Pension and Other Postretirement Benefits - Components of Pension Expense for Defined Benefit Pension Plan (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Pension benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost - benefits earned during the period	$ 2	$ 1
Interest cost on projected benefit obligation	14	9
Expected return on assets	(18)	(10)
Recognized losses	11	5
Pension expense	9	5
Other Postretirement Benefit Plans, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost - benefits earned during the period	0	0
Interest cost on projected benefit obligation	2	1
Pension expense	$ 2	$ 1