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Stock-Based Compensation (Summary Of Share Based Compensation Shares Authorized Under Stock Option Plans By Exercise Price Range) (Detail) - USD ($)
$ / shares in Units, shares in Thousands, $ in Millions
6 Months Ended 12 Months Ended
Jun. 30, 2018
Dec. 31, 2017
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]    
Share-based Compensation Arrangements by Share-based Payment Award, Options, Grants in Period, Weighted Average Exercise Price $ 0.00  
Share-based Compensation Arrangement by Share-based Payment Award, Options, Grants in Period, Gross  
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Number 87 273
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Exercise Price $ 0.79 $ 7.08
Share-based Compensation Arrangements by Share-based Payment Award, Options, Forfeitures in Period, Weighted Average Exercise Price $ 0.00  
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercises in Period, Intrinsic Value $ 7  
Share-based Compensation Arrangement by Share-based Payment Award, Options, Forfeitures in Period 0  
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercises in Period (186)  
Share-based Compensation Arrangements by Share-based Payment Award, Options, Exercises in Period, Weighted Average Exercise Price $ 9.99  
Options outstanding, intrinsic value $ 3 $ 10
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Remaining Contractual Term 7 months 1 year 8 months
Share-based Compensation Arrangement by Share-based Payment Award, Options, Grants in Period, Net of Forfeitures 0