Unpaid Losses and Loss Expenses - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014 Manufacturers_Distributors Areas	Dec. 31, 2013	Dec. 31, 2012
Insurance [Abstract]
Favorable development on net unpaid losses and loss expenses established as of the previous year end	$ 636	$ 712	$ 614
Favorable development experienced in the professional liability classes other than fidelity	320	260	200
Aggregate favorable development experienced in the personal and commercial liability classes	205	160	250
Adverse development for losses related to asbestos and toxic waste claims	100	106	83
Aggregate favorable development experienced in the personal and commercial property classes	60	265	125
Favorable development experienced in workers' compensation class	50
Amount related to catastrophes		30
Adverse development experienced in the fidelity class		50	60
Favorable development experienced in the personal automobile business		35	40
Favorable development experienced in the surety business		30	25
Favorable development experienced in the runoff of the reinsurance assumed business			$ 45
Number of manufacturers and distributors of asbestos products that filed for bankruptcy protection	110
Number of areas of exposure for P&C Group's insureds	2