Schedule III - Real Estate and Accumulated Depreciation and Amortization Part 15 (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] | Brewster One [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	789
Land	789
Total Investment in Real Estate	$ 789
Date of Initial Leasehold or Acquisition Investment	2011
NEW YORK | Accumulated Depreciation And Amortization Description [Member] | Briarcliff Manor One [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 652
Cost Capitalized Subsequent to Initial Investment	564
Land	502
Building and Improvements	714
Total Investment in Real Estate	1,216
Accumulated Depreciation and Amortization	$ 467
Date of Initial Leasehold or Acquisition Investment	1976
NEW YORK | Accumulated Depreciation And Amortization Description [Member] | Bronx One
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 423
Land	423
Total Investment in Real Estate	$ 423
Date of Initial Leasehold or Acquisition Investment	2013
NEW YORK | Accumulated Depreciation And Amortization Description [Member] | Bronx 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	54
Land	251
Building and Improvements	193
Total Investment in Real Estate	444
Accumulated Depreciation and Amortization	$ 165
Date of Initial Leasehold or Acquisition Investment	1985
NEW YORK | Accumulated Depreciation And Amortization Description [Member] | Bronx Three [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 877
Land	877
Total Investment in Real Estate	$ 877
Date of Initial Leasehold or Acquisition Investment	2013
NEW YORK | Accumulated Depreciation And Amortization Description [Member] | Bronx Four [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 884
Land	884
Total Investment in Real Estate	$ 884
Date of Initial Leasehold or Acquisition Investment	2013
NEW YORK | Accumulated Depreciation And Amortization Description [Member] | Bronx Five [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 953
Land	953
Total Investment in Real Estate	$ 953
Date of Initial Leasehold or Acquisition Investment	2013
NEW YORK | Accumulated Depreciation And Amortization Description [Member] | Bronx Six [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 1,049
Land	485
Building and Improvements	564
Total Investment in Real Estate	1,049
Accumulated Depreciation and Amortization	$ 152
Date of Initial Leasehold or Acquisition Investment	2013
NEW YORK | Accumulated Depreciation And Amortization Description [Member] | Bronx Seven [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 1,910
Land	1,349
Building and Improvements	561
Total Investment in Real Estate	1,910
Accumulated Depreciation and Amortization	$ 158
Date of Initial Leasehold or Acquisition Investment	2013
NEW YORK | Accumulated Depreciation And Amortization Description [Member] | Bronx Eight [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 2,408
Land	1,712
Building and Improvements	696
Total Investment in Real Estate	2,408
Accumulated Depreciation and Amortization	$ 176
Date of Initial Leasehold or Acquisition Investment	2013
NEW YORK | Accumulated Depreciation And Amortization Description [Member] | Bronxville [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 1,232
Land	1,232
Total Investment in Real Estate	$ 1,232
Date of Initial Leasehold or Acquisition Investment	2011
NEW YORK | Accumulated Depreciation And Amortization Description [Member] | Brooklyn One [Member]
Schedule Of Investments In Real Estate [Line Items]
Cost Capitalized Subsequent to Initial Investment	$ 396
Building and Improvements	396
Total Investment in Real Estate	396
Accumulated Depreciation and Amortization	$ 220
Date of Initial Leasehold or Acquisition Investment	1970
NEW YORK | Accumulated Depreciation And Amortization Description [Member] | Brooklyn Two [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 100
Cost Capitalized Subsequent to Initial Investment	345
Land	67
Building and Improvements	378
Total Investment in Real Estate	445
Accumulated Depreciation and Amortization	$ 202
Date of Initial Leasehold or Acquisition Investment	1972
NEW YORK | Accumulated Depreciation And Amortization Description [Member] | Brooklyn Three [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 76
Cost Capitalized Subsequent to Initial Investment	382
Land	31
Building and Improvements	427
Total Investment in Real Estate	458
Accumulated Depreciation and Amortization	$ 237
Date of Initial Leasehold or Acquisition Investment	1967
NEW YORK | Accumulated Depreciation And Amortization Description [Member] | Brooklyn Four [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 148
Cost Capitalized Subsequent to Initial Investment	388
Land	104
Building and Improvements	432
Total Investment in Real Estate	536
Accumulated Depreciation and Amortization	$ 257
Date of Initial Leasehold or Acquisition Investment	1972
NEW YORK | Accumulated Depreciation And Amortization Description [Member] | Brooklyn Five [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 236
Cost Capitalized Subsequent to Initial Investment	374
Land	154
