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Stock-based Compensation - Compensation Costs Recognized (Details) - USD ($)
$ in Millions
12 Months Ended
Sep. 30, 2019
Sep. 30, 2018
Sep. 30, 2017
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]      
Total compensation cost $ 8.6 $ 6.9 $ 7.4
Compensation cost capitalized (1.4) (1.3) (3.3)
Prior period disallowed stock compensation capitalization 0.0 6.9 0.0
Compensation cost recognized in net income 7.2 12.5 4.1
Income tax benefit recognized in net income (1.7) (4.0) (1.5)
Compensation cost recognized in net income, net of income tax $ 5.5 $ 8.5 $ 2.6