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Mortgages Payable (Narrative) (Details) |  | 5 Months Ended |  |
--- | --- | --- | --- | ---
Jun. 01, 2017 USD ($) option | May 31, 2017 USD ($) | Dec. 31, 2017 USD ($) | Dec. 31, 2016 USD ($)
Debt Instrument [Line Items] |  |  |  |
Mortgages payable, net of deferred debt issuance costs |  |  | $ 1,240,222,000 | $ 1,052,359,000
731 Lexington Avenue | Office space | Mortgages |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Mortgages payable, net of deferred debt issuance costs | $ 500,000,000 | $ 300,000,000 |  |
Maturity date | Jun. 01, 2020 |  |  |
Number of one year extension options | option | 4 |  |  |
Derivative liability, notional amount | $ 500,000,000 |  |  |
731 Lexington Avenue | Office space | Mortgages | LIBOR |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Basis spread over LIBOR | 0.90% | 0.95% |  |
Derivative, cap interest rate | 6.00% |  |  |

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- Definition Date when the debt instrument is scheduled to be fully repaid, in CCYY-MM-DD format. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.22(a)(2)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
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