Components of Postretirement Benefit Expense (Detail) (Postretirement Benefit Expense, USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011
Postretirement Benefit Expense
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 0	$ 0	$ 1	$ 1
Interest cost	5	6	10	11
Actuarial loss	3	3	5	5
Amortization of prior service credit	(7)	(7)	(14)	(13)
Net periodic benefit expense	$ 1	$ 2	$ 2	$ 4