Claim and Claim Adjustment Expense Reserves (Net prior year development) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Segment Reporting Information [Line Items]
Total pretax (favorable) unfavorable development	$ (62)	$ (115)	$ (160)	$ (227)
Specialty
Segment Reporting Information [Line Items]
Total pretax (favorable) unfavorable development	(53)	(99)	(127)	(134)
Commercial
Segment Reporting Information [Line Items]
Total pretax (favorable) unfavorable development	(5)	(17)	(27)	(94)
International
Segment Reporting Information [Line Items]
Total pretax (favorable) unfavorable development	(2)	1	(4)	1
Corporate & Other
Segment Reporting Information [Line Items]
Total pretax (favorable) unfavorable development	$ (2)	$ 0	$ (2)	$ 0