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Investments in Unconsolidated Joint Ventures - Acquisition, Development and Construction Arrangements/Sale of Joint Venture Interest or Property (Details) - USD ($) $ in Thousands | 1 Months Ended | 3 Months Ended | 9 Months Ended |  |  |
--- | --- | --- | --- | --- | --- | ---
Jul. 31, 2018 | Jun. 30, 2018 | May 31, 2018 | Apr. 30, 2018 | Mar. 31, 2018 | Feb. 28, 2018 | Sep. 30, 2018 | Sep. 30, 2017 | Sep. 30, 2018 | Sep. 30, 2017 | Aug. 31, 2018 | Dec. 31, 2017 | Oct. 31, 2016
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Equity method investments |  |  |  |  |  |  | $ 3,070,825 |  | $ 3,070,825 |  |  | $ 2,362,989 |
Equity in net gain on sale of interest in unconsolidated joint venture/real estate |  |  |  |  |  |  | 70,937 | $ 1,030 | 136,522 | $ 16,166 |  |  |
Stonehenge Portfolio |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Ownership Interest |  |  |  |  |  |  |  |  |  |  |  |  | 90.00%
Preferred Equity due September 2018 |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Equity method investments |  |  |  |  |  |  | 145,007 |  | 145,007 |  |  | 0 |
Mezzanine loan due February 2022 |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Equity method investments |  |  |  |  |  |  | 44,420 |  | 44,420 |  |  | 44,823 |
Mezzanine loan due July 2036 |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Equity method investments |  |  |  |  |  |  | 0 |  | 0 |  |  | 26,716 |
Mezzanine Loan and Preferred Equity, Due March 2018 |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Equity method investments |  |  |  |  |  |  | 0 |  | 0 |  |  | 100,000 |
Mezzanine Loans And Preferred Equity |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Equity method investments |  |  |  |  |  |  | $ 189,427 |  | $ 189,427 |  |  | $ 171,539 |
1274 Fifth Avenue |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Ownership Interest |  |  |  |  |  | 9.83% |  |  |  |  |  |  |
Gross Asset Valuation |  |  |  |  |  | $ 44,100 |  |  |  |  |  |  |
Equity in net gain on sale of interest in unconsolidated joint venture/real estate |  |  |  |  |  | $ (362) |  |  |  |  |  |  |
1515 Broadway |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Ownership Interest |  |  |  |  |  | 13.00% |  |  |  |  |  |  |
Gross Asset Valuation |  |  |  |  |  | $ 1,950,000 |  |  |  |  |  |  |
Equity in net gain on sale of interest in unconsolidated joint venture/real estate |  |  |  |  |  | $ 0 |  |  |  |  |  |  |
Stonehenge Portfolio |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Ownership Interest |  |  |  |  | 5.00% |  |  |  |  |  |  |  |
Gross Asset Valuation |  |  |  |  | $ 287,000 |  |  |  |  |  |  |  |
Equity in net gain on sale of interest in unconsolidated joint venture/real estate |  |  |  |  | $ (5,701) |  |  |  |  |  |  |  |
175-225 Third Street Brooklyn, New York |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Ownership Interest |  |  |  | 95.00% |  |  |  |  |  |  |  |  |
Gross Asset Valuation |  |  |  | $ 115,000 |  |  |  |  |  |  |  |  |
Equity in net gain on sale of interest in unconsolidated joint venture/real estate |  |  |  | 19,483 |  |  |  |  |  |  |  |  |
Employee-related liabilities |  |  |  | $ 1,500 |  |  |  |  |  |  |  |  |
1745 Broadway |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Ownership Interest |  |  | 56.87% |  |  |  |  |  |  |  |  |  |
Gross Asset Valuation |  |  | $ 633,000 |  |  |  |  |  |  |  |  |  |
Equity in net gain on sale of interest in unconsolidated joint venture/real estate |  |  | 52,038 |  |  |  |  |  |  |  |  |  |
Employee-related liabilities |  |  | $ 2,200 |  |  |  |  |  |  |  |  |  |
Jericho Plaza |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Ownership Interest |  | 11.67% |  |  |  |  |  |  |  |  |  |  |
Gross Asset Valuation |  | $ 117,400 |  |  |  |  |  |  |  |  |  |  |
Equity in net gain on sale of interest in unconsolidated joint venture/real estate |  | $ 147 |  |  |  |  |  |  |  |  |  |  |
724 Fifth Avenue |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Ownership Interest | 49.90% |  |  |  |  |  |  |  |  |  |  |  |
Gross Asset Valuation | $ 365,000 |  |  |  |  |  |  |  |  |  |  |  |
Equity in net gain on sale of interest in unconsolidated joint venture/real estate | 64,587 |  |  |  |  |  |  |  |  |  |  |  |
Employee-related liabilities | $ 2,400 |  |  |  |  |  |  |  |  |  |  |  |
Mezzanine Loan |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Ownership Interest |  |  |  |  |  |  |  |  |  |  | 33.33% |  |
Gross Asset Valuation |  |  |  |  |  |  |  |  |  |  | $ 15,000 |  |

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