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Pension and Other Postretirement Benefits - Components of Net Periodic Benefit Cost for Postretirement Medical Plan Benefits Other Than Pensions (Detail) (Other postretirement benefits [Member], USD $)
In Millions, unless otherwise specified
3 Months Ended 9 Months Ended
Sep. 30, 2012
Sep. 30, 2011
Sep. 30, 2012
Sep. 30, 2011
Other postretirement benefits [Member]
       
Defined Benefit Plan Disclosure [Line Items]        
Service cost - benefits earned during the period            
Interest cost 2 1 5 3
Pension expense $ 2 $ 1 $ 5 $ 3