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Goodwill - Narrative (Details) - USD ($)
12 Months Ended
Sep. 30, 2021
Sep. 30, 2020
Sep. 30, 2019
Segment Reporting Information [Line Items]      
Accumulated goodwill impairment loss $ 168,600,000 $ 168,600,000  
Impairment of long-lived assets $ 0 $ 0 $ 0