Related Party Transactions - Receivables from and Payables to Related Broker-Dealers (Details) - Related Party - USD ($) $ in Millions	Sep. 30, 2024	Dec. 31, 2023
Transactions with Freedom
Related Party Transaction [Line Items]
Receivables from related parties	$ 1.5	$ 1.4
Open Derivative Contracts with Cantor
Related Party Transaction [Line Items]
Receivables from related parties	3.8	2.7
Payables to related parties	5.4	4.9
Fails and Pending Trades with Cantor
Related Party Transaction [Line Items]
Payables to related parties	$ 23.2	$ 0.8