Fair Values of Financial Instruments (Reconciliation of Beginning and Ending Balances of Level 3 Liabilities) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning balance	$ 6,563,288	$ 5,983,622
Premiums less benefits	2,052,985	434,621
Change in fair value, net	174,154	145,045
Ending balance	8,790,427	6,563,288
Coinsurance ceded, fixed index annuities embedded derivatives	4,858,289	4,639,492
Fixed Index Annuities - Embedded Derivatives [Member]
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Coinsurance ceded, fixed index annuities embedded derivatives	$ 539,700	$ 398,100