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Goodwill and Intangible Assets - Goodwill (Detail) - USD ($)
$ in Millions
12 Months Ended
Sep. 30, 2021
Sep. 30, 2020
Sep. 30, 2019
Goodwill And Intangible Assets [Line Items]      
Changes in carrying value of goodwill   $ 0.0  
Increase in goodwill $ 21.3    
Increase in intangible assets 18.4    
Maximum      
Goodwill And Intangible Assets [Line Items]      
Total amortization expense related to intangible assets $ 0.5 $ 0.2 $ 0.2