Cash and Securities Segregated Under Federal Regulations and Other Requirements (Details) - USD ($) $ in Millions |
Sep. 30, 2026 |
Dec. 31, 2025 |
|---|---|---|
| Cash | ||
| Marketable Securities [Line Items] | ||
| Cash and securities in special reserve bank accounts for exclusive benefit of brokerage customers | $ 200.0 | |
| U.S. Treasury Bills | ||
| Marketable Securities [Line Items] | ||
| Cash and securities in special reserve bank accounts for exclusive benefit of brokerage customers | $ 379.2 | $ 498.6 |