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Equity And Earnings Per Share (Narrative) (Details) - USD ($) $ / shares in Units, $ in Millions |  |  | 3 Months Ended | 9 Months Ended | 36 Months Ended | 120 Months Ended | 173 Months Ended |
--- | --- | --- | --- | --- | --- | --- | --- | ---
Jul. 30, 2021 | Jul. 27, 2020 | Sep. 30, 2021 | Jun. 30, 2021 | Mar. 31, 2021 | Sep. 30, 2021 | Sep. 30, 2020 | Dec. 31, 2010 | Dec. 31, 2020 | Sep. 30, 2021 | 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 |
Preferred stock shares issued |  |  | 0 |  |  | 0 |  |  | 0 | 0 |
Preferred stock shares outstanding |  |  | 0 |  |  | 0 |  |  | 0 | 0 |
Amortization period DTA (years) |  |  |  |  |  | 15 years |  |  |  |  |
Unamortized deferred tax asset arising from equity offerings |  |  | $ 216 |  |  | $ 216 |  |  | $ 190 | $ 216 |
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 |  |  | 634 |  |  | $ 634 |  |  |  | 634 |
Tax savings owed to IBG Holdings LLC |  |  |  |  |  |  |  |  |  | 539 |
Tax savings retained by IBG Inc. |  |  |  |  |  |  |  |  |  | 95 |
Tax savings paid to IBG Holdings LLC |  |  |  |  |  |  |  |  |  | $ 205 |
Shares reserved for future issuance |  |  |  |  |  |  |  |  |  |  | 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 |  |  |  |  |  |  |  |  | $ 1,100 |  |
Stock registered, number of common stock |  | 990,000 |  |  |  |  |  |  |  |  |
Shares Issued | 6,079,542 |  |  |  |  | 100,000 |  |  | 28,127,765 |  |
Shares issued, value | $ 376 |  |  |  |  |  |  |  |  |  |
Distribution from IBG LLC |  |  |  |  |  | $ 297 |  |  |  |  |
Cash distribution to IBG, Inc. |  |  |  |  |  | $ 67 |  |  |  |  |
Dividend per share |  |  |  |  |  | $ 0.10 |  |  |  |  |
Dividends paid to common shareholders |  |  | $ 10 | $ 9 | $ 9 | $ 28 | $ 23 |  |  |  |
Declaration Date |  |  |  |  |  | Oct. 19, 2021 |  |  |  |  |
Payment Date |  |  |  |  |  | Dec. 14, 2021 |  |  |  |  |
Record Date |  |  |  |  |  | Dec. 01, 2021 |  |  |  |  |
Mr. Thomas Peterffy [Member] |  |  |  |  |  |  |  |  |  |  |
Equity And Earnings Per Share [Line Items] |  |  |  |  |  |  |  |  |  |  |
Majority stakeholder percentage |  |  | 90.60% |  |  | 90.60% |  |  |  | 90.60% | 84.60%
IBG LLC [Member] |  |  |  |  |  |  |  |  |  |  |
Equity And Earnings Per Share [Line Items] |  |  |  |  |  |  |  |  |  |  |
IBG Inc. ownership % of IBG LLC |  |  | 23.50% |  |  | 23.50% |  |  |  | 23.50% | 10.00%
IBG Holdings ownership % of IBG LLC |  |  | 76.50% |  |  | 76.50% |  |  |  | 76.50% |
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 |
Shares authorized |  |  | 1,000,000,000 |  |  | 1,000,000,000 |  |  | 1,000,000,000 | 1,000,000,000 |
Common stock, shares issued |  |  | 98,305,396 |  |  | 98,305,396 |  |  | 90,909,889 | 98,305,396 |
Common stock, shares outstanding |  |  | 98,162,756 |  |  | 98,162,756 |  |  | 90,773,105 | 98,162,756 |
Common Class B [Member] |  |  |  |  |  |  |  |  |  |  |
Equity And Earnings Per Share [Line Items] |  |  |  |  |  |  |  |  |  |  |
Common stock, par value |  |  | $ 0.01 |  |  | $ 0.01 |  |  | $ 0.01 | $ 0.01 |
Shares authorized |  |  | 100 |  |  | 100 |  |  | 100 | 100 |
Common stock, shares issued |  |  | 100 |  |  | 100 |  |  | 100 | 100 |
Common stock, shares outstanding |  |  | 100 |  |  | 100 |  |  | 100 | 100 |

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