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Property Acquisitions - 2018 Acquisitions (Details) $ in Millions | 1 Months Ended |
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Dec. 31, 2018 USD ($) ft² | Nov. 30, 2018 ft² | Oct. 31, 2018 USD ($) ft² | Aug. 31, 2018 USD ($) a parcel | Jul. 31, 2018 USD ($) ft² | May 31, 2018 USD ($) ft² | Dec. 31, 2019 ft²
Business Acquisition [Line Items] |  |  |  |  |  |  |
Approximate Square Feet | ft² |  |  |  |  |  |  | 27,607,971
2 Herald Square |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |
Approximate Square Feet | ft² |  |  |  |  |  | 369,000 |
Acquisition Price (in millions) | $ |  |  |  |  |  | $ 266.0 |
Purchase price fair value adjustment | $ |  |  |  |  |  | $ 8.1 |
1231 Third Avenue |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |
Approximate Square Feet | ft² |  |  |  |  | 39,000 |  |
Acquisition Price (in millions) | $ |  |  |  |  | $ 55.4 |  |
Upper East Side Residential |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |
Approximate Square Feet | a |  |  |  | 0.2 |  |  |
Acquisition Price (in millions) | $ |  |  |  | $ 30.2 |  |  |
Number of land parcels, fee interest acquired | parcel |  |  |  | 3 |  |  |
113 Greene Street |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |
Approximate Square Feet | ft² |  |  | 6,425 |  |  |  |
Acquisition Price (in millions) | $ |  |  | $ 31.0 |  |  |  |
712 Madison Avenue |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |
Approximate Square Feet | ft² | 6,600 |  |  |  |  |  |
Acquisition Price (in millions) | $ | $ 58.0 |  |  |  |  |  |
2 Herald Square |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |
Approximate Square Feet | ft² |  | 369,000 |  |  |  |  |
Ownership percentage in disposed asset |  | 49.00% |  |  |  |  |

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