Building and Improvements	456
Total Investment in Real Estate	610
Accumulated Depreciation and Amortization	$ 195
Date of Initial Leasehold or Acquisition Investment	1985
NEW YORK | Accumulated Depreciation And Amortization Description [Member] | Brooklyn Six
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 282
Cost Capitalized Subsequent to Initial Investment	457
Land	176
Building and Improvements	563
Total Investment in Real Estate	739
Accumulated Depreciation and Amortization	$ 387
Date of Initial Leasehold or Acquisition Investment	1967
NEW YORK | Accumulated Depreciation And Amortization Description [Member] | Brooklyn Seven [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 422
Cost Capitalized Subsequent to Initial Investment	334
Land	275
Building and Improvements	481
Total Investment in Real Estate	756
Accumulated Depreciation and Amortization	$ 279
Date of Initial Leasehold or Acquisition Investment	1985
NEW YORK | Accumulated Depreciation And Amortization Description [Member] | Brooklyn Eight [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 476
Cost Capitalized Subsequent to Initial Investment	320
Land	306
Building and Improvements	490
Total Investment in Real Estate	796
Accumulated Depreciation and Amortization	$ 285
Date of Initial Leasehold or Acquisition Investment	1985
NEW YORK | Accumulated Depreciation And Amortization Description [Member] | Brooklyn Nine [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 627
Cost Capitalized Subsequent to Initial Investment	313
Land	408
Building and Improvements	532
Total Investment in Real Estate	940
Accumulated Depreciation and Amortization	$ 308
Date of Initial Leasehold or Acquisition Investment	1985
NEW YORK | Accumulated Depreciation And Amortization Description [Member] | Buffalo [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 313
Cost Capitalized Subsequent to Initial Investment	241
Land	151
Building and Improvements	403
Total Investment in Real Estate	554
Accumulated Depreciation and Amortization	$ 198
Date of Initial Leasehold or Acquisition Investment	2000
NEW YORK | Accumulated Depreciation And Amortization Description [Member] | Byron [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 969
Land	669
Building and Improvements	300
Total Investment in Real Estate	969
Accumulated Depreciation and Amortization	$ 142
Date of Initial Leasehold or Acquisition Investment	2006
NEW YORK | Accumulated Depreciation And Amortization Description [Member] | Chester [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 1,158
Land	1,158
Total Investment in Real Estate	$ 1,158
Date of Initial Leasehold or Acquisition Investment	2011
NEW YORK | Accumulated Depreciation And Amortization Description [Member] | Dobbs Ferry One [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 670
Cost Capitalized Subsequent to Initial Investment	34
Land	434
Building and Improvements	270
Total Investment in Real Estate	704
Accumulated Depreciation and Amortization	$ 223
Date of Initial Leasehold or Acquisition Investment	1985
NEW YORK | Accumulated Depreciation And Amortization Description [Member] | Churchville [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 1,012
Land	602
Building and Improvements	410
Total Investment in Real Estate	1,012
Accumulated Depreciation and Amortization	$ 194
Date of Initial Leasehold or Acquisition Investment	2006
NEW YORK | Accumulated Depreciation And Amortization Description [Member] | Dobbs FerryTwo [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 1,345
Land	1,345
Total Investment in Real Estate	$ 1,345
Date of Initial Leasehold or Acquisition Investment	2011
NEW YORK | Accumulated Depreciation And Amortization Description [Member] | Corona One [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 115
Cost Capitalized Subsequent to Initial Investment	300
Land	113
Building and Improvements	302
Total Investment in Real Estate	415
Accumulated Depreciation and Amortization	$ 302
Date of Initial Leasehold or Acquisition Investment	1965
NEW YORK | Accumulated Depreciation And Amortization Description [Member] | East Hampton One [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 660
Cost Capitalized Subsequent to Initial Investment	39
Land	428
Building and Improvements	271
Total Investment in Real Estate	699
Accumulated Depreciation and Amortization	$ 224
Date of Initial Leasehold or Acquisition Investment	1985
NEW YORK | Accumulated Depreciation And Amortization Description [Member] | Corona Two [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 2,543
Land	1,903
Building and Improvements	640
Total Investment in Real Estate	2,543
Accumulated Depreciation and Amortization	$ 165
Date of Initial Leasehold or Acquisition Investment	2013
NEW YORK | Accumulated Depreciation And Amortization Description [Member] | Cortland Manor [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 1,872
Land	1,872
Total Investment in Real Estate	$ 1,872
Date of Initial Leasehold or Acquisition Investment	2011
