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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, 2017
Jun. 30, 2016
Jun. 30, 2017
Jun. 30, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Granted (in dollars per share) $ 25.21 $ 9.58 $ 14.32 $ 10.84
Fair value of restricted stock awards vested $ 2.9 $ 0.3 $ 14.8 $ 14.3
Withholding taxes remitted     4.6 4.4
Withholding taxes paid     0.1 4.3
Proceeds from employees to satisfy tax obligation     $ 4.6 $ 0.1