Claim and Claim Adjustment Expense Reserves (Net prior year development) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Segment Reporting Information [Line Items]
Total pretax (favorable) unfavorable development	$ (59)	$ (55)	$ (98)	$ (112)
Specialty
Segment Reporting Information [Line Items]
Total pretax (favorable) unfavorable development	(44)	(23)	(74)	(35)
Commercial
Segment Reporting Information [Line Items]
Total pretax (favorable) unfavorable development	(13)	(34)	(22)	(77)
International
Segment Reporting Information [Line Items]
Total pretax (favorable) unfavorable development	(2)	2	(2)	0
Corporate & Other
Segment Reporting Information [Line Items]
Total pretax (favorable) unfavorable development	$ 0	$ 0	$ 0	$ 0