Marketable Securities and Derivative Instruments (Marketable securities portfolio) (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Available-for-sale Securities
Available-for-sale Securities, Equity Securities	$ 203,704	$ 150,997
Available-for-sale Equity Securities, Amortized Cost Basis	73,199	72,549
Available-for-sale Securities, Gross Unrealized Gain	130,505	78,448
Lexington Realty Trust
Available-for-sale Securities
Available-for-sale Securities, Equity Securities	199,465	147,752
Available-for-sale Equity Securities, Amortized Cost Basis	72,549	72,549
Available-for-sale Securities, Gross Unrealized Gain	126,916	75,203
Other Equity Securities
Available-for-sale Securities
Available-for-sale Securities, Equity Securities	4,239	3,245
Available-for-sale Equity Securities, Amortized Cost Basis	650	0
Available-for-sale Securities, Gross Unrealized Gain	$ 3,589	$ 3,245