Current Expected Credit Losses (CECL) - Narrative (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2022	Jun. 30, 2021	Jun. 30, 2022	Jun. 30, 2021	Mar. 31, 2022	Dec. 31, 2021	Mar. 31, 2021	Dec. 31, 2020
Financing Receivable, Allowance for Credit Loss [Line Items]
Current-period provision for expected credit losses	$ 5,800	$ (100)	$ 12,600	$ 100
Current expected credit losses reserve	15,000	2,700	15,000	2,700	$ 9,200	$ 2,400	$ 2,800	$ 2,600
Change in CECL reserve				0
Russia's invasion of Ukraine
Financing Receivable, Allowance for Credit Loss [Line Items]
Current-period provision for expected credit losses			3,900
Accrued commissions and other receivables, net
Financing Receivable, Allowance for Credit Loss [Line Items]
Current-period provision for expected credit losses	3,200	0	4,100	0
Current expected credit losses reserve	4,800	1,000	4,800	1,000	1,600	700	1,000	1,000
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	1,200	(100)	1,600	100
Current expected credit losses reserve	3,300	1,700	3,300	1,700	2,100	1,700	1,800	1,600
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	1,400	0	6,900	0
Current expected credit losses reserve	$ 6,900	0	$ 6,900	$ 0	$ 5,500	$ 0	$ 0	$ 0
Change in CECL reserve		$ 0