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Stock-Based Compensation (Narrative) (Detail) - USD ($)
$ in Millions
3 Months Ended 6 Months Ended
Jun. 30, 2018
Jun. 30, 2017
Jun. 30, 2018
Jun. 30, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Stock-based compensation expense $ 7 $ 4 $ 12 $ 5
Stock-based compensation expense (net of tax) $ 5 $ 3 $ 9 $ 3
Document Period End Date     Jun. 30, 2018  
Document Fiscal Year Focus     2018