Current Expected Credit Losses (CECL) - Narrative (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Financing Receivable, Allowance for Credit Loss [Line Items]
Current-period provision for expected credit losses	$ 3.3	$ 11.3	$ 12.5
Current expected credit losses reserve	27.2	26.2	14.9	$ 2.4
Release of allowance for expected credit losses	(2.3)
Accrued commissions and other receivables, net
Financing Receivable, Allowance for Credit Loss [Line Items]
Current-period provision for expected credit losses	1.2	(0.4)	4.7
Current expected credit losses reserve	6.2	5.0	5.4	0.7
Release of allowance for expected credit losses	0.0
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	0.0	(0.2)	0.8
Current expected credit losses reserve	0.0	2.3	2.5	1.7
Release of allowance for expected credit losses	(2.3)
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	2.1	11.9	7.0
Current expected credit losses reserve	21.0	$ 18.9	$ 7.0	$ 0.0
Release of allowance for expected credit losses	0.0
Receivables from broker-dealers, clearing organizations, customers and related broker-dealers | Russia/Ukraine Conflict
Financing Receivable, Allowance for Credit Loss [Line Items]
Current expected credit losses reserve	$ 21.0