Real Estate Activities - Dispositions (Details) - USD ($) $ in Thousands		1 Months Ended	3 Months Ended		9 Months Ended		12 Months Ended
	Jun. 30, 2021	May 31, 2021	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020	Aug. 31, 2021
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
(Loss) gain on sales of real estate, net			$ (217)	$ (94)	$ (1,191)	$ 11,169
Cash proceeds from sales of real estate					4,076	21,951
Mortgage loans payable, net			856,986		856,986		$ 884,503
Gain on derecognition of property							8,100
Impairment of real estate assets					1,564	$ 0
Moorestown Mall
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Disposal group, consideration		$ 10,100
Lease termination fee		9,000
(Loss) gain on sales of real estate, net		(1,000)
Cash proceeds from sales of real estate		$ 800
Valley View Mall | Mortgage Loan
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Mortgage loans payable, net			$ 27,200		$ 27,200		$ 27,200
Valley View Center
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Disposal group, consideration								$ 3,500
Impairment of real estate assets	$ 1,300