Current Expected Credit Losses (CECL) - Schedule of Current Expected Credit Losses (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2024	Sep. 30, 2023
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	$ 27.7	$ 18.5	$ 26.2	$ 14.9
Current-period provision for expected credit losses	(0.1)	1.6	3.7	5.2
Ending balance	27.6	20.1	27.6	20.1
Accrued commissions and other receivables, net
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	6.0	5.1	5.0	5.4
Current-period provision for expected credit losses	0.1	(0.2)	1.1	(0.5)
Ending balance	6.1	4.9	6.1	4.9
Loans, forgivable loans and other receivables from employees and partners, net
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	0.0	2.4	2.3	2.5
Current-period provision for expected credit losses	0.0	(0.2)	0.0	(0.3)
Ending balance	0.0	2.2	0.0	2.2
Receivables from broker-dealers, clearing organizations, customers and related broker-dealers
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	21.7	11.0	18.9	7.0
Current-period provision for expected credit losses	$ (0.2)		$ 2.6	6.0
Ending balance		$ 13.0		$ 13.0