Current Expected Credit Losses (CECL) - Schedule of Current Expected Credit Losses (Details) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	$ 27,700,000	$ 16,800,000	$ 26,200,000	$ 14,900,000
Current-period provision for expected credit losses	2,300,000	1,700,000	3,800,000	3,600,000
Release of allowance for expected credit losses	(2,300,000)		(2,300,000)
Ending balance	27,700,000	18,500,000	27,700,000	18,500,000
Accrued commissions and other receivables, net
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	5,900,000	5,400,000	5,000,000.0	5,400,000
Current-period provision for expected credit losses	100,000	(300,000)	1,000,000	(300,000)
Release of allowance for expected credit losses	0		0
Ending balance	6,000,000.0	5,100,000	6,000,000.0	5,100,000
Loans, forgivable loans and other receivables from employees and partners, net
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	2,300,000	2,400,000	2,300,000	2,500,000
Current-period provision for expected credit losses	0	0	0	(100,000)
Release of allowance for expected credit losses	(2,300,000)		(2,300,000)
Ending balance	0	2,400,000	0	2,400,000
Receivables from broker-dealers, clearing organizations, customers and related broker-dealers
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	19,500,000	9,000,000.0	18,900,000	7,000,000.0
Current-period provision for expected credit losses	2,200,000		2,800,000	4,000,000.0
Release of allowance for expected credit losses	$ 0		$ 0
Ending balance		$ 11,000,000.0		$ 11,000,000.0