Current Expected Credit Losses (CECL) - Narrative (Details) - USD ($)	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023	Dec. 31, 2023	Dec. 31, 2022
Financing Receivable, Allowance for Credit Loss [Line Items]
Current-period provision for expected credit losses	$ 1,500,000	$ 1,900,000
Current expected credit losses reserve	27,700,000	16,800,000	$ 26,200,000	$ 14,900,000
Accrued commissions and other receivables, net
Financing Receivable, Allowance for Credit Loss [Line Items]
Current-period provision for expected credit losses	900,000	0
Current expected credit losses reserve	5,900,000	5,400,000	5,000,000.0	5,400,000
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	(100,000)
Current expected credit losses reserve	2,300,000	2,400,000	2,300,000	2,500,000
Receivables from broker-dealers, clearing organizations, customers and related broker-dealers
Financing Receivable, Allowance for Credit Loss [Line Items]
Current expected credit losses reserve		9,000,000.0	$ 18,900,000	$ 7,000,000.0
Receivables from broker-dealers, clearing organizations, customers and related broker-dealers | Russia/Ukraine Conflit
Financing Receivable, Allowance for Credit Loss [Line Items]
Current-period provision for expected credit losses	600,000	$ 2,000,000.0
Current expected credit losses reserve	$ 19,500,000