Real Estate Activities - Dispositions (Details) $ in Thousands		1 Months Ended	3 Months Ended						12 Months Ended
	Sep. 26, 2019 USD ($)	Nov. 30, 2019 Property	Dec. 31, 2019 USD ($)	Sep. 30, 2019 USD ($)	Jun. 30, 2019 USD ($)	Mar. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Jun. 30, 2018 USD ($)	Dec. 31, 2020 USD ($) Property	Dec. 31, 2019 USD ($) Property	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Number of real estate properties sold | Property		14								3
Gains (losses) on sales of investment real estate			$ 100	$ 1,200	$ 1,500		$ 800	$ 700		$ 2,744	$ 1,722	$ (27)
Total Impairment of Assets										5,017	137,487	55,793
Gain on debt extinguishment, net										24,859	0	0
Wyoming Valley Mall
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Total Impairment of Assets											32,177
Wyoming Valley Mall | Pennsylvania
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Total Impairment of Assets								32,200
Wyoming Valley Mall | Commercial Real Estate | Mortgage Loan
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Debt default, amount	$ 72,800
Total Impairment of Assets										32,200
Written off remaining carrying value of property										43,200
Gain on debt extinguishment, net	$ 29,600									29,600
Undeveloped Land Parcel, Gainesville, Florida
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Disposal group, consideration			10,000			$ 15,000				10,000
Proceeds from sale of real estate						5,000
Gains (losses) on sales of investment real estate						$ (1,500)
Undeveloped Land Parcel, New Garden township, Pennsylvania
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Disposal group, consideration			11,000							11,000
Proceeds from sale of real estate										8,250
Gains (losses) on sales of investment real estate										200
Contingent liability			1,250							1,250
Undeveloped Land Parcel, New Garden township, Pennsylvania | Preferred Stock
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Disposal group, consideration transferred, equity instruments										2,750
Whole Foods Store Adjacent To Exton Square Mall
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Disposal group, consideration			22,100							22,100
Gains (losses) on sales of investment real estate										1,300
Valley View Mall In La Crosse Wisconsin
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Disposal group, consideration			$ 1,400							1,400
Gains (losses) on sales of investment real estate										$ 1,200
Exton Square Mall in Exton | Pennsylvania
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Disposal group, consideration							10,300				10,300
Gains (losses) on sales of investment real estate							8,100
Valley Mall in Hagerstown | Maryland
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Disposal group, consideration							2,400				2,400
Gains (losses) on sales of investment real estate							1,000
Magnolia Mall in Florence | South Carolina
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Disposal group, consideration							$ 1,700				$ 1,700
Gains (losses) on sales of investment real estate								$ 700
Beaver Valley Mall, Exton Square Mall and Valley Mall
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Disposal group, consideration												6,400
Gains (losses) on sales of investment real estate												$ 1,300
Scenario Forecast | Properties Under Sale-leaseback Transaction
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Number of real estate properties sold | Property									5
Disposal group, consideration									$ 153,600
Scenario Forecast | Land Parcels | Multifamily Development
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Disposal group, consideration									$ 125,300