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Mortgages Payable (Outstanding mortgages payable) (Details) - USD ($) $ in Thousands |  | 5 Months Ended | 12 Months Ended |
--- | --- | --- | --- | ---
Jun. 01, 2017 | May 31, 2017 | Dec. 31, 2017 | Dec. 31, 2016
Mortgage Loans on Real Estate |  |  |  |
Percentage of cash mortgage collateralized |  |  | 100.00% |
Deferred debt issuance costs, net of accumulated amortization of $6,315 and $6,824, respectively |  |  | $ (12,218) |
Notes Payable (in US dollars) |  |  | 1,240,222 | $ 1,052,359
Mortgages | 731 Lexington Avenue | Office space |  |  |  |
Mortgage Loans on Real Estate |  |  |  |
Maturity date | Jun. 01, 2020 |  |  |
Notes Payable (in US dollars) | $ 500,000 | $ 300,000 |  |
Mortgages | 731 Lexington Avenue | Office space | LIBOR |  |  |  |
Mortgage Loans on Real Estate |  |  |  |
Basis spread over LIBOR | 0.90% | 0.95% |  |
Mortgages | Secured loan |  |  |  |
Mortgage Loans on Real Estate |  |  |  |
Notes payable, gross |  |  | 1,252,440 | 1,056,147
Deferred debt issuance costs, net of accumulated amortization of $6,315 and $6,824, respectively |  |  | (12,218) | (3,788)
Notes Payable (in US dollars) |  |  | 1,240,222 | 1,052,359
Deferred debt issuance costs, accumulated amortization |  |  | $ 6,315 | $ 6,824
Mortgages | Secured loan | Rego Park I |  |  |  |
Mortgage Loans on Real Estate |  |  |  |
Maturity date |  |  | Mar. 10, 2018 |
Interest rate |  |  | 0.35% |
Percentage of cash mortgage collateralized |  |  | 100.00% | 100.00%
Notes payable, gross |  |  | $ 78,246 | $ 78,246
Mortgages | Secured loan | Paramus |  |  |  |
Mortgage Loans on Real Estate |  |  |  |
Maturity date |  |  | Oct. 05, 2018 |
Interest rate |  |  | 2.90% |
Notes payable, gross |  |  | $ 68,000 | 68,000
Mortgages | Secured loan | Rego Park II |  |  |  |
Mortgage Loans on Real Estate |  |  |  |
Maturity date |  |  | Nov. 30, 2018 |
Interest rate |  |  | 3.42% |
Notes payable, gross |  |  | $ 256,194 | 259,901
Mortgages | Secured loan | Rego Park II | LIBOR |  |  |  |
Mortgage Loans on Real Estate |  |  |  |
Basis spread over LIBOR |  |  | 1.85% |
Mortgages | Secured loan | 731 Lexington Avenue | Retail space |  |  |  |
Mortgage Loans on Real Estate |  |  |  |
Maturity date |  |  | Aug. 05, 2022 |
Interest rate |  |  | 2.78% |
Notes payable, gross |  |  | $ 350,000 | 350,000
Mortgages | Secured loan | 731 Lexington Avenue | Retail space | LIBOR |  |  |  |
Mortgage Loans on Real Estate |  |  |  |
Basis spread over LIBOR |  |  | 1.40% |
Mortgages | Secured loan | 731 Lexington Avenue | Office space |  |  |  |
Mortgage Loans on Real Estate |  |  |  |
Maturity date |  |  | Jun. 11, 2024 |
Interest rate |  |  | 2.38% |
Notes payable, gross |  |  | $ 500,000 | $ 300,000
Mortgages | Secured loan | 731 Lexington Avenue | Office space | LIBOR |  |  |  |
Mortgage Loans on Real Estate |  |  |  |
Basis spread over LIBOR |  |  | 0.90% |

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