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Investments in Unconsolidated Joint Ventures - Additional Information (Details) $ in Thousands |  | 1 Months Ended | 2 Months Ended | 3 Months Ended | 9 Months Ended |
--- | --- | --- | --- | --- | --- | ---
Jan. 01, 2018 USD ($) | Feb. 28, 2018 | Nov. 30, 2017 | Oct. 31, 2016 | Mar. 31, 2018 property | Sep. 30, 2018 USD ($) ft² unit | Sep. 30, 2017 USD ($) | Sep. 30, 2018 USD ($) ft² unit | Sep. 30, 2017 USD ($) | Dec. 31, 2017 USD ($)
General information on each joint venture |  |  |  |  |  |  |  |  |  |
Equity method investments |  |  |  |  |  | $ 3,070,825 |  | $ 3,070,825 |  | $ 2,362,989
Net equity investment in VIEs in which the entity is not primary beneficiary |  |  |  |  |  | $ 722,400 |  | $ 722,400 |  | 606,200
Unaudited Approximate Square Feet (sqft) | ft² |  |  |  |  |  | 30,584,045 |  | 30,584,045 |  |
Purchase price and other fair value adjustments |  |  |  |  |  | $ (3,057) | $ 0 | $ 57,385 | $ 0 |
100 Park Avenue |  |  |  |  |  |  |  |  |  |
General information on each joint venture |  |  |  |  |  |  |  |  |  |
Ownership Interest |  |  |  |  |  | 49.90% |  | 49.90% |  |
Unaudited Approximate Square Feet (sqft) | ft² |  |  |  |  |  | 834,000 |  | 834,000 |  |
Acquisition Price |  |  |  |  |  | $ 95,800 |  | $ 95,800 |  |
717 Fifth Avenue |  |  |  |  |  |  |  |  |  |
General information on each joint venture |  |  |  |  |  |  |  |  |  |
Ownership Interest |  |  |  |  |  | 10.92% |  | 10.92% |  |
Unaudited Approximate Square Feet (sqft) | ft² |  |  |  |  |  | 119,500 |  | 119,500 |  |
Acquisition Price |  |  |  |  |  | $ 251,900 |  | $ 251,900 |  |
800 Third Avenue |  |  |  |  |  |  |  |  |  |
General information on each joint venture |  |  |  |  |  |  |  |  |  |
Ownership Interest |  |  |  |  |  | 60.52% |  | 60.52% |  |
Unaudited Approximate Square Feet (sqft) | ft² |  |  |  |  |  | 526,000 |  | 526,000 |  |
Acquisition Price |  |  |  |  |  | $ 285,000 |  | $ 285,000 |  |
11 West 34th Street |  |  |  |  |  |  |  |  |  |
General information on each joint venture |  |  |  |  |  |  |  |  |  |
Ownership Interest |  |  |  |  |  | 30.00% |  | 30.00% |  |
Unaudited Approximate Square Feet (sqft) | ft² |  |  |  |  |  | 17,150 |  | 17,150 |  |
Acquisition Price |  |  |  |  |  | $ 10,800 |  | $ 10,800 |  |
3 Columbus Circle |  |  |  |  |  |  |  |  |  |
General information on each joint venture |  |  |  |  |  |  |  |  |  |
Ownership Interest |  |  |  |  |  | 48.90% |  | 48.90% |  |
Unaudited Approximate Square Feet (sqft) | ft² |  |  |  |  |  | 741,500 |  | 741,500 |  |
Acquisition Price |  |  |  |  |  | $ 500,000 |  | $ 500,000 |  |
280 Park Avenue |  |  |  |  |  |  |  |  |  |
General information on each joint venture |  |  |  |  |  |  |  |  |  |
Ownership Interest |  |  |  |  |  | 50.00% |  | 50.00% |  |
Unaudited Approximate Square Feet (sqft) | ft² |  |  |  |  |  | 1,219,158 |  | 1,219,158 |  |
Acquisition Price |  |  |  |  |  | $ 400,000 |  | $ 400,000 |  |
