Marketable Securities (Marketable securities portfolio) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Marketable Securities [Roll Forward]
Beginning balance			$ 33,313
Decrease in fair value of marketable securities	$ 0	$ 1,312	(4,938)	$ 1,773
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	$ 1,313	(4,938)	$ (14,336)
Ending balance	$ 0		$ 0