Commitments and Contingencies - Additional Information (Detail)	1 Months Ended		3 Months Ended		6 Months Ended		12 Months Ended
	Mar. 31, 2016 USD ($)	Feb. 28, 2014 fire	Jun. 30, 2018 USD ($) ft²	Jun. 30, 2017 USD ($)	Jun. 30, 2018 USD ($) ft²	Jun. 30, 2017 USD ($)	Dec. 31, 2021 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2001 a
Loss Contingencies [Line Items]
Insurance maximum coverage per occurrence					$ 300,000,000
Insurance maximum coverage per property					300,000,000
Mortgages payable, net of deferred debt issuance costs			$ 1,162,632,000		1,162,632,000			$ 1,240,222,000
Annual property rental			38,618,000	$ 38,264,000	76,859,000	$ 76,537,000
Deferred leasing cost, net			43,234,000		43,234,000			$ 45,790,000
Standby letters of credit, outstanding			$ 1,040,000		$ 1,040,000
Rego Park I | Sears
Loss Contingencies [Line Items]
Lease expiration date					2021-03
Number of fires | fire		2
Area of property | ft²			195,000		195,000
Annual property rental					$ 10,400,000
Straight line rent receivable			$ 3,271,000
Deferred leasing cost, net			$ 343,000		343,000
Rego Park I | Sears | Minimum
Loss Contingencies [Line Items]
Loss contingency, damages sought, value					$ 4,000,000
Paramus | IKEA | Tenant Occupant
Loss Contingencies [Line Items]
Area of land | a									30.3
Fixed interest rate on the debt			2.90%		2.90%
Triple-net rent, annual amount					$ 700,000
Mortgages payable, net of deferred debt issuance costs			$ 68,000,000		$ 68,000,000
Debt instrument maturity date					Oct. 05, 2018
All Risk Property and Rental Value
Loss Contingencies [Line Items]
Insurance maximum coverage per occurrence					$ 1,700,000,000
Terrorism Coverage Including NBCR
Loss Contingencies [Line Items]
Insurance maximum coverage per occurrence					1,700,000,000
Insurance maximum coverage in aggregate					$ 1,700,000,000
NBCR
Loss Contingencies [Line Items]
Insurance coverage end date					Dec. 01, 2020
Self insured responsibility			18.00%		18.00%
Federal government responsibility			82.00%		82.00%
NBCR | FNSIC
Loss Contingencies [Line Items]
Insurance deductible					$ 306,000
Forecast | Paramus | IKEA | Tenant Occupant
Loss Contingencies [Line Items]
Lease purchase option amount							$ 75,000,000
Purchase option not exercised amount included in triple net rent over remainder of lease							68,000,000
Purchase option exercised, net cash proceeds from sale of land							7,000,000
Purchase option exercised, gain on sale of land							$ 60,000,000
Lease term range as Lessor							20 years
Loan amortization period							20 years
Estimated Future Damages | Rego Park I | Sears
Loss Contingencies [Line Items]
Loss contingency, damages sought, value	$ 650,000
Estimated Future Damages | Rego Park I | Sears | Minimum
Loss Contingencies [Line Items]
Loss contingency, damages sought, value					25,000,000
Estimated Future Damages | Rego Park I | Sears | Maximum
Loss Contingencies [Line Items]
Reasonably possible losses			$ 650,000		$ 650,000