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		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 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	$ (55)	$ (98)	$ (112)	$ (150)
Specialty
Liability for Claims and Claims Adjustment Expense [Line Items]
Medical Professional Liability	3	(23)	4	(30)
Other Professional Liability and Management Liability	(37)	(41)	(69)	(50)
Surety	0	0	0	0
Warranty	6	3	6	5
Other	5	(4)	5	(24)
Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development	$ (23)	$ (65)	$ (54)	$ (99)