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Stock-Based Compensation (Narrative) (Detail) - USD ($)
$ in Millions
3 Months Ended 9 Months Ended
Sep. 30, 2018
Sep. 30, 2017
Sep. 30, 2018
Sep. 30, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Stock-based compensation expense $ 7 $ 1 $ 18 $ 6
Stock-based compensation expense (net of tax) $ 5 $ 1 $ 14 $ 4
Document Period End Date     Sep. 30, 2018  
Document Fiscal Year Focus     2018