Marketable Securities (Marketable Securities Portfolio) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Marketable Securities [Roll Forward]
Beginning balance			$ 33,313
Decrease in fair value of marketable securities	$ 0	$ (4,868)	(4,938)	$ (3,095)
Ending balance	0		0
PREIT
Marketable Securities [Roll Forward]
Beginning balance			33,313
Sale of marketable securities on January 23, 2020			(28,375)
Decrease in fair value of marketable securities	0	$ (4,875)	(4,938)	$ (19,211)
Ending balance	$ 0		$ 0