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Stock-based Compensation - Compensation Costs Recognized (Details) - USD ($)
$ in Millions
12 Months Ended
Sep. 30, 2021
Sep. 30, 2020
Sep. 30, 2019
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]      
Total compensation cost $ 16.6 $ 9.4 $ 8.6
Compensation cost capitalized (2.7) (0.6) (1.4)
Compensation cost recognized in net income 13.9 8.8 7.2
Income tax benefit recognized in net income (3.2) (2.1) (1.7)
Compensation cost recognized in net income, net of income tax $ 10.7 $ 6.7 $ 5.5