [Member] | Pendleton One [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 766
Land	122
Building and Improvements	644
Total Investment in Real Estate	766
Accumulated Depreciation and Amortization	$ 100
Date of Initial Leasehold or Acquisition Investment	2015
OREGON | Accumulated Depreciation And Amortization Description [Member] | Portland One [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 4,416
Land	3,368
Building and Improvements	1,048
Total Investment in Real Estate	4,416
Accumulated Depreciation and Amortization	$ 154
Date of Initial Leasehold or Acquisition Investment	2015