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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 9 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]        
Granted (in dollars per share) $ 25.71 $ 25.21 $ 33.31 $ 14.32
Fair value of restricted stock awards vested $ 2.7 $ 2.9 $ 21.8 $ 14.8
Withholding taxes remitted     7.2 4.6
Withholding taxes paid     0.0 0.1
Proceeds from employees to satisfy tax obligation     $ 7.2 $ 4.6