Allowance for Doubtful Accounts - Narrative (Details) - USD ($)	Dec. 31, 2020	Sep. 30, 2020
Receivables [Abstract]
Allowance for doubtful accounts - deposits with and receivables from broker-dealers, clearings organizations, and counterparties	$ 1,300,000	$ 1,300,000
Allowance for doubtful accounts receivable	35,500,000	25,800,000
Allowance for doubtful accounts related to notes receivable	$ 0	$ 0