Claim and Claim Adjustment Expense Reserves (Net prior year claim and allocated claim adjustment expense reserve development for Commercial segment) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development, excluding Life & Group	$ (288)	$ (210)	$ (21)
Commercial [Member]
Net prior year claim and allocated claim adjustment expense reserve development, Commercial Auto	(46)	(22)	31
Net prior year claim and allocated claim adjustment expense reserve development, General Liability	(28)	(33)	45
Net prior year claim and allocated claim adjustment expense reserve development, Workers' Compensation	150	80	139
Property and Other	(21)	(40)	(39)
Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development, excluding Life & Group	$ 55	$ (15)	$ 176