Components of Postretirement Benefit Expense (Detail) (Postretirement benefit expense, USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Postretirement benefit expense
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 1	$ 0	$ 1	$ 1
Interest cost	3	5	7	10
Actuarial loss	0	3	3	5
Amortization of prior service credit	(6)	(7)	(12)	(14)
Net periodic benefit expense	$ (2)	$ 1	$ (1)	$ 2