Significant Accounting Policies and Pronouncements (Impact of Change in Accounting for Certain Reinsurance Contracts) (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Change in Accounting Estimate [Line Items]
Decrease in Policy charges and fee income									$ 254,542	$ 787,195	$ 2,156,387
Decrease in Policyholders' benefits									(38,380)	(260,200)	299,150
Increase in Amortization of deferred policy acquisition costs									95,007	628,101	659,169
Pre-tax charge to income	$ 28,584	$ (217,945)	$ 338,003	$ 21,621	$ (84,388)	$ 49,004	$ 729,039	$ (372,307)	170,263	321,348	497,362
Other liabilities	1,060,123				849,698				1,060,123	849,698
Reinsurance recoverables	32,555,500				28,674,226				32,555,500	28,674,226
Policyholders’ account balances	20,036,134				18,894,893				20,036,134	18,894,893
Deferred policy acquisition costs	$ 1,376,211				$ 1,341,093				$ 1,376,211	$ 1,341,093	$ 5,129,931	$ 5,081,938
Individual Life | Accounting for Certain Reinsurance Contracts
Change in Accounting Estimate [Line Items]
Decrease in Policy charges and fee income			(236,000)
Decrease in Policyholders' benefits			253,000
Increase in Amortization of deferred policy acquisition costs			(19,000)
Pre-tax charge to income			(2,000)
Other liabilities			284,000
Reinsurance recoverables			247,000
Policyholders’ account balances			(48,000)
Deferred policy acquisition costs			(19,000)
Future policy benefits			$ (6,000)