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Mortgages Payable (Narrative) (Details) - USD ($) |  | 12 Months Ended |  |  |  |
--- | --- | --- | --- | --- | --- | ---
Dec. 11, 2018 | Dec. 31, 2018 | Dec. 12, 2018 | Oct. 03, 2018 | Oct. 02, 2018 | Dec. 31, 2017
Debt Instrument [Line Items] |  |  |  |  |  |
Mortgages payable, net of deferred debt issuance costs |  | $ 1,161,534,000 |  |  |  | $ 1,240,222,000
Refinance date |  | May 11, 2018 |  |  |  |
Repayment date |  | Jun. 06, 2018 |  |  |  |
Paramus |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |
Interest rate |  |  |  |  | 2.90% |
Matures in October 2021 | Paramus |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |
Mortgages payable, net of deferred debt issuance costs |  |  |  | $ 68,000,000 |  |
Interest rate |  | 4.72% |  |  |  |
Matures in December 2025 | Rego Park II |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |
Mortgages payable, net of deferred debt issuance costs |  |  | $ 252,544,000 |  |  |
Interest rate |  | 3.87% |  |  |  |
Matures in December 2025 | Rego Park II | LIBOR |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |
Basis spread over LIBOR |  | 1.35% |  |  |  |
Matures in January 2019 | Rego Park II | LIBOR |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |
Basis spread over LIBOR | 1.85% |  |  |  |  |

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