Reinsurance (amounts included in the Statements of Financial Position) (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Effects of Reinsurance [Line Items]
Reinsurance recoverables	$ 31,696,534	$ 32,555,500
Policy loans	(1,169,847)	(1,161,101)
Deferred policy acquisition costs	(1,432,029)	(1,376,211)
Other assets	433,592	465,467
Future policy benefits	17,431,953	18,593,130
Other liabilities	1,013,952	1,060,123
Impacts of Reinsurance
Effects of Reinsurance [Line Items]
Reinsurance recoverables	31,696,534	32,555,500
Policy loans	(125,587)	(124,843)
Deferred policy acquisition costs	(6,968,267)	(6,832,729)
Deferred sales inducements	(616,795)	(638,065)
Other assets	193,113	205,430
Policyholders’ account balances	5,004,098	5,004,885
Future policy benefits	3,219,983	3,301,841
Other liabilities	587,909	626,306
Unaffiliated activity
Effects of Reinsurance [Line Items]
Other assets	100	100
Other liabilities	$ 42,000	$ 73,000