1552-1560 Broadway |  |  |  |  |  |  |  |  |  |
General information on each joint venture |  |  |  |  |  |  |  |  |  |
Ownership Interest |  |  |  |  |  | 50.00% |  | 50.00% |  |
Unaudited Approximate Square Feet (sqft) | ft² |  |  |  |  |  | 57,718 |  | 57,718 |  |
Acquisition Price |  |  |  |  |  | $ 136,550 |  | $ 136,550 |  |
10 East 53rd Street |  |  |  |  |  |  |  |  |  |
General information on each joint venture |  |  |  |  |  |  |  |  |  |
Ownership Interest |  |  |  |  |  | 55.00% |  | 55.00% |  |
Unaudited Approximate Square Feet (sqft) | ft² |  |  |  |  |  | 354,300 |  | 354,300 |  |
Acquisition Price |  |  |  |  |  | $ 252,500 |  | $ 252,500 |  |
521 Fifth Avenue |  |  |  |  |  |  |  |  |  |
General information on each joint venture |  |  |  |  |  |  |  |  |  |
Ownership Interest |  |  |  |  |  | 50.50% |  | 50.50% |  |
Unaudited Approximate Square Feet (sqft) | ft² |  |  |  |  |  | 460,000 |  | 460,000 |  |
Acquisition Price |  |  |  |  |  | $ 315,000 |  | $ 315,000 |  |
21 East 66th Street |  |  |  |  |  |  |  |  |  |
General information on each joint venture |  |  |  |  |  |  |  |  |  |
Ownership Interest |  |  |  |  |  | 32.28% |  | 32.28% |  |
Unaudited Approximate Square Feet (sqft) | ft² |  |  |  |  |  | 13,069 |  | 13,069 |  |
Acquisition Price |  |  |  |  |  | $ 75,000 |  | $ 75,000 |  |
650 Fifth Avenue |  |  |  |  |  |  |  |  |  |
General information on each joint venture |  |  |  |  |  |  |  |  |  |
Ownership Interest |  |  |  |  |  | 50.00% |  | 50.00% |  |
Unaudited Approximate Square Feet (sqft) | ft² |  |  |  |  |  | 69,214 |  | 69,214 |  |
Acquisition Price |  |  |  |  |  | $ 0 |  | $ 0 |  |
121 Greene Street |  |  |  |  |  |  |  |  |  |
General information on each joint venture |  |  |  |  |  |  |  |  |  |
Ownership Interest |  |  |  |  |  | 50.00% |  | 50.00% |  |
Unaudited Approximate Square Feet (sqft) | ft² |  |  |  |  |  | 7,131 |  | 7,131 |  |
Acquisition Price |  |  |  |  |  | $ 27,400 |  | $ 27,400 |  |
55 West 46th Street |  |  |  |  |  |  |  |  |  |
General information on each joint venture |  |  |  |  |  |  |  |  |  |
Ownership Interest |  |  |  |  |  | 25.00% |  | 25.00% |  |
Unaudited Approximate Square Feet (sqft) | ft² |  |  |  |  |  | 347,000 |  | 347,000 |  |
Acquisition Price |  |  |  |  |  | $ 295,000 |  | $ 295,000 |  |
Stonehenge Portfolio |  |  |  |  |  |  |  |  |  |
General information on each joint venture |  |  |  |  |  |  |  |  |  |
Ownership Interest |  |  |  | 90.00% |  |  |  |  |  |
Unaudited Approximate Square Feet (sqft) | ft² |  |  |  |  |  | 1,439,016 |  | 1,439,016 |  |
Acquisition Price |  |  |  |  |  | $ 36,668 |  | $ 36,668 |  |
Number of properties sold | property |  |  |  |  | 2 |  |  |  |  |
131-137 Spring Street |  |  |  |  |  |  |  |  |  |
General information on each joint venture |  |  |  |  |  |  |  |  |  |
Ownership Interest |  |  |  |  |  | 20.00% |  | 20.00% |  |
Unaudited Approximate Square Feet (sqft) | ft² |  |  |  |  |  | 68,342 |  | 68,342 |  |
