Unpaid Losses and Loss Expenses - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012 Manufacturers_Distributors	Dec. 31, 2011	Dec. 31, 2010
Favorable development on net unpaid losses and loss expenses established as of the previous year end	$ 614	$ 767	$ 746
Aggregate favorable development experienced in the personal and commercial liability classes	250	355	265
Adverse development for losses related to asbestos and toxic waste claims	83	72
Favorable development experienced in the professional liability classes other than fidelity	200	310	315
Aggregate favorable development experienced in the personal and commercial property classes	125	80	110
Adverse development experienced in the fidelity class	60	70	70
Favorable development experienced in the runoff of the reinsurance assumed business	45	30	25
Favorable development experienced in the personal automobile business	40	30	40
Favorable development experienced in the surety business	25	15	40
Favorable development experienced in the professional liability classes other than fidelity, outside the United States			190
Adverse development for losses related to toxic waste claims			$ 61
Number of manufacturers and distributors of asbestos products that filed for bankruptcy protection	95