Derivative Investments (Balance Sheet) (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Derivatives, Fair Value [Line Items]
Asset Derivatives	$ 116,356	$ 194,731
Liability Derivatives	134,293	211,159
Derivatives not designated as hedging instruments
Derivatives, Fair Value [Line Items]
Asset Derivatives	116,356	194,731
Liability Derivatives	134,293	211,159
Derivatives not designated as hedging instruments | Equity index options | Derivatives, index options
Derivatives, Fair Value [Line Items]
Asset Derivatives	116,356	194,731
Derivatives not designated as hedging instruments | Fixed-index products | Universal Life and Annuity Contracts
Derivatives, Fair Value [Line Items]
Liability Derivatives	$ 134,293	$ 211,159