Acquisition Price |  |  |  |  |  | $ 277,750 |  | $ 277,750 |  |
605 West 42nd Street |  |  |  |  |  |  |  |  |  |
General information on each joint venture |  |  |  |  |  |  |  |  |  |
Ownership Interest |  |  |  |  |  | 20.00% |  | 20.00% |  |
Unaudited Approximate Square Feet (sqft) | ft² |  |  |  |  |  | 927,358 |  | 927,358 |  |
Acquisition Price |  |  |  |  |  | $ 759,000 |  | $ 759,000 |  |
11 Madison Avenue |  |  |  |  |  |  |  |  |  |
General information on each joint venture |  |  |  |  |  |  |  |  |  |
Ownership Interest |  |  |  |  |  | 60.00% |  | 60.00% |  |
Unaudited Approximate Square Feet (sqft) | ft² |  |  |  |  |  | 2,314,000 |  | 2,314,000 |  |
Acquisition Price |  |  |  |  |  | $ 2,605,000 |  | $ 2,605,000 |  |
333 East 22nd St |  |  |  |  |  |  |  |  |  |
General information on each joint venture |  |  |  |  |  |  |  |  |  |
Ownership Interest |  |  |  |  |  | 33.33% |  | 33.33% |  |
Economic Interest (as a percent) |  |  |  |  |  | 33.33% |  | 33.33% |  |
Unaudited Approximate Square Feet (sqft) | ft² |  |  |  |  |  | 26,926 |  | 26,926 |  |
Acquisition Price |  |  |  |  |  | $ 0 |  | $ 0 |  |
East 400 Street 57 |  |  |  |  |  |  |  |  |  |
General information on each joint venture |  |  |  |  |  |  |  |  |  |
Ownership Interest |  |  |  | 51.00% |  | 51.00% |  | 51.00% |  |
Unaudited Approximate Square Feet (sqft) | ft² |  |  |  |  |  | 290,482 |  | 290,482 |  |
Acquisition Price |  |  |  |  |  | $ 170,000 |  | $ 170,000 |  |
1552 Broadway |  |  |  |  |  |  |  |  |  |
General information on each joint venture |  |  |  |  |  |  |  |  |  |
Unaudited Approximate Square Feet (sqft) | ft² |  |  |  |  |  | 13,045 |  | 13,045 |  |
One Vanderbilt |  |  |  |  |  |  |  |  |  |
General information on each joint venture |  |  |  |  |  |  |  |  |  |
Ownership Interest |  |  |  |  |  | 71.01% |  | 71.01% |  |
Unaudited Approximate Square Feet (sqft) | ft² |  |  |  |  |  | 0 |  | 0 |  |
Acquisition Price |  |  |  |  |  | $ 3,310,000 |  | $ 3,310,000 |  |
Aggregate equity committed by partners |  |  |  |  |  | $ 525,000 |  | $ 525,000 |  |
Maximum ownership percentage of partners |  |  |  |  |  | 29.00% |  | 29.00% |  |
Partners' ownership percentage |  |  |  |  |  | 17.10% |  | 17.10% |  |
Worldwide Plaza |  |  |  |  |  |  |  |  |  |
General information on each joint venture |  |  |  |  |  |  |  |  |  |
Ownership Interest |  |  |  |  |  | 24.35% |  | 24.35% |  |
Economic Interest (as a percent) |  |  |  |  |  | 24.35% |  | 24.35% |  |
Unaudited Approximate Square Feet (sqft) | ft² |  |  |  |  |  | 2,048,725 |  | 2,048,725 |  |
Acquisition Price |  |  |  |  |  | $ 1,725,000 |  | $ 1,725,000 |  |
1515 Broadway |  |  |  |  |  |  |  |  |  |
General information on each joint venture |  |  |  |  |  |  |  |  |  |
Ownership Interest |  |  |  |  |  | 56.87% |  | 56.87% |  |
Economic Interest (as a percent) |  |  |  |  |  | 56.87% |  | 56.87% |  |
Unaudited Approximate Square Feet (sqft) | ft² |  |  |  |  |  | 1,750,000 |  | 1,750,000 |  |
