Current Expected Credit Losses (CECL) - Schedule of Current Expected Credit Losses (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2023	Mar. 31, 2022
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	$ 14.9	$ 2.4
Current-period provision for expected credit losses	1.9	6.8
Ending balance	16.8	9.2
Accrued commissions and other receivables, net
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	5.4	0.7
Current-period provision for expected credit losses	0.0	0.9
Ending balance	5.4	1.6
Loans, forgivable loans and other receivables from employees and partners, net
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	2.5	1.7
Current-period provision for expected credit losses	(0.1)	0.4
Ending balance	2.4	2.1
Receivables from broker-dealers, clearing organizations, customers and related broker-dealers
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	7.0	0.0
Current-period provision for expected credit losses	2.0	5.5
Ending balance	$ 9.0	$ 5.5