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Equity Incentive Plans - Restricted Stock Unit Activity - Additional Information (Details) - Restricted Stock Units (RSUs) - USD ($)
$ / shares in Units, $ in Millions
12 Months Ended
Sep. 30, 2019
Sep. 30, 2018
Sep. 30, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Granted (in dollars per share) $ 30.47 $ 33.28 $ 14.43
Fair value of restricted stock awards vested $ 34.8 $ 22.0 $ 15.0
Withholding taxes remitted 15.3 7.3 4.7
Withholding taxes paid 0.0 0.0 0.1
Proceeds from employees to satisfy tax obligation $ 15.3 $ 7.3 $ 4.6