Current Expected Credit Losses (CECL) - Narrative (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2025	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Financing Receivable, Allowance for Credit Loss [Line Items]
Current-period provision for expected credit losses	$ 1.4	$ 3.3	$ 11.3
Release of allowance for expected credit losses	4.4	2.3
Current expected credit losses reserve	24.2	27.2	26.2	$ 14.9
Accrued commissions and other receivables, net
Financing Receivable, Allowance for Credit Loss [Line Items]
Allowance for credit losses, period increase (decrease)	0.0
Current-period provision for expected credit losses	0.0	1.2	(0.4)
Release of allowance for expected credit losses	0.0	0.0
Current expected credit losses reserve	6.2	6.2	5.0	5.4
Loans, forgivable loans and other receivables from employees and partners, net
Financing Receivable, Allowance for Credit Loss [Line Items]
Allowance for credit losses, period increase (decrease)	0.0
Current-period provision for expected credit losses	0.0	0.0	(0.2)
Release of allowance for expected credit losses	0.0	2.3
Current expected credit losses reserve	0.0	0.0	2.3	2.5
Receivables from broker-dealers, clearing organizations, customers and related broker-dealers
Financing Receivable, Allowance for Credit Loss [Line Items]
Allowance for credit losses, period increase (decrease)	(3.0)
Current-period provision for expected credit losses	1.4	2.1	11.9
Release of allowance for expected credit losses	4.4	0.0
Current expected credit losses reserve	18.0	$ 21.0	$ 18.9	$ 7.0
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	$ 18.0