Commitments and Contingencies (Litigation and contingencies) (Details) ft² in Thousands, $ in Thousands	1 Months Ended		12 Months Ended
	Mar. 31, 2016 USD ($)	Feb. 28, 2014 fire	Dec. 31, 2021 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2018 USD ($)	Apr. 04, 2017 ft²	Dec. 31, 2001 a
Commitments And Contingencies Disclosure
Mortgages payable, carrying value				$ 970,961			$ 965,826
Standby letters of credit, outstanding				$ 1,030
Paramus, NJ | IKEA | Tenant Occupant
Commitments And Contingencies Disclosure
Area of land | a									30.3
Debt instrument, interest rate, stated percentage				4.72%
Triple-net rent, annual amount				$ 700
Maturity date				Oct. 01, 2021
Mortgages payable, carrying value				$ 68,000
Paramus, NJ | IKEA | Forecast | Tenant Occupant
Commitments And Contingencies Disclosure
Property purchase option exercisable by lessee			$ 75,000
Purchase option not exercised amount included in triple net rent over remainder of lease			68,000
Purchase option exercised, net cash proceeds from sale of land			7,000
Gain (loss) on disposition of real estate, discontinued operations			$ 60,000
Lease term range as Lessor			20 years
Loan amortization period			20 years
Rego Park I | Sears
Commitments And Contingencies Disclosure
Number of fires | fire		2
Area of property (in square feet) | ft²								195
Property rentals					$ 10,300
Minimum | Rego Park I | Sears
Commitments And Contingencies Disclosure
Loss contingency, damages sought, value						$ 4,000
Estimated Future Damages | Rego Park I | Sears
Commitments And Contingencies Disclosure
Loss contingency, damages sought, value	$ 650
Estimated Future Damages | Minimum | Rego Park I | Sears
Commitments And Contingencies Disclosure
Loss contingency, damages sought, value						$ 25,000
Estimated Future Damages | Maximum | Rego Park I | Sears
Commitments And Contingencies Disclosure
Loss contingency, estimate of possible loss				$ 650