Marketable Securities (Marketable securities portfolio) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Marketable Securities [Roll Forward]
Beginning balance	$ 33,313
Decrease in fair value of marketable securities	(4,938)	$ 461
Ending balance	0
PREIT
Marketable Securities [Roll Forward]
Beginning balance	33,313
Sale of marketable securities	(28,375)
Decrease in fair value of marketable securities	(4,938)	$ (15,649)
Ending balance	$ 0