Allowance for Doubtful Accounts - Narrative (Details) - USD ($)	Jun. 30, 2023	Sep. 30, 2022
Receivables [Abstract]
Allowance for doubtful accounts - deposits with and receivables from broker-dealers, clearings organizations, and counterparties	$ 100,000	$ 1,400,000
Allowance for doubtful accounts receivable	53,200,000	46,400,000
Allowance for doubtful accounts related to notes receivable	$ 0	$ 0