Claim and Claim Adjustment Expense Reserves (Net prior year development) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Segment Reporting Information [Line Items]
Total pretax (favorable) unfavorable development	$ (39)	$ (57)
Specialty
Segment Reporting Information [Line Items]
Total pretax (favorable) unfavorable development	(30)	(12)
Commercial
Segment Reporting Information [Line Items]
Total pretax (favorable) unfavorable development	(9)	(43)
International
Segment Reporting Information [Line Items]
Total pretax (favorable) unfavorable development	0	(2)
Corporate & Other revenues
Segment Reporting Information [Line Items]
Total pretax (favorable) unfavorable development	$ 0	$ 0