NEW YORK | Accumulated Depreciation And Amortization Description [Member] | East Pembroke One [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 787
Land	537
Building and Improvements	250
Total Investment in Real Estate	787
Accumulated Depreciation and Amortization	$ 118
Date of Initial Leasehold or Acquisition Investment	2006
NEW YORK | Accumulated Depreciation And Amortization Description [Member] | Eastchester One [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 993
Land	578
Building and Improvements	415
Total Investment in Real Estate	993
Accumulated Depreciation and Amortization	$ 23
Date of Initial Leasehold or Acquisition Investment	2017
NEW YORK | Accumulated Depreciation And Amortization Description [Member] | Elmont One [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 389
Cost Capitalized Subsequent to Initial Investment	319
Land	231
Building and Improvements	477
Total Investment in Real Estate	708
Accumulated Depreciation and Amortization	$ 310
Date of Initial Leasehold or Acquisition Investment	1978
NEW YORK | Accumulated Depreciation And Amortization Description [Member] | Elmsford One [Member]
Schedule Of Investments In Real Estate [Line Items]
Cost Capitalized Subsequent to Initial Investment	$ 949
Land	581
Building and Improvements	368
Total Investment in Real Estate	949
Accumulated Depreciation and Amortization	$ 220
Date of Initial Leasehold or Acquisition Investment	1971
NEW YORK | Accumulated Depreciation And Amortization Description [Member] | Elmsford Two [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 1,453
Land	1,453
Total Investment in Real Estate	$ 1,453
Date of Initial Leasehold or Acquisition Investment	2011
NEW YORK | Accumulated Depreciation And Amortization Description [Member] | Fishkill One [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 1,793
Land	1,793
Total Investment in Real Estate	$ 1,793
Date of Initial Leasehold or Acquisition Investment	2011
NEW YORK | Accumulated Depreciation And Amortization Description [Member] | Eastchester Two
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 1,724
Land	1,724
Total Investment in Real Estate	$ 1,724
Date of Initial Leasehold or Acquisition Investment	2011
NEW YORK | Accumulated Depreciation And Amortization Description [Member] | Floral Park One [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 617
Cost Capitalized Subsequent to Initial Investment	170
Land	356
Building and Improvements	431
Total Investment in Real Estate	787
Accumulated Depreciation and Amortization	$ 247
Date of Initial Leasehold or Acquisition Investment	1998
NEW YORK | Accumulated Depreciation And Amortization Description [Member] | Flushing One [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 516
Cost Capitalized Subsequent to Initial Investment	241
Land	320
Building and Improvements	437
Total Investment in Real Estate	757
Accumulated Depreciation and Amortization	$ 226
Date of Initial Leasehold or Acquisition Investment	1998
NEW YORK | Accumulated Depreciation And Amortization Description [Member] | Flushing Two [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 1,936
Land	1,413
Building and Improvements	523
Total Investment in Real Estate	1,936
Accumulated Depreciation and Amortization	$ 140
Date of Initial Leasehold or Acquisition Investment	2013
NEW YORK | Accumulated Depreciation And Amortization Description [Member] | Flushing Three [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 1,947
Land	1,405
Building and Improvements	542
Total Investment in Real Estate	1,947
Accumulated Depreciation and Amortization	$ 134
Date of Initial Leasehold or Acquisition Investment	2013
NEW YORK | Accumulated Depreciation And Amortization Description [Member] | Flushing Four [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 2,478
Land	1,801
Building and Improvements	677
Total Investment in Real Estate	2,478
Accumulated Depreciation and Amortization	$ 167
Date of Initial Leasehold or Acquisition Investment	2013
NEW YORK | Accumulated Depreciation And Amortization Description [Member] | Forrest Hill One [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 1,273
Land	1,273
Total Investment in Real Estate	$ 1,273
Date of Initial Leasehold or Acquisition Investment	2013
NEW YORK | Accumulated Depreciation And Amortization Description [Member] | Franklin Square One [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 153
Cost Capitalized Subsequent to Initial Investment	331
Land	137
Building and Improvements	347
Total Investment in Real Estate	484
Accumulated Depreciation and Amortization	$ 170
Date of Initial Leasehold or Acquisition Investment	1978
NEW YORK | Accumulated Depreciation And Amortization Description [Member] | Friendship One [Member]
Schedule Of Investments In Real Estate [Line Items]
Initial Cost of Leasehold or Acquisition Investment to Company	$ 393
Land	43
Building and Improvements	350
Total Investment in Real Estate	393
Accumulated Depreciation and Amortization	$ 166
Date of Initial Leasehold or Acquisition Investment	2006