Current Expected Credit Losses (CECL) - Changes in CECL Allowance Reserve (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Financing Receivable, Allowance for Credit Loss [Line Items]
Beginning balance	$ 2.4	$ 2.6	$ 1.9
Current-period provision for expected credit losses	12.5	(0.2)	0.7
Ending balance	14.9	2.4	2.6
Accrued commissions and other receivables, net
Financing Receivable, Allowance for Credit Loss [Line Items]
Beginning balance	0.7	1.0	0.8
Current-period provision for expected credit losses	4.7	(0.3)	0.2
Ending balance	5.4	0.7	1.0
Loans, forgivable loans and other receivables from employees and partners, net
Financing Receivable, Allowance for Credit Loss [Line Items]
Beginning balance	1.7	1.6	1.1
Current-period provision for expected credit losses	0.8	0.1	0.5
Ending balance	2.5	1.7	1.6
Receivables from broker-dealers, clearing organizations, customers and related broker-dealers
Financing Receivable, Allowance for Credit Loss [Line Items]
Beginning balance	0.0	0.0	0.0
Current-period provision for expected credit losses	7.0	0.0	0.0
Ending balance	$ 7.0	$ 0.0	$ 0.0