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Leases (Narratives) (Details) | 3 Months Ended | 12 Months Ended |
--- | --- | --- | ---
Dec. 31, 2018 USD ($) option | Sep. 30, 2018 USD ($) | Jun. 30, 2018 USD ($) | Mar. 31, 2018 USD ($) | Dec. 31, 2017 USD ($) | Sep. 30, 2017 USD ($) | Jun. 30, 2017 USD ($) | Mar. 31, 2017 USD ($) | Dec. 31, 2018 USD ($) option | Dec. 31, 2017 USD ($) | Dec. 31, 2016 USD ($) | Jan. 01, 2019 USD ($)
As Lessor |  |  |  |  |  |  |  |  |  |  |  |
Additional rent based on percentage of tenant's sales |  |  |  |  |  |  |  |  | $ 234,000 | $ 174,000 | $ 182,000 |
Revenues | $ 57,567,000 | $ 59,125,000 | $ 58,253,000 | $ 57,880,000 | $ 58,061,000 | $ 58,094,000 | $ 57,190,000 | $ 57,229,000 | $ 232,825,000 | 230,574,000 | 226,936,000 |
Minimum |  |  |  |  |  |  |  |  |  |  |  |
Leases Lease Terms |  |  |  |  |  |  |  |  |  |  |  |
Lease term range as Lessor |  |  |  |  |  |  |  |  | 5 years |  |  |
Maximum |  |  |  |  |  |  |  |  |  |  |  |
Leases Lease Terms |  |  |  |  |  |  |  |  |  |  |  |
Lease term range as Lessor |  |  |  |  |  |  |  |  | 25 years |  |  |
Alexander apartment tower | Residence space | Minimum |  |  |  |  |  |  |  |  |  |  |  |
Leases Lease Terms |  |  |  |  |  |  |  |  |  |  |  |
Lease term range as Lessor |  |  |  |  |  |  |  |  | 1 year |  |  |
Alexander apartment tower | Residence space | Maximum |  |  |  |  |  |  |  |  |  |  |  |
Leases Lease Terms |  |  |  |  |  |  |  |  |  |  |  |
Lease term range as Lessor |  |  |  |  |  |  |  |  | 2 years |  |  |
Flushing property |  |  |  |  |  |  |  |  |  |  |  |
As Lessee |  |  |  |  |  |  |  |  |  |  |  |
Rent expense |  |  |  |  |  |  |  |  | $ 746,000 | 746,000 | 746,000 |
Flushing property | Ten year extension option |  |  |  |  |  |  |  |  |  |  |  |
As Lessee |  |  |  |  |  |  |  |  |  |  |  |
Lease extension period |  |  |  |  |  |  |  |  | 10 years |  |  |
Number of extension options | option | 1 |  |  |  |  |  |  |  | 1 |  |  |
Customer Concentration Risk | Bloomberg | Sales Revenue Services Net |  |  |  |  |  |  |  |  |  |  |  |
As Lessor |  |  |  |  |  |  |  |  |  |  |  |
Revenues |  |  |  |  |  |  |  |  | $ 107,356,000 | $ 105,224,000 | $ 104,590,000 |
Percentage rent contributed by tenant |  |  |  |  |  |  |  |  | 46.00% | 46.00% | 46.00% |
Subsequent Event | Accounting Standards Update 2016-02 |  |  |  |  |  |  |  |  |  |  |  |
As Lessee |  |  |  |  |  |  |  |  |  |  |  |
Finance lease, right-of-use asset |  |  |  |  |  |  |  |  |  |  |  | $ 5,400,000
Finance lease, liability |  |  |  |  |  |  |  |  |  |  |  | $ 5,400,000

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