Current Expected Credit Losses (CECL) - Narrative (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022	Jun. 30, 2023	Dec. 31, 2022	Jun. 30, 2022	Dec. 31, 2021
Financing Receivable, Allowance for Credit Loss [Line Items]
Current-period provision for expected credit losses	$ 1,600,000	$ (200,000)	$ 5,200,000	$ 12,400,000
Current expected credit losses reserve	20,100,000	14,800,000	20,100,000	14,800,000	$ 18,500,000	$ 14,900,000	$ 15,000,000.0	$ 2,400,000
Russia/Ukraine Conflit
Financing Receivable, Allowance for Credit Loss [Line Items]
Current expected credit losses reserve	3,900,000	3,800,000	3,900,000	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)	(200,000)	(500,000)	3,900,000
Current expected credit losses reserve	4,900,000	4,600,000	4,900,000	4,600,000	5,100,000	5,400,000	4,800,000	700,000
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	(200,000)	(100,000)	(300,000)	1,500,000
Current expected credit losses reserve	2,200,000	3,200,000	2,200,000	3,200,000	2,400,000	2,500,000	3,300,000	1,700,000
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	2,000,000	100,000	6,000,000.0	7,000,000
Current expected credit losses reserve	$ 13,000,000	$ 7,000,000.0	$ 13,000,000	$ 7,000,000.0	$ 11,000,000.0	$ 7,000,000.0	$ 6,900,000	$ 0