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Mortgages Payable - Additional Information (Detail) - USD ($) $ in Thousands |  |  | 12 Months Ended |  |
--- | --- | --- | --- | --- | ---
Sep. 14, 2020 | Feb. 14, 2020 | Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018 | Oct. 23, 2020 | Sep. 13, 2020
Mortgage Loans on Real Estate |  |  |  |  |  |  |
Repayments of long-term debt |  |  | $ 50,000 | $ 0 | $ 160,142 |  |
Net carrying value of real estate collateralizing the debt |  |  | 657,800 |  |  |  |
Mortgages |  |  |  |  |  |  |
Mortgage Loans on Real Estate |  |  |  |  |  |  |
Notes payable, gross |  |  | 1,164,544 | 974,836 |  |  |
Rego Park II | Retail space | Mortgages |  |  |  |  |  |  |
Mortgage Loans on Real Estate |  |  |  |  |  |  |
Loan participation balance |  | $ 50,000 |  |  |  |  |
Proceeds from loan participation |  | $ 145,000 |  |  |  |  |
Notes payable, gross |  |  | $ 202,544 | 56,836 |  |  |
Interest rate |  |  | 1.50% |  |  |  |
Debt instrument, description of variable rate basis |  |  | LIBOR |  |  |  |
Rego Park II | Retail space | Mortgages | LIBOR |  |  |  |  |  |  |
Mortgage Loans on Real Estate |  |  |  |  |  |  |
Basis spread over LIBOR |  |  | 1.35% |  |  |  |
731 Lexington Avenue | Retail space |  |  |  |  |  |  |
Mortgage Loans on Real Estate |  |  |  |  |  |  |
Repayments of long-term debt | $ 50,000 |  |  |  |  |  |
731 Lexington Avenue | Retail space | Mortgages |  |  |  |  |  |  |
Mortgage Loans on Real Estate |  |  |  |  |  |  |
Notes payable, gross | $ 300,000 |  | $ 300,000 | 350,000 |  |  | $ 350,000
Basis spread over LIBOR | 1.40% |  |  |  |  |  |
Interest rate |  |  | 1.55% |  |  |  |
731 Lexington Avenue | Retail space | Mortgages | Interest Rate Swap |  |  |  |  |  |  |
Mortgage Loans on Real Estate |  |  |  |  |  |  |
Interest rate |  |  | 1.72% |  |  |  |
Debt instrument, description of variable rate basis |  |  | LIBOR |  |  |  |
731 Lexington Avenue | Retail space | Mortgages | LIBOR | Interest Rate Swap |  |  |  |  |  |  |
Mortgage Loans on Real Estate |  |  |  |  |  |  |
Basis spread over LIBOR |  |  | 1.40% |  |  |  |
731 Lexington Avenue | Office space | Mortgages |  |  |  |  |  |  |
Mortgage Loans on Real Estate |  |  |  |  |  |  |
Notes payable, gross |  |  | $ 500,000 | 500,000 |  |  |
Interest rate |  |  | 1.06% |  |  |  |
Debt instrument, description of variable rate basis |  |  | LIBOR |  |  |  |
731 Lexington Avenue | Office space | Mortgages | LIBOR |  |  |  |  |  |  |
Mortgage Loans on Real Estate |  |  |  |  |  |  |
Basis spread over LIBOR |  |  | 0.90% |  |  |  |
The Alexander | Residence space | Mortgages |  |  |  |  |  |  |
Mortgage Loans on Real Estate |  |  |  |  |  |  |
Notes payable, gross |  |  |  |  |  | $ 94,000 |
Interest rate |  |  |  |  |  | 2.63% |
Participation Agreement | Rego Park II |  |  |  |  |  |  |
Mortgage Loans on Real Estate |  |  |  |  |  |  |
Loan participation balance |  |  | $ 50,000 | $ 195,708 |  |  |

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