Mortgages Payable - Additional Information (Detail) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2019	Dec. 31, 2018
731 Lexington Avenue | Retail Site | Mortgages
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Debt instrument, description of variable rate basis	LIBOR
731 Lexington Avenue | Retail Site | Mortgages | LIBOR
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Basis spread over LIBOR	1.40%
731 Lexington Avenue | Office Space | Mortgages
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Debt instrument, description of variable rate basis	LIBOR
731 Lexington Avenue | Office Space | Mortgages | LIBOR
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Basis spread over LIBOR	0.90%
Rego Park II | Retail Site | Mortgages
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Debt instrument, description of variable rate basis	LIBOR
Rego Park II | Retail Site | Mortgages | LIBOR
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Basis spread over LIBOR	1.35%
Participation Agreement | Rego Park II
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Loan participation balance	$ 195,708	$ 195,708