Marketable Securities (Marketable securities portfolio) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Available-for-sale Securities
Fair Value	$ 157,951		$ 157,951		$ 182,752
Increase (decrease) in fair value of marketable securities	(7,699)	$ 0	(24,801)	$ 0
Lexington Realty Trust
Available-for-sale Securities
Fair Value	153,292		153,292		178,226
Increase (decrease) in fair value of marketable securities			(24,934)
Other
Available-for-sale Securities
Fair Value	$ 4,659		4,659		$ 4,526
Increase (decrease) in fair value of marketable securities			$ 133