Pension and Other Postretirement Benefits - Components of Pension Expense for Defined Benefit Pension Plan (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Pension benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost - benefits earned during the period	$ 2	$ 3	$ 4	$ 4
Interest cost on projected benefit obligation	14	12	28	21
Expected return on assets	(18)	(17)	(36)	(27)
Recognized losses	11	7	22	12
Pension expense	9	5	18	10
Other Postretirement Benefit Plans, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost - benefits earned during the period	0	0	0	0
Interest cost on projected benefit obligation	2	2	4	3
Pension expense	$ 2	$ 2	$ 4	$ 3