Policyholders' Liabilities (Policyholders' Account Balances) (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Liability for Policyholders' Account Balances, by Product Segment [Line Items]
Total policyholders’ account balances	$ 20,036,134	$ 18,894,893
Interest-sensitive life contracts
Liability for Policyholders' Account Balances, by Product Segment [Line Items]
Total policyholders’ account balances	15,460,543	14,593,376
Individual annuities
Liability for Policyholders' Account Balances, by Product Segment [Line Items]
Total policyholders’ account balances	3,132,751	2,861,882
Guaranteed interest accounts
Liability for Policyholders' Account Balances, by Product Segment [Line Items]
Total policyholders’ account balances	264,239	310,321
Other
Liability for Policyholders' Account Balances, by Product Segment [Line Items]
Total policyholders’ account balances	$ 1,178,601	$ 1,129,314