Mortgages Payable - Additional Information (Detail) - Mortgages	6 Months Ended
	Jun. 30, 2018
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%