Reinsurance (Balance Sheet Reinsurance Results) (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Effects of Reinsurance [Line Items]
Reinsurance recoverables	$ 32,555,500	$ 28,674,226
Policy loans	(1,161,101)	(1,166,456)
Deferred policy acquisition costs	(1,376,211)	(1,341,093)	$ (5,129,931)	$ (5,081,938)
Deferred sales inducements	0	0	$ (684,844)	$ (836,791)
Other assets	465,467	279,222
Other liabilities	1,060,123	849,698
Impacts of Reinsurance
Effects of Reinsurance [Line Items]
Reinsurance recoverables	32,555,500	28,674,226
Policy loans	(124,843)	(87,112)
Deferred policy acquisition costs	(6,832,729)	(6,482,889)
Deferred sales inducements	(638,065)	(615,117)
Other assets	205,430	226,347
Policyholders’ account balances	5,004,885	4,978,859
Future policy benefits	3,301,841	2,833,327
Other liabilities	626,306	410,376
Unaffiliated
Effects of Reinsurance [Line Items]
Other assets	100	100
Other liabilities	$ 73,000	$ 28,000