Acquisition Price |  |  |  |  |  | $ 1,950,000 |  | $ 1,950,000 |  |
Cumulative effect on retained earnings | $ 600,000 |  |  |  |  |  |  |  |  |
East 400 Street 57 |  |  |  |  |  |  |  |  |  |
General information on each joint venture |  |  |  |  |  |  |  |  |  |
Ownership percentage in disposed asset |  |  |  | 49.00% |  |  |  |  |  |
21 East 66th Street | Three Retail and Two Residential Units |  |  |  |  |  |  |  |  |  |
General information on each joint venture |  |  |  |  |  |  |  |  |  |
Ownership Interest |  |  |  |  |  | 3228.00% |  | 3228.00% |  |
Number of stores | unit |  |  |  |  |  | 3 |  | 3 |  |
Number of residential units | unit |  |  |  |  |  | 2 |  | 2 |  |
21 East 66th Street | Three Residential Units |  |  |  |  |  |  |  |  |  |
General information on each joint venture |  |  |  |  |  |  |  |  |  |
Ownership Interest |  |  |  |  |  | 1614.00% |  | 1614.00% |  |
Number of residential units | unit |  |  |  |  |  | 3 |  | 3 |  |
Allianz Real Estate | 1515 Broadway |  |  |  |  |  |  |  |  |  |
General information on each joint venture |  |  |  |  |  |  |  |  |  |
Ownership percentage in disposed asset |  | 13.00% | 30.00% |  |  |  |  |  |  |
Joint venture |  |  |  |  |  |  |  |  |  |
General information on each joint venture |  |  |  |  |  |  |  |  |  |
Equity method investments |  |  |  |  |  | $ 3,070,825 |  | $ 3,070,825 |  | $ 2,362,989
Equity method investments with negative book value |  |  |  |  |  | $ 81,900 |  | $ 81,900 |  |
Joint venture | 100 Park Avenue |  |  |  |  |  |  |  |  |  |
General information on each joint venture |  |  |  |  |  |  |  |  |  |
Economic Interest (as a percent) |  |  |  |  |  | 49.90% |  | 49.90% |  |
Joint venture | 717 Fifth Avenue |  |  |  |  |  |  |  |  |  |
General information on each joint venture |  |  |  |  |  |  |  |  |  |
Economic Interest (as a percent) |  |  |  |  |  | 10.92% |  | 10.92% |  |
Joint venture | 800 Third Avenue |  |  |  |  |  |  |  |  |  |
General information on each joint venture |  |  |  |  |  |  |  |  |  |
Economic Interest (as a percent) |  |  |  |  |  | 60.52% |  | 60.52% |  |
Joint venture | 919 Third Avenue |  |  |  |  |  |  |  |  |  |
General information on each joint venture |  |  |  |  |  |  |  |  |  |
Ownership Interest |  |  |  |  |  | 51.00% |  | 51.00% |  |
Economic Interest (as a percent) |  |  |  |  |  | 51.00% |  | 51.00% |  |
Unaudited Approximate Square Feet (sqft) | ft² |  |  |  |  |  | 1,454,000 |  | 1,454,000 |  |
Acquisition Price |  |  |  |  |  | $ 1,256,727 |  | $ 1,256,727 |  |
Joint venture | 11 West 34th Street |  |  |  |  |  |  |  |  |  |
General information on each joint venture |  |  |  |  |  |  |  |  |  |
Economic Interest (as a percent) |  |  |  |  |  | 30.00% |  | 30.00% |  |
Joint venture | 3 Columbus Circle |  |  |  |  |  |  |  |  |  |
General information on each joint venture |  |  |  |  |  |  |  |  |  |
Economic Interest (as a percent) |  |  |  |  |  | 48.90% |  | 48.90% |  |
Joint venture | 280 Park Avenue |  |  |  |  |  |  |  |  |  |
General information on each joint venture |  |  |  |  |  |  |  |  |  |
Economic Interest (as a percent) |  |  |  |  |  | 50.00% |  | 50.00% |  |
Joint venture | 1552-1560 Broadway |  |  |  |  |  |  |  |  |  |
General information on each joint venture |  |  |  |  |  |  |  |  |  |
Economic Interest (as a percent) |  |  |  |  |  | 50.00% |  | 50.00% |  |
Joint venture | 10 East 53rd Street |  |  |  |  |  |  |  |  |  |
General information on each joint venture |  |  |  |  |  |  |  |  |  |
Economic Interest (as a percent) |  |  |  |  |  | 55.00% |  | 55.00% |  |
Joint venture | 521 Fifth Avenue |  |  |  |  |  |  |  |  |  |
General information on each joint venture |  |  |  |  |  |  |  |  |  |
Economic Interest (as a percent) |  |  |  |  |  | 50.50% |  | 50.50% |  |
Joint venture | 21 East 66th Street |  |  |  |  |  |  |  |  |  |
General information on each joint venture |  |  |  |  |  |  |  |  |  |
Economic Interest (as a percent) |  |  |  |  |  | 32.28% |  | 32.28% |  |
Joint venture | 650 Fifth Avenue |  |  |  |  |  |  |  |  |  |
General information on each joint venture |  |  |  |  |  |  |  |  |  |
Economic Interest (as a percent) |  |  |  |  |  | 50.00% |  | 50.00% |  |
Joint venture | 121 Greene Street |  |  |  |  |  |  |  |  |  |
General information on each joint venture |  |  |  |  |  |  |  |  |  |
Economic Interest (as a percent) |  |  |  |  |  | 50.00% |  | 50.00% |  |
Joint venture | 55 West 46th Street |  |  |  |  |  |  |  |  |  |
General information on each joint venture |  |  |  |  |  |  |  |  |  |
Economic Interest (as a percent) |  |  |  |  |  | 25.00% |  | 25.00% |  |
Joint venture | Stonehenge Portfolio |  |  |  |  |  |  |  |  |  |
General information on each joint venture |  |  |  |  |  |  |  |  |  |
Ownership Interest |  |  |  | 10.00% |  |  |  |  |  |
Joint venture | 131-137 Spring Street |  |  |  |  |  |  |  |  |  |
General information on each joint venture |  |  |  |  |  |  |  |  |  |
Economic Interest (as a percent) |  |  |  |  |  | 20.00% |  | 20.00% |  |
Joint venture | 605 West 42nd Street |  |  |  |  |  |  |  |  |  |
General information on each joint venture |  |  |  |  |  |  |  |  |  |
Economic Interest (as a percent) |  |  |  |  |  | 20.00% |  | 20.00% |  |
Joint venture | 11 Madison Avenue |  |  |  |  |  |  |  |  |  |
General information on each joint venture |  |  |  |  |  |  |  |  |  |
Economic Interest (as a percent) |  |  |  |  |  | 60.00% |  | 60.00% |  |
Joint venture | East 400 Street 57 |  |  |  |  |  |  |  |  |  |
General information on each joint venture |  |  |  |  |  |  |  |  |  |
Economic Interest (as a percent) |  |  |  |  |  | 41.00% |  | 41.00% |  |
Joint venture | One Vanderbilt |  |  |  |  |  |  |  |  |  |
General information on each joint venture |  |  |  |  |  |  |  |  |  |
Economic Interest (as a percent) |  |  |  |  |  | 71.01% |  | 71.01% |  |

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