Marketable Securities (Marketable securities portfolio) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Marketable Securities [Roll Forward]
Beginning balance	$ 182,752
(Decrease) increase in fair value of marketable securities	(26,453)	$ 0	$ 0
Sale of marketable securities	(4,101)	0	$ (3,937)
Ending balance	152,198	182,752
Lexington Realty Trust
Marketable Securities [Roll Forward]
Beginning balance	178,226
(Decrease) increase in fair value of marketable securities	(26,596)
Sale of marketable securities	0
Ending balance	151,630	178,226
Other Equity Securities
Marketable Securities [Roll Forward]
Beginning balance	4,526
(Decrease) increase in fair value of marketable securities	143
Sale of marketable securities	(4,101)
Ending balance	$ 568	$ 4,526