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Equity And Earnings Per Share (Narrative) (Details) - USD ($) $ / shares in Units, $ in Millions |  |  |  |  | 1 Months Ended | 3 Months Ended | 6 Months Ended | 9 Months Ended | 36 Months Ended | 81 Months Ended | 168 Months Ended | 221 Months Ended |
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Oct. 16, 2025 | Jul. 30, 2025 | Jul. 26, 2023 | Oct. 07, 2019 | Mar. 31, 2025 | Sep. 30, 2025 | Jun. 30, 2025 | Mar. 31, 2025 | Sep. 30, 2024 | Jun. 30, 2024 | Mar. 31, 2024 | Sep. 30, 2025 | Sep. 30, 2025 | Sep. 30, 2024 | Dec. 31, 2010 | Sep. 30, 2025 | Dec. 31, 2024 | Sep. 30, 2025 | May 03, 2007
Equity And Earnings Per Share [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
IBG Holdings Redemption of IBG LLC |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 10.00%
IBG Holdings LLC Ownership Percentage of Class B Common Stock |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 100.00%
Amortization period DTA (years) |  |  |  |  |  |  |  |  |  |  |  |  | 15 years |  |  |  |  |  |
Unamortized deferred tax asset arising from equity offerings |  |  |  |  |  | $ 223 |  |  |  |  |  | $ 223 | $ 223 |  |  | $ 223 | $ 196 | $ 223 |
Percent of tax savings owed to IBG Holdings LLC |  |  |  |  |  |  |  |  |  |  |  |  | 85.00% |  |  |  |  |  |
Percentage of tax savings retained by IBG Inc. |  |  |  |  |  |  |  |  |  |  |  |  | 15.00% |  |  |  |  |  |
Deferred tax asset from common stock offerings |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 727 |
Tax savings owed to IBG Holdings LLC |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 618 |
Tax savings retained by IBG Inc. |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 109 |
Tax savings paid to IBG Holdings LLC |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 308 |
Shares reserved for future issuance |  |  |  |  |  | 1,440,000,000 |  |  |  |  |  | 1,440,000,000 | 1,440,000,000 |  |  | 1,440,000,000 |  | 1,440,000,000 | 360,000,000
Shares redeemed by IBG Holdings from IBG LLC |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 20,053,036 |  |  |  |
Cash Redemptions IBG Holdings |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 114 |  |  |  |
Fair Value of Issued Shares in Exchange for Membership Interests |  | $ 254 |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 2,000 |  |
Stock registered, number of common stock |  |  | 2,520,000 | 4,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Shares Issued |  | 3,836,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 161,777,780 |  |
Distribution from IBG LLC |  |  |  |  |  |  |  |  |  |  |  |  | $ 918 |  |  |  |  |  |
Cash distribution to IBG, Inc. |  |  |  |  |  |  |  |  |  |  |  |  | 238 |  |  |  |  |  |
Dividend per share |  |  |  |  | $ 0.0625 | $ 0.08 | $ 0.08 | $ 0.0625 | $ 0.0625 | $ 0.0625 | $ 0.025 |  |  |  |  |  |  |  |
Dividends paid to common shareholders |  |  |  |  | $ 27 |  |  |  |  |  |  | $ 71 | $ 98 | $ 65 |  |  |  |  |
Subsequent Event [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Equity And Earnings Per Share [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Dividend per share | $ 0.08 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Subsequent Event [Member] | O 2025 Q4 Dividends [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Equity And Earnings Per Share [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Declaration Date | Oct. 16, 2025 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Payment Date | Dec. 12, 2025 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Record Date | Dec. 01, 2025 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
IBG LLC [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Equity And Earnings Per Share [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
IBG Inc. ownership % of IBG LLC |  |  |  |  |  | 26.30% |  |  |  |  |  | 26.30% | 26.30% |  |  | 26.30% |  | 26.30% | 10.00%
IBG Holdings ownership % of IBG LLC |  |  |  |  |  | 73.70% |  |  |  |  |  | 73.70% | 73.70% |  |  | 73.70% |  | 73.70% | 90.00%
Shares Issued |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 2,880,000 |  |  |
IBG LLC [Member] | Thomas Peterffy And Affiliates [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Equity And Earnings Per Share [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Majority stakeholder percentage |  |  |  |  |  | 91.60% |  |  |  |  |  | 91.60% | 91.60% |  |  | 91.60% |  | 91.60% | 84.60%
Common Class A [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Equity And Earnings Per Share [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Common stock, par value |  |  |  |  |  | $ 0.01 |  |  |  |  |  | $ 0.01 | $ 0.01 |  |  | $ 0.01 | $ 0.01 | $ 0.01 |
Common Class B [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Equity And Earnings Per Share [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Common stock, par value |  |  |  |  |  | $ 0.01 |  |  |  |  |  | $ 0.01 | $ 0.01 |  |  | $ 0.01 | $ 0.01 | $ 0.01 |

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