Reinsurance (Reinsurance amounts included in the Statements of Financial Position) (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Effects of Reinsurance [Line Items]
Reinsurance recoverables	$ 31,430,148	$ 32,521,264
Policy loans	(1,217,608)	(1,161,101)
Deferred policy acquisition costs	(1,530,072)	(1,376,211)
Other assets	387,078	465,467
Future policy benefits	16,498,119	18,561,550
Other liabilities	1,084,739	1,038,972
Impacts of Reinsurance
Effects of Reinsurance [Line Items]
Reinsurance recoverables	31,430,148	32,521,264
Policy loans	(129,636)	(124,843)
Deferred policy acquisition costs	(7,175,536)	(6,832,729)
Deferred sales inducements	(570,514)	(638,065)
Other assets	168,397	205,430
Policyholders’ account balances	4,997,598	5,004,885
Future policy benefits	3,277,715	3,301,841
Other liabilities	628,105	605,155
Unaffiliated activity
Effects of Reinsurance [Line Items]
Other assets	100	100
Other liabilities	$ 73,000	$ 73,000