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Stock-Based Compensation - Stock Option Activity (Narrative) (Details)
shares in Millions
9 Months Ended
Sep. 30, 2018
$ / shares
shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]  
Stock options vested and expected to vest (in shares) | shares 19.4
Stock options  
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]  
Stock options granted, weighted average grant date fair value (in usd per share) | $ / shares $ 6.84
Stock options | Minimum  
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]  
Stock options, employees, vesting period 3 years
Stock options | Maximum  
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]  
Stock options, employees, vesting period 4 years