Derivative Instruments (Changes in Estimated Fair Value Related to Embedded Derivatives) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Embedded Derivatives In Modified Coinsurance Or Funds Withheld Arrangements And Variable Annuity Contracts [Member]
Changes In Estimated Fair Value Related To Embedded Derivatives [Line Items]
Gain (Loss) on Derivative Instruments, Net, Pretax	$ 144,723	$ 54,169	$ 98,792
Derivative, Gain (Loss) on Derivative, Net	48,316	9,160	(26,025)
Embedded Derivatives In Variable Annuity Contracts [Member]
Changes In Estimated Fair Value Related To Embedded Derivatives [Line Items]
Gain (Loss) on Derivative Instruments, Net, Pretax	32,166	7,835	(33,192)
Derivative, Gain (Loss) on Derivative, Net	53,645	(41,201)	(29,008)
Embedded Derivatives In Equity Indexed Annuities [Member]
Changes In Estimated Fair Value Related To Embedded Derivatives [Line Items]
Gain (Loss) on Derivative Instruments, Net, Pretax	(80,062)	10,708	19,440
Derivative, Gain (Loss) on Derivative, Net	$ (68,808)	$ (4,148)	$ 6,204