Related Party Transactions - Receivables from and Payables to Related Broker-Dealers (Details) - Related Party - USD ($) $ in Millions	Jun. 30, 2025	Dec. 31, 2024
Transactions with Freedom
Related Party Transaction [Line Items]
Receivables from related parties	$ 2.1	$ 1.3
Open Derivative Contracts with Cantor
Related Party Transaction [Line Items]
Receivables from related parties	0.9	4.8
Payables to related parties	1.0	4.0
Fails and Pending Trades with Cantor
Related Party Transaction [Line Items]
Receivables from related parties	$ 3.6	$ 0.1