Current Expected Credit Losses - Changes in CECL Allowance Reserve (Details) - USD ($)	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Financing Receivable, Allowance for Credit Loss [Line Items]
Beginning balance	$ 14,900,000	$ 2,400,000	$ 2,600,000
Current-period provision for expected credit losses	11,300,000	12,500,000	(200,000)
Ending balance	26,200,000	14,900,000	2,400,000
Accrued commissions and other receivables, net
Financing Receivable, Allowance for Credit Loss [Line Items]
Beginning balance	5,400,000	700,000	1,000,000.0
Current-period provision for expected credit losses	(400,000)	4,700,000	(300,000)
Ending balance	5,000,000.0	5,400,000	700,000
Loans, forgivable loans and other receivables from employees and partners, net
Financing Receivable, Allowance for Credit Loss [Line Items]
Beginning balance	2,500,000	1,700,000	1,600,000
Current-period provision for expected credit losses	(200,000)	800,000	100,000
Ending balance	2,300,000	2,500,000	1,700,000
Receivables from broker-dealers, clearing organizations, customers and related broker-dealers
Financing Receivable, Allowance for Credit Loss [Line Items]
Beginning balance	7,000,000.0	0	0
Current-period provision for expected credit losses	11,900,000	7,000,000.0	0
Ending balance	$ 18,900,000	$ 7,000,000.0	$ 0