Current Expected Credit Losses (CECL) - Narrative (Details) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2023	Jun. 30, 2022	Jun. 30, 2023	Jun. 30, 2022	Mar. 31, 2023	Dec. 31, 2022	Mar. 31, 2022	Dec. 31, 2021
Financing Receivable, Allowance for Credit Loss [Line Items]
Current-period provision for expected credit losses	$ 1,700,000	$ 5,800,000	$ 3,600,000	$ 12,600,000
Current expected credit losses reserve	18,500,000	15,000,000.0	18,500,000	15,000,000.0	$ 16,800,000	$ 14,900,000	$ 9,200,000	$ 2,400,000
Accrued commissions and other receivables, net
Financing Receivable, Allowance for Credit Loss [Line Items]
Current-period provision for expected credit losses	(300,000)	3,200,000	(300,000)	4,100,000
Current expected credit losses reserve	5,100,000	4,800,000	5,100,000	4,800,000	5,400,000	5,400,000	1,600,000	700,000
Accrued commissions and other receivables, net | Russia/Ukraine Conflit
Financing Receivable, Allowance for Credit Loss [Line Items]
Current expected credit losses reserve	3,900,000	3,900,000	3,900,000	3,900,000
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	0	1,200,000	(100,000)	1,600,000
Current expected credit losses reserve	2,400,000	3,300,000	2,400,000	3,300,000	2,400,000	2,500,000	2,100,000	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	2,000,000.0	1,400,000	4,000,000.0	6,900,000
Current expected credit losses reserve	$ 11,000,000.0	$ 6,900,000	$ 11,000,000.0	$ 6,900,000	$ 9,000,000.0	$ 7,000,000.0	$ 5,500,000	$ 0