Claim and Claim Adjustment Expense Reserves (Net prior year development) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Segment Reporting Information [Line Items]
Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development, excluding Life & Group	$ (308)	$ (288)	$ (210)
Pretax (favorable) unfavorable premium development, excluding Life & Group	(4)	(26)	(8)
Total pretax (favorable) unfavorable net prior year development, excluding Life & Group	(312)	(314)	(218)
Specialty
Segment Reporting Information [Line Items]
Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development, excluding Life & Group	(202)	(287)	(141)
Pretax (favorable) unfavorable premium development, excluding Life & Group	(14)	(18)	(11)
Total pretax (favorable) unfavorable net prior year development, excluding Life & Group	(216)	(305)	(152)
Commercial
Segment Reporting Information [Line Items]
Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development, excluding Life & Group	(87)	55	(15)
Pretax (favorable) unfavorable premium development, excluding Life & Group	28	(2)	(15)
Total pretax (favorable) unfavorable net prior year development, excluding Life & Group	(59)	53	(30)
International
Segment Reporting Information [Line Items]
Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development, excluding Life & Group	(9)	(58)	(54)
Pretax (favorable) unfavorable premium development, excluding Life & Group	(18)	(6)	18
Total pretax (favorable) unfavorable net prior year development, excluding Life & Group	(27)	(64)	(36)
Corporate & Other
Segment Reporting Information [Line Items]
Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development, excluding Life & Group	(10)	2	0
Pretax (favorable) unfavorable premium development, excluding Life & Group	0	0	0
Total pretax (favorable) unfavorable net prior year development, excluding Life & Group	$ (10)	$ 2	$ 0