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Equity And Earnings Per Share (Narrative) (Details) - USD ($) $ / shares in Units, $ in Millions |  |  |  | 1 Months Ended | 9 Months Ended | 36 Months Ended | 108 Months Ended | 161 Months Ended |  |  |
--- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | ---
Oct. 30, 2020 | Aug. 20, 2020 | Jul. 27, 2020 | Sep. 30, 2020 | Jun. 30, 2020 | Mar. 31, 2020 | Sep. 30, 2020 | Sep. 30, 2019 | Dec. 31, 2010 | Dec. 31, 2019 | Sep. 30, 2020 | Oct. 31, 2020 | Oct. 29, 2020 | 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 |  |  |  | $ 107 |  |  | $ 107 |  |  | $ 116 | $ 107 |  |  |
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 |  |  |  | $ 506 |  |  | $ 506 |  |  |  | 506 |  |  |
Tax savings owed to IBG Holdings LLC |  |  |  |  |  |  |  |  |  |  | 430 |  |  |
Tax savings retained by IBG Inc. |  |  