Mortgages Payable - Summary of Outstanding Mortgages Payable (Parenthetical) (Detail) - USD ($) $ in Thousands		5 Months Ended	9 Months Ended
	Jun. 01, 2017	May 31, 2017	Sep. 30, 2017	Dec. 31, 2016
Rego Park I [Member]
Mortgage Loans on Real Estate [Line Items]
Percentage of cash mortgage collateralized			100.00%
Mortgages [Member]
Mortgage Loans on Real Estate [Line Items]
Deferred debt issuance costs, accumulated amortization			$ 4,995	$ 6,824
Mortgages [Member] | Retail Space [Member] | Rego Park I [Member]
Mortgage Loans on Real Estate [Line Items]
Percentage of cash mortgage collateralized			100.00%
Mortgage loan extension date			2016-03
Duration of mortgage loan extension			2 years
Mortgages [Member] | Retail Space [Member] | Rego Park II [Member]
Mortgage Loans on Real Estate [Line Items]
Debt Instrument, Description of Variable Rate Basis			LIBOR
Mortgages [Member] | Retail Space [Member] | Rego Park II [Member] | LIBOR [Member]
Mortgage Loans on Real Estate [Line Items]
Basis spread over LIBOR			1.85%
Mortgages [Member] | Retail Space [Member] | 731 Lexington Avenue [Member]
Mortgage Loans on Real Estate [Line Items]
Debt Instrument, Description of Variable Rate Basis			LIBOR
Mortgages [Member] | Retail Space [Member] | 731 Lexington Avenue [Member] | LIBOR [Member]
Mortgage Loans on Real Estate [Line Items]
Basis spread over LIBOR			1.40%
Mortgages [Member] | Office Space [Member] | 731 Lexington Avenue [Member]
Mortgage Loans on Real Estate [Line Items]
Debt Instrument, Description of Variable Rate Basis	LIBOR	LIBOR	LIBOR
Mortgages [Member] | Office Space [Member] | 731 Lexington Avenue [Member] | LIBOR [Member]
Mortgage Loans on Real Estate [Line Items]
Basis spread over LIBOR	0.90%	0.95%	0.90%