Marketable Securities (Marketable securities portfolio) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Marketable Securities [Roll Forward]
Beginning balance			$ 152,198
Sale of marketable securities			(168,314)
Transfer of PREIT investment balance at Conversion Date			54,962
(Decrease) increase in fair value of marketable securities	$ (4,868)	$ (7,699)	(3,095)	$ (24,801)
Ending balance	35,751		35,751
Lexington Realty Trust
Marketable Securities [Roll Forward]
Beginning balance			151,630
Sale of marketable securities			(167,698)
Transfer of PREIT investment balance at Conversion Date			0
(Decrease) increase in fair value of marketable securities	0	(7,942)	16,068	(24,934)
Ending balance	0		0
PREIT
Marketable Securities [Roll Forward]
Beginning balance			0
Sale of marketable securities			0
Transfer of PREIT investment balance at Conversion Date			54,962
(Decrease) increase in fair value of marketable securities	(4,875)	$ 0	(19,211)	$ 0
Ending balance	35,751		35,751
Other
Marketable Securities [Roll Forward]
Beginning balance			568
Sale of marketable securities			(616)
Transfer of PREIT investment balance at Conversion Date			0
(Decrease) increase in fair value of marketable securities			48
Ending balance	$ 0		$ 0