Marketable Securities (Marketable securities portfolio) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Available-for-sale Securities
Fair Value	$ 149,766		$ 182,752
Decrease in fair value of marketable securities (see Note 7 for details)	(32,986)	$ 0
Lexington Realty Trust
Available-for-sale Securities
Fair Value	145,351		178,226
Decrease in fair value of marketable securities (see Note 7 for details)	(32,875)
Other
Available-for-sale Securities
Fair Value	4,415		$ 4,526
Decrease in fair value of marketable securities (see Note 7 for details)	$ (111)