Claim and Claim Adjustment Expense Reserves - Net prior year claim and allocated claim adjustment expense reserve development for Specialty segment (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Liability for Claims and Claims Adjustment Expense [Line Items]
Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development	$ (115)	$ (132)	$ (227)	$ (282)
Specialty
Liability for Claims and Claims Adjustment Expense [Line Items]
Medical Professional Liability	1	13	5	(17)
Other Professional Liability and Management Liability	(27)	(48)	(96)	(98)
Surety	(82)	(63)	(82)	(63)
Warranty	(1)	2	5	7
Other	0	(16)	5	(40)
Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development	$ (109)	$ (112)	$ (163)	$ (211)