Mortgages Payable - Summary of Outstanding Mortgages Payable Narrative (Detail) - Mortgages - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2018	Dec. 31, 2017
Mortgage Loans on Real Estate [Line Items]
Deferred debt issuance costs, accumulated amortization	$ 8,915	$ 6,315
Paramus
Mortgage Loans on Real Estate [Line Items]
Maturity date	Oct. 05, 2018
Retail Space | Rego Park I
Mortgage Loans on Real Estate [Line Items]
Percentage of cash mortgage collateralized	100.00%
Refinance date	May 11, 2018
Repayment date	Jun. 06, 2018
Retail Space | Rego Park II
Mortgage Loans on Real Estate [Line Items]
Maturity date	Nov. 30, 2018
Retail Space | 731 Lexington Avenue
Mortgage Loans on Real Estate [Line Items]
Maturity date	Aug. 01, 2022