Mortgages Payable - Additional Information (Detail) - Mortgages	1 Months Ended	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2018
Rego Park I | Retail Site
Mortgage Loans on Real Estate [Line Items]
Duration of mortgage loan extension	2 months
Rego Park II | Retail Site
Mortgage Loans on Real Estate [Line Items]
Debt Instrument, Description of Variable Rate Basis		LIBOR
Rego Park II | Retail Site | LIBOR
Mortgage Loans on Real Estate [Line Items]
Basis spread over LIBOR		1.85%
731 Lexington Avenue | Retail Site
Mortgage Loans on Real Estate [Line Items]
Debt Instrument, Description of Variable Rate Basis		LIBOR
731 Lexington Avenue | Retail Site | LIBOR
Mortgage Loans on Real Estate [Line Items]
Basis spread over LIBOR		1.40%
731 Lexington Avenue | Office Space
Mortgage Loans on Real Estate [Line Items]
Debt Instrument, Description of Variable Rate Basis		LIBOR
731 Lexington Avenue | Office Space | LIBOR
Mortgage Loans on Real Estate [Line Items]
Basis spread over LIBOR		0.90%