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
	Mar. 31, 2017	Mar. 31, 2016
Liability for Claims and Claims Adjustment Expense [Line Items]
Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development	$ (57)	$ (52)
Specialty
Liability for Claims and Claims Adjustment Expense [Line Items]
Medical Professional Liability	1	(7)
Other Professional Liability and Management Liability	(32)	(9)
Surety	0	0
Warranty	0	2
Other	0	(20)
Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development	$ (31)	$ (34)