Marketable Securities (Marketable securities portfolio) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Marketable Securities [Roll Forward]
Beginning balance	$ 152,198	$ 182,752
(Decrease) increase in fair value of marketable securities	(5,533)	(26,453)	$ 0
Sale of marketable securities	(168,314)	(4,101)
Transfer of PREIT investment balance at Conversion Date	54,962
Ending balance	33,313	152,198	182,752
Lexington Realty Trust
Marketable Securities [Roll Forward]
Beginning balance	151,630	178,226
(Decrease) increase in fair value of marketable securities	16,068	(26,596)	0
Sale of marketable securities	(167,698)	0
Transfer of PREIT investment balance at Conversion Date	0
Ending balance	0	151,630	178,226
PREIT
Marketable Securities [Roll Forward]
Beginning balance	0	0
(Decrease) increase in fair value of marketable securities	(21,649)	0	0
Sale of marketable securities	0	0
Transfer of PREIT investment balance at Conversion Date	54,962
Ending balance	33,313	0	0
Other
Marketable Securities [Roll Forward]
Beginning balance	568	4,526
(Decrease) increase in fair value of marketable securities	48	143
Sale of marketable securities	(616)	(4,101)
Transfer of PREIT investment balance at Conversion Date	0
Ending balance	$ 0	$ 568	$ 4,526