Current Expected Credit Losses (CECL) - Current Expected Credit Losses (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2025	Sep. 30, 2024
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	$ 25.2	$ 27.7	$ 27.2	$ 26.2
Current-period provision for expected credit losses	(0.4)	(0.1)	1.6	3.7
Release of allowance for expected credit losses			(4.0)	(2.3)
Ending balance	24.8	27.6	24.8	27.6
Accrued commissions and other receivables, net
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	7.1	6.0	6.2	5.0
Current-period provision for expected credit losses	0.0	0.1	0.9	1.1
Release of allowance for expected credit losses			0.0	0.0
Ending balance	7.1	6.1	7.1	6.1
Loans, forgivable loans and other receivables from employees and partners, net
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	0.0	0.0	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)
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	18.1	21.7	21.0	18.9
Current-period provision for expected credit losses	(0.4)	(0.2)	0.7	2.6
Release of allowance for expected credit losses			(4.0)	0.0
Ending balance	$ 17.7	$ 21.5	$ 17.7	$ 21.5