Current Expected Credit Losses (CECL) - Narrative (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2025	Sep. 30, 2024	Jun. 30, 2025	Dec. 31, 2024	Jun. 30, 2024	Dec. 31, 2023
Financing Receivable, Allowance for Credit Loss [Line Items]
Current-period provision for expected credit losses	$ (0.4)	$ (0.1)	$ 1.6	$ 3.7
Current expected credit losses reserve	24.8	27.6	24.8	27.6	$ 25.2	$ 27.2	$ 27.7	$ 26.2
Accrued commissions and other receivables, net
Financing Receivable, Allowance for Credit Loss [Line Items]
Current-period provision for expected credit losses	0.0	0.1	0.9	1.1
Current expected credit losses reserve	7.1	6.1	7.1	6.1	7.1	6.2	6.0	5.0
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	0.0	0.0	0.0
Current expected credit losses reserve	0.0	0.0	0.0	0.0	0.0	0.0	0.0	2.3
Financing receivable, allowance for credit loss, period increase (decrease)	0.0	0.0	0.0	(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	(0.4)	(0.2)	0.7	2.6
Current expected credit losses reserve	$ 17.7	$ 21.5	17.7	$ 21.5	$ 18.1	$ 21.0	$ 21.7	$ 18.9
Financing receivable, allowance for credit loss, period increase (decrease)			$ (3.3)