Claim and Claim Adjustment Expense Reserves - Net Prior Year Claim and Allocated Claim Adjustment Expense Reserve Development (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
Specialty [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Medical professional liability	$ 14
Other professional liability and management liability	(3)	(6)
Surety	1	1
Other	(10)	2
Total pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development	2	(3)
Commercial [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Commercial auto		20
General liability	4
Workers' compensation	(1)	11
Property and other	(8)	(13)
Total pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development	(5)	18
International [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Medical professional liability		1
Other professional liability		(1)
General liability	(5)	(2)
Property & marine	(6)	8
Other	7	(6)
Commutations		10
Total pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development	$ (4)	$ 10