Claim and Claim Adjustment Expense Reserves - Net prior year development (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Segment Reporting Information [Line Items]
Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development	$ (115)	$ (132)	$ (227)	$ (282)
Pretax (favorable) unfavorable premium development	(19)	(5)	(2)	(27)
Total pretax (favorable) unfavorable net prior year development	(134)	(137)	(229)	(309)
Specialty
Segment Reporting Information [Line Items]
Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development	(109)	(112)	(163)	(211)
Pretax (favorable) unfavorable premium development	(3)	0	(13)	(18)
Total pretax (favorable) unfavorable net prior year development	(112)	(112)	(176)	(229)
Commercial
Segment Reporting Information [Line Items]
Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development	(7)	(5)	(65)	(37)
Pretax (favorable) unfavorable premium development	(11)	(3)	27	(7)
Total pretax (favorable) unfavorable net prior year development	(18)	(8)	(38)	(44)
International
Segment Reporting Information [Line Items]
Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development	1	(15)	1	(34)
Pretax (favorable) unfavorable premium development	(5)	(2)	(16)	(2)
Total pretax (favorable) unfavorable net prior year development	(4)	(17)	(15)	(36)
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	0	0
Pretax (favorable) unfavorable premium development	0	0	0	0
Total pretax (favorable) unfavorable net prior year development	$ 0	$ 0	$ 0	$ 0