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Current Expected Credit Losses (CECL) - Narrative (Details) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023	Mar. 31, 2024	Dec. 31, 2023	Mar. 31, 2023	Dec. 31, 2022
Financing Receivable, Allowance for Credit Loss [Line Items]
Current-period provision for expected credit losses	$ 2,300,000	$ 1,700,000	$ 3,800,000	$ 3,600,000
Current expected credit losses reserve	27,700,000	18,500,000	27,700,000	18,500,000	$ 27,700,000	$ 26,200,000	$ 16,800,000	$ 14,900,000
Release of allowance for expected credit losses	(2,300,000)		(2,300,000)
Accrued commissions and other receivables, net
Financing Receivable, Allowance for Credit Loss [Line Items]
Current-period provision for expected credit losses	100,000	(300,000)	1,000,000	(300,000)
Current expected credit losses reserve	6,000,000.0	5,100,000	6,000,000.0	5,100,000	5,900,000	5,000,000.0	5,400,000	5,400,000
Release of allowance for expected credit losses	0		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	0	0	0	(100,000)
Current expected credit losses reserve	0	2,400,000	0	2,400,000	2,300,000	2,300,000	2,400,000	2,500,000
Release of allowance for expected credit losses	(2,300,000)		(2,300,000)
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	2,200,000		2,800,000	4,000,000.0
Current expected credit losses reserve		11,000,000.0		$ 11,000,000.0	$ 19,500,000	$ 18,900,000	$ 9,000,000.0	$ 7,000,000.0
Release of allowance for expected credit losses	0		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		$ 2,000,000.0
Current expected credit losses reserve	$ 21,700,000		$ 21,700,000