Employee Benefits - Funded Status of Postretirement Plans Recognized in Balance Sheets (Detail) (Other Postretirement Benefit Plans, Defined Benefit [Member], USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011
Other Postretirement Benefit Plans, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Funded status	$ (23)	$ (24)
Accrued postretirement benefit cost recognized	$ (23)	$ (24)