Commitments and Contingencies (Litigation and contingencies) (Details)	1 Months Ended		3 Months Ended		12 Months Ended
	Mar. 31, 2016 USD ($)	Feb. 28, 2014 fire	Mar. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2021 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Sep. 19, 2017 ft²	Apr. 04, 2017 ft²	Dec. 31, 2001 a
Commitments And Contingencies Disclosure
Property rentals						$ 152,857,000	$ 151,444,000	$ 138,688,000
Straight line rent adjustments						(4,250,000)	(2,347,000)	1,418,000
Unamortized deferred leasing costs				$ 45,790,000		45,790,000	48,387,000
Depreciation and amortization						34,925,000	33,807,000	31,086,000
Mortgages payable, carrying value				1,240,222,000		1,240,222,000	1,052,359,000
Standby letters of credit, outstanding				1,474,000		1,474,000
Proceeds from legal settlements						396,000	$ 825,000	$ 2,100,000
Rego Park I | Sears
Commitments And Contingencies Disclosure
Area of property (in square feet) | ft²										195,000
Property rentals						$ 10,600,000
Lease termination date						Mar. 01, 2021
Straight line rent adjustments						$ 3,865,000
Unamortized deferred leasing costs				406,000		$ 406,000
Number of fires | fire		2
Maturity date						Mar. 01, 2020
731 Lexington Avenue | Le Cirque
Commitments And Contingencies Disclosure
Area of property (in square feet) | ft²									13,000
Property rentals						$ 1,200,000
Lease termination date						Jan. 05, 2018
Original lease termination date						May 01, 2021
Lease, cost						$ 2,780,000
Depreciation and amortization				$ 2,650,000
731 Lexington Avenue | Le Cirque | Forecast
Commitments And Contingencies Disclosure
Depreciation and amortization			$ 130,000
Paramus | Ikea | Tenant Occupant
Commitments And Contingencies Disclosure
Area of land | a											30.3
Debt instrument, interest rate, stated percentage				2.90%		2.90%
Triple-net rent, annual amount						$ 700,000
Maturity date						Oct. 05, 2018
Mortgages payable, carrying value				$ 68,000,000		$ 68,000,000
Paramus | Ikea | Forecast | Tenant Occupant
Commitments And Contingencies Disclosure
Property purchase option exercisable by lessee					$ 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
Gain (loss) on disposition of real estate, discontinued operations					$ 60,000,000
Lease term range as Lessor					20 years
Loan amortization period					20 years
The Kings Plaza Regional Shopping Center
Commitments And Contingencies Disclosure
Loss contingency, damages sought, value						$ 22,910,000
Minimum
Commitments And Contingencies Disclosure
Lease term range as Lessor						5 years
Minimum | Rego Park I | Sears
Commitments And Contingencies Disclosure
Loss contingency, damages sought, value						$ 4,000,000
Maximum
Commitments And Contingencies Disclosure
Lease term range as Lessor						25 years
Estimated Future Damages | Rego Park I | Sears
Commitments And Contingencies Disclosure
Loss contingency, damages sought, value	$ 650,000
Estimated Future Damages | Minimum | Rego Park I | Sears
Commitments And Contingencies Disclosure
Loss contingency, damages sought, value						$ 25,000,000
Estimated Future Damages | Maximum | Rego Park I | Sears
Commitments And Contingencies Disclosure
Loss contingency, estimate of possible loss				$ 650,000		$ 650,000