Claim and Claim Adjustment Expense Reserves (Narrative) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2010
Liability for Claims and Claims Adjustment Expense [Line Items]
Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development Life & Group	$ (19)	$ (72)	$ (46)
Asbestos and Environmental Reserves
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	178	60	$ 200
Cumulative amounts ceded under AEP Loss Portfolio Transfer	3,100	2,900
Deferred reinsurance benefit yet to be recognized	374	326
Fair value of collateral trust account established by NICO under A&EP Loss Portfolio Transfer	$ 2,700	$ 3,100