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Current Expected Credit Losses (CECL) - Narrative (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2022	Sep. 30, 2021	Sep. 30, 2022	Sep. 30, 2021	Jun. 30, 2022	Dec. 31, 2021	Jun. 30, 2021	Dec. 31, 2020
Financing Receivable, Allowance for Credit Loss [Line Items]
Current-period provision for expected credit losses	$ (200,000)	$ (100,000)	$ 12,400,000	$ 0
Current expected credit losses reserve	14,800,000	2,600,000	14,800,000	2,600,000	$ 15,000,000.0	$ 2,400,000	$ 2,700,000	$ 2,600,000
Change in CECL reserve				0
Russia's invasion of Ukraine
Financing Receivable, Allowance for Credit Loss [Line Items]
Current-period provision for expected credit losses			3,800,000
Accrued commissions and other receivables, net
Financing Receivable, Allowance for Credit Loss [Line Items]
Current-period provision for expected credit losses	(200,000)	(100,000)	3,900,000	(100,000)
Current expected credit losses reserve	4,600,000	900,000	4,600,000	900,000	4,800,000	700,000	1,000,000.0	1,000,000.0
Loans, forgivable loans and other receivables from employees and partners, net
Financing Receivable, Allowance for Credit Loss [Line Items]
Current-period provision for expected credit losses	(100,000)	0	1,500,000	100,000
Current expected credit losses reserve	3,200,000	1,700,000	3,200,000	1,700,000	3,300,000	1,700,000	1,700,000	1,600,000
Receivables from broker-dealers, clearing organizations, customers and related broker-dealers
Financing Receivable, Allowance for Credit Loss [Line Items]
Current-period provision for expected credit losses	100,000	0	7,000,000.0	0
Current expected credit losses reserve	$ 7,000,000.0	0	$ 7,000,000.0	$ 0	$ 6,900,000	$ 0	$ 0	$ 0
Change in CECL reserve		$ 0