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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 6 Months Ended
Mar. 31, 2018
Mar. 31, 2017
Mar. 31, 2018
Mar. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Granted (in dollars per share) $ 25.41 $ 20.76 $ 33.61 $ 14.32
Fair value of restricted stock awards vested $ 0.1 $ 0.6 $ 19.1 $ 11.9
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