Claim and Claim Adjustment Expense Reserves (Narrative) (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Sep. 30, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2011
Liability for Claims and Claims Adjustment Expense [Line Items]
Catastrophe losses net of reinsurance		$ 380	$ 165	$ 141
Payments for Reinsurance		4
Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development Life & Group		(79)	(45)	(50)
Loss Portfolio Transfer
Liability for Claims and Claims Adjustment Expense [Line Items]
Net A&EP claim and allocated claim adjustment expense reserves ceded to NICO					$ 1,600
Aggregate limit under A&EP Loss Portfolio Transfer					4,000
A&EP claim and allocated claim adjustment expense reserves ceded under existing third party reinsurance contracts transferred to NICO under A&EP Loss Portfolio Transfer					1,200
Reinsurance premium paid to NICO under A&EP Loss Portfolio Transfer					2,000
Net reinsurance receivables transferrred to NICO under A&EP Loss Portfolio Transfer					215
Total consideration paid to NICO under AEP Loss Portfolio Transfer					$ 2,200
Net A&EP adverse development before consideration of LPT		60	200	$ 150
Cumulative amounts ceded under AEP Loss Portfolio Transfer		2,900	2,800
Deferred reinsurance benefit yet to be recognized		326	334
Fair value of collateral trust account established by NICO under A&EP Loss Portfolio Transfer		3,100	$ 2,800
Hurricane Harvey, Irma and Maria
Liability for Claims and Claims Adjustment Expense [Line Items]
Catastrophe losses net of reinsurance	$ 188	$ 256