Claim and Claim Adjustment Expense Reserves - Net prior year development (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Segment Reporting Information [Line Items]
Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development	$ (57)	$ (52)
Pretax (favorable) unfavorable premium development	25	(14)
Total pretax (favorable) unfavorable net prior year development	(32)	(66)
Specialty
Segment Reporting Information [Line Items]
Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development	(31)	(34)
Pretax (favorable) unfavorable premium development	(5)	(11)
Total pretax (favorable) unfavorable net prior year development	(36)	(45)
Commercial
Segment Reporting Information [Line Items]
Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development	(24)	(14)
Pretax (favorable) unfavorable premium development	37	(2)
Total pretax (favorable) unfavorable net prior year development	13	(16)
International
Segment Reporting Information [Line Items]
Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development	(2)	(4)
Pretax (favorable) unfavorable premium development	(7)	(1)
Total pretax (favorable) unfavorable net prior year development	(9)	(5)
Corporate & Other Non-Core
Segment Reporting Information [Line Items]
Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development	0	0
Pretax (favorable) unfavorable premium development	0	0
Total pretax (favorable) unfavorable net prior year development	$ 0	$ 0