Marketable Securities (Marketable securities portfolio) (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Available-for-sale Securities
Fair Value	$ 193,145	$ 203,704
GAAP Cost	73,199	73,199
Unrealized Gain	119,946	130,505
Lexington Realty Trust
Available-for-sale Securities
Fair Value	188,753	199,465
GAAP Cost	72,549	72,549
Unrealized Gain	116,204	126,916
Other
Available-for-sale Securities
Fair Value	4,392	4,239
GAAP Cost	650	650
Unrealized Gain	$ 3,742	$ 3,589