Claim and Claim Adjustment Expense Reserves (Narrative) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended		48 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014	Dec. 31, 2013	Dec. 31, 2014	Dec. 31, 2010
Catastrophe losses net of reinsurance	$ 14	$ 17	$ 103	$ 147
Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development, excluding Life & Group	(175)	(26)	(187)	23
Specialty [Member]
Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development, excluding Life & Group	(130)	(79)	(141)	(123)
Commercial [Member]
Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development, excluding Life & Group	(11)	71	0	179
Net Prior Year Claim and Allocated Claim Adjustment Expense Reserve Development, Workers' Compensation Reinsurance Pool		(26)		(26)
Loss Portfolio Transfer [Member]
Net A&EP claim and allocated claim adjustment expense reserves ceded to NICO							$ 1,600
Aggregate limit under AEP 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 A&EP Loss Portfolio Transfer							$ 2,200
Net AEP adverse development before consideration of LPT	0	$ 0	150	$ 0	$ 900
Cumulative amounts ceded under A&EP Loss Portfolio Transfer	2,600		2,600			$ 2,500
Deferred reinsurance benefit yet to be recognized	251		251			176
Fair value of collateral trust account established by NICO under A&EP Loss Portfolio Transfer	$ 2,800		$ 2,800			$ 3,400