Claim and Claim Adjustment Expense Reserves (Narrative) (Details) - USD ($) $ in Millions	12 Months Ended			48 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2013	Dec. 31, 2010
Catastrophe losses net of reinsurance	$ 141	$ 156	$ 169
Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development Life & Group	(50)	(14)	(9)
Commercial [Member]
Net Prior Year Claim and Allocated Claim Adjustment Expense Reserve Development, Workers' Compensation Reinsurance Pool		(26)
Loss Portfolio Transfer [Member]
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 AEP adverse development before consideration of LPT	150	0	$ 363	$ 900
Cumulative amounts ceded under AEP Loss Portfolio Transfer	2,600	2,500
Deferred reinsurance benefit yet to be recognized	241	176
Fair value of collateral trust account established by NICO under A&EP Loss Portfolio Transfer	$ 2,800	$ 3,400