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Properties Held for Sale and Property Dispositions (Details) $ in Millions | 1 Months Ended | 3 Months Ended
--- | --- | ---
Jul. 31, 2018 USD ($) ft² | Jun. 30, 2018 USD ($) ft² | May 31, 2018 USD ($) ft² | Jan. 31, 2018 USD ($) ft² | Sep. 30, 2018 USD ($) ft²
Properties Sold [Abstract] |  |  |  |  |
Approximate Square Feet | ft² |  |  |  |  | 30,584,045
1231 Third Avenue |  |  |  |  |
Properties Sold [Abstract] |  |  |  |  |
Sales Price |  |  |  |  | $ 143.8
Upper East Side Residential |  |  |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |
Real estate held-for-sale |  |  |  |  | $ 6.7
600 Lexington Ave |  |  |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |
Employee-related liabilities |  |  |  | $ 1.3 |
Properties Sold [Abstract] |  |  |  |  |
Approximate Square Feet | ft² |  |  |  | 303,515 |
Sales Price |  |  |  | $ 305.0 |
Gain on Sale (Loss) |  |  |  | $ 23.8 |
115-117 Stevens Avenue |  |  |  |  |
Properties Sold [Abstract] |  |  |  |  |
Approximate Square Feet | ft² |  |  | 178,000 |  |
Sales Price |  |  | $ 12.0 |  |
Gain on Sale (Loss) |  |  | $ (0.7) |  |
635 Madison Avenue |  |  |  |  |
Properties Sold [Abstract] |  |  |  |  |
Approximate Square Feet | ft² |  | 176,530 |  |  |
Sales Price |  | $ 153.0 |  |  |
Gain on Sale (Loss) |  | $ (14.1) |  |  |
1-6 International Drive |  |  |  |  |
Properties Sold [Abstract] |  |  |  |  |
Approximate Square Feet | ft² | 540,000 |  |  |  |
Sales Price | $ 55.0 |  |  |  |
Gain on Sale (Loss) | $ (2.6) |  |  |  |

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