Current Expected Credit Losses (CECL) - Narrative (Details) - USD ($)	3 Months Ended	9 Months Ended
Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2024	Sep. 30, 2023
Financing Receivable, Allowance for Credit Loss [Line Items]
Beginning balance	$ 27,700,000	$ 18,500,000	$ 26,200,000	$ 14,900,000
Current-period provision for expected credit losses	(100,000)	1,600,000	3,700,000	5,200,000
Release of allowance for expected credit losses			(2,300,000)
Ending balance	27,600,000	20,100,000	27,600,000	20,100,000
Accrued commissions and other receivables, net
Financing Receivable, Allowance for Credit Loss [Line Items]
Beginning balance	6,000,000.0	5,100,000	5,000,000.0	5,400,000
Current-period provision for expected credit losses	100,000	(200,000)	1,100,000	(500,000)
Release of allowance for expected credit losses			0
Ending balance	6,100,000	4,900,000	6,100,000	4,900,000
Loans, forgivable loans and other receivables from employees and partners, net
Financing Receivable, Allowance for Credit Loss [Line Items]
Beginning balance	0	2,400,000	2,300,000	2,500,000
Current-period provision for expected credit losses	0	(200,000)	0	(300,000)
Release of allowance for expected credit losses	0		(2,300,000)
Ending balance	0	2,200,000	0	2,200,000
Receivables from broker-dealers, clearing organizations, customers and related broker-dealers
Financing Receivable, Allowance for Credit Loss [Line Items]
Beginning balance	21,700,000	11,000,000.0	18,900,000	7,000,000.0
Current-period provision for expected credit losses	(200,000)		2,600,000	6,000,000.0
Release of allowance for expected credit losses			0
Ending balance		13,000,000.0		$ 13,000,000.0
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		$ 2,000,000.0
Ending balance	$ 21,500,000		$ 21,500,000

Amount of allowance for credit loss on financing receivable. Excludes allowance for financing receivable covered under loss sharing agreement.

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