Unpaid Losses and Loss Expenses - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2013 Manufacturers_Distributors	Dec. 31, 2012	Dec. 31, 2011
Insurance [Abstract]
Favorable development on net unpaid losses and loss expenses established as of the previous year end	$ 712	$ 614	$ 767
Aggregate favorable development experienced in the personal and commercial liability classes	265	250	355
Amount related to catastrophes	30
Favorable development experienced in the professional liability classes other than fidelity	260	200	310
Aggregate favorable development experienced in the personal and commercial property classes	160	125	80
Adverse development for losses related to asbestos and toxic waste claims	106	83	72
Adverse development experienced in the fidelity class	50	60	70
Favorable development experienced in the personal automobile business	35	40	30
Favorable development experienced in the surety business	30	25	15
Favorable development experienced in the runoff of the reinsurance assumed business		$ 45	$ 30
Number of manufacturers and distributors of asbestos products that filed for bankruptcy protection	100