Marketable Securities (Marketable securities portfolio) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Marketable Securities [Roll Forward]
Beginning balance	$ 33,313	$ 152,198
Sale of marketable securities	(28,375)	(168,314)
Transfer of PREIT investment balance		54,962
Decrease in fair value of marketable securities	(4,938)	(5,533)	$ (26,453)
Ending balance	$ 0	$ 33,313	$ 152,198