Marketable Securities (Marketable securities portfolio) (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Available-for-sale Securities
Fair Value	$ 182,752	$ 203,704
GAAP Cost	73,199	73,199
Unrealized Gain	109,553	130,505
Lexington Realty Trust
Available-for-sale Securities
Fair Value	178,226	199,465
GAAP Cost	72,549	72,549
Unrealized Gain	105,677	126,916
Other Equity Securities
Available-for-sale Securities
Fair Value	4,526	4,239
GAAP Cost	650	650
Unrealized Gain	$ 3,876	$ 3,589