Investments (Other Invested Assets) (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Other Investments [Abstract]
Other invested assets	$ 1,605,484	$ 1,591,940
Equity Securities [Member]
Other Investments [Abstract]
Other invested assets	100,152	275,361
Limited Partner [Member]
Other Investments [Abstract]
Other invested assets	781,124	687,522
Gain (Loss) on Derivatives [Member]
Other Investments [Abstract]
Other invested assets	137,613	229,108
Fair Value Option Contractholder Directed Unit Linked Investments [Member]
Other Investments [Abstract]
Other invested assets	218,541	190,120
Other [Member]
Other Investments [Abstract]
Other invested assets	$ 368,054	$ 209,829