Components of Postretirement Benefit Expense (Detail) (Postretirement Benefit Expense, USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Postretirement Benefit Expense
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 1	$ 0	$ 2	$ 1
Interest cost	5	5	15	16
Actuarial loss	3	2	8	7
Amortization of prior service credit	(8)	(6)	(22)	(19)
Net periodic benefit expense	$ 1	$ 1	$ 3	$ 5