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Equity And Earnings Per Share (Narrative) (Details) - USD ($) $ / shares in Units, $ in Millions |  |  |  |  | 1 Months Ended | 3 Months Ended | 7 Months Ended | 9 Months Ended | 36 Months Ended | 48 Months Ended | 144 Months Ended | 197 Months Ended |  |
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Oct. 17, 2023 | Oct. 04, 2023 | Jul. 27, 2023 | Jul. 26, 2023 | Sep. 30, 2023 | Jun. 30, 2023 | Mar. 31, 2023 | Sep. 30, 2023 | Jun. 30, 2023 | Mar. 31, 2023 | Sep. 30, 2022 | Jun. 30, 2022 | Mar. 31, 2022 | Sep. 30, 2023 | Sep. 30, 2023 | Sep. 30, 2022 | Dec. 31, 2010 | Sep. 30, 2023 | Dec. 31, 2022 | Sep. 30, 2023 | Oct. 07, 2019 | 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%
Preferred stock shares authorized |  |  |  |  | 10,000 |  |  | 10,000 |  |  |  |  |  | 10,000 | 10,000 |  |  | 10,000 | 10,000 | 10,000 |  |
Preferred stock shares issued |  |  |  |  | 0 |  |  | 0 |  |  |  |  |  | 0 | 0 |  |  | 0 | 0 | 0 |  |
Preferred stock shares outstanding |  |  |  |  | 0 |  |  | 0 |  |  |  |  |  | 0 | 0 |  |  | 0 | 0 | 0 |  |
Amortization period DTA (years) |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 15 years |  |  |  |  |  |  |
Unamortized deferred tax asset arising from equity offerings |  |  |  |  | $ 207 |  |  | $ 207 |  |  |  |  |  | $ 207 | $ 207 |  |  | $ 207 | $ 193 | $ 207 |  |
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 |  |  |  |  | $ 681 |  |  | $ 681 |  |  |  |  |  | 681 | $ 681 |  |  | $ 681 |  | 681 |  |
Tax savings owed to IBG Holdings LLC |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 579 |  |
Tax savings retained by IBG Inc. |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 102 |  |
Tax savings paid to IBG Holdings LLC |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 268 |  |
Shares reserved for future issuance |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 1,000,000 | 360,000,000
Shares redeemed by IBG Holdings from IBG LLC |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 5,013,259 |  |  |  |  |
Cash Redemptions IBG Holdings |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 114 |  |  |  |  |
Fair Value of Issued Shares in Exchange for Membership Interests |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 1,700 |  |  |
Stock registered, number of common stock |  |  |  | 630,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Shares Issued |  |  | 2,632,748 |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 370,000 | 37,478,697 |  |  |
Shares issued, value |  |  | $ 229 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Distribution from IBG LLC |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 603 |  |  |  |  |  |  |
Cash distribution to IBG, Inc. |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 150 |  |  |  |  |  |  |
Dividend per share |  |  |  |  | $ 0.10 | $ 0.10 | $ 0.10 | $ 0.10 | $ 0.10 | $ 0.10 | $ 0.10 | $ 0.10 | $ 0.10 |  |  |  |  |  |  |  |  |
Dividends paid to common shareholders |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 31 | $ 31 | $ 30 |  |  |  |  |  |
Declaration Date |  |  |  |  |  |  |  |  |  |  |  |  |  |  | Oct. 17, 2023 |  |  |  |  |  |  |
Payment Date |  |  |  |  |  |  |  |  |  |  |  |  |  |  | Dec. 14, 2023 |  |  |  |  |  |  |
Record Date |  |  |  |  |  |  |  |  |  |  |  |  |  |  | Dec. 01, 2023 |  |  |  |  |  |  |
Mr. Thomas Peterffy [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Equity And Earnings Per Share [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Majority stakeholder percentage |  |  | 91.30% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Subsequent Event [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Equity And Earnings Per Share [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Shares Issued |  | 50,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Dividend per share | $ 0.10 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
IBG LLC [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Equity And Earnings Per Share [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
IBG Inc. ownership % of IBG LLC |  |  |  |  | 25.40% |  |  | 25.40% |  |  |  |  |  | 25.40% | 25.40% |  |  | 25.40% |  | 25.40% |  | 10.00%
IBG Holdings ownership % of IBG LLC |  |  | 25.40% |  | 24.80% |  |  | 24.80% |  |  |  |  |  | 24.80% | 24.80% |  |  | 24.80% |  | 24.80% |  |
IBG LLC [Member] | Holdings [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Equity And Earnings Per Share [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
IBG Holdings ownership % of IBG LLC |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 90.00%
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 | $ 0.01 |  |
Shares authorized |  |  |  |  | 1,000,000,000 | 1,000,000,000 |  | 1,000,000,000 | 1,000,000,000 |  |  |  |  | 1,000,000,000 | 1,000,000,000 |  |  | 1,000,000,000 | 1,000,000,000 | 1,000,000,000 |  |
Common stock, shares issued |  |  |  |  | 107,125,489 |  |  | 107,125,489 |  |  |  |  |  | 107,125,489 | 107,125,489 |  |  | 107,125,489 | 103,057,148 | 107,125,489 |  |
Common stock, shares outstanding |  |  |  |  | 106,992,860 |  |  | 106,992,860 |  |  |  |  |  | 106,992,860 | 106,992,860 |  |  | 106,992,860 | 102,887,728 | 106,992,860 |  |
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 | $ 0.01 |  |
Shares authorized |  |  |  |  | 100 | 100 |  | 100 | 100 |  |  |  |  | 100 | 100 |  |  | 100 | 100 | 100 |  |
Common stock, shares issued |  |  |  |  | 100 | 100 |  | 100 | 100 |  |  |  |  | 100 | 100 |  |  | 100 | 100 | 100 |  |
Common stock, shares outstanding |  |  |  |  | 100 | 100 |  | 100 | 100 |  |  |  |  | 100 | 100 |  |  | 100 | 100 | 100 |  |

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