Allowance for Doubtful Accounts - Narrative (Details) - USD ($)	Mar. 31, 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	48,700,000	46,400,000
Allowance for doubtful accounts related to notes receivable	$ 0	$ 0

Amount of allowance for credit loss on financing receivable. Excludes allowance for financing receivable covered under loss sharing agreement.

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Name:	us-gaap_FinancingReceivableAllowanceForCreditLosses
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant