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Allowance for Doubtful Accounts - Narrative (Details) - USD ($)
$ in Millions
Jun. 30, 2025
Sep. 30, 2024
Receivables [Abstract]    
Allowance for doubtful accounts - deposits with and receivables from broker-dealers, clearings organizations, and counterparties $ 0.1 $ 0.0
Allowance for doubtful accounts receivable $ 54.0 $ 51.9