Current Expected Credit Losses (CECL) - Schedule of Changes in CECL Allowance Reserve (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	$ 26.2	$ 14.9	$ 2.4
Current-period provision for expected credit losses	3.3	11.3	12.5
Release of allowance for expected credit losses	(2.3)
Ending balance	27.2	26.2	14.9
Accrued commissions and other receivables, net
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	5.0	5.4	0.7
Current-period provision for expected credit losses	1.2	(0.4)	4.7
Release of allowance for expected credit losses	0.0
Ending balance	6.2	5.0	5.4
Loans, forgivable loans and other receivables from employees and partners, net
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	2.3	2.5	1.7
Current-period provision for expected credit losses	0.0	(0.2)	0.8
Release of allowance for expected credit losses	(2.3)
Ending balance	0.0	2.3	2.5
Receivables from broker-dealers, clearing organizations, customers and related broker-dealers
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	18.9	7.0	0.0
Current-period provision for expected credit losses	2.1	11.9	7.0
Release of allowance for expected credit losses	0.0
Ending balance	$ 21.0	$ 18.9	$ 7.0