Current Expected Credit Losses (CECL) - Schedule of Current Expected Credit Losses (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2025	Jun. 30, 2024	Jun. 30, 2025	Jun. 30, 2024
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	$ 28.3	$ 27.7	$ 27.2	$ 26.2
Current-period provision for expected credit losses	(0.9)	(2.3)	(2.0)	(3.8)
Release of allowance for expected credit losses	(4.0)	(2.3)	(4.0)	(2.3)
Ending balance	25.2	27.7	25.2	27.7
Accrued commissions and other receivables, net
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	6.2	5.9	6.2	5.0
Current-period provision for expected credit losses	(0.9)	(0.1)	(0.9)	(1.0)
Release of allowance for expected credit losses	0.0	0.0	0.0	0.0
Ending balance	7.1	6.0	7.1	6.0
Loans, forgivable loans and other receivables from employees and partners, net
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	0.0	2.3	0.0	2.3
Current-period provision for expected credit losses	0.0	0.0	0.0	0.0
Release of allowance for expected credit losses	0.0	(2.3)	0.0	(2.3)
Ending balance	0.0	0.0	0.0	0.0
Receivables from broker-dealers, clearing organizations, customers and related broker-dealers
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	22.1	19.5	21.0	18.9
Current-period provision for expected credit losses	0.0	(2.2)	(1.1)	(2.8)
Release of allowance for expected credit losses	(4.0)	0.0	(4.0)	0.0
Ending balance	$ 18.1	$ 21.7	$ 18.1	$ 21.7