Current Expected Credit Losses (CECL) - Schedule of Current Expected Credit Losses (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2022	Sep. 30, 2021	Sep. 30, 2022	Sep. 30, 2021
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	$ 15,000,000.0	$ 2,700,000	$ 2,400,000	$ 2,600,000
Current-period provision for expected credit losses	(200,000)	(100,000)	12,400,000	0
Ending balance	14,800,000	2,600,000	14,800,000	2,600,000
Accrued commissions and other receivables, net
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	4,800,000	1,000,000.0	700,000	1,000,000.0
Current-period provision for expected credit losses	(200,000)	(100,000)	3,900,000	(100,000)
Ending balance	4,600,000	900,000	4,600,000	900,000
Loans, forgivable loans and other receivables from employees and partners, net
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	3,300,000	1,700,000	1,700,000	1,600,000
Current-period provision for expected credit losses	(100,000)	0	1,500,000	100,000
Ending balance	3,200,000	1,700,000	3,200,000	1,700,000
Receivables from broker-dealers, clearing organizations, customers and related broker-dealers
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	6,900,000	0	0	0
Current-period provision for expected credit losses	100,000	0	7,000,000.0	0
Ending balance	$ 7,000,000.0	$ 0	$ 7,000,000.0	$ 0