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Stock-Based Compensation - Restricted Stock Unit Activity - Additional Information (Details) - Restricted Stock Units (RSUs) - USD ($)
$ / shares in Units, $ in Millions
3 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Granted (in dollars per share) $ 34.25 $ 13.93
Fair value of restricted stock awards vested $ 19.0 $ 11.3
Withholding taxes remitted 6.3 3.8
Withholding taxes paid 0.0 0.1
Proceeds from employees to satisfy tax obligation $ 6.3 $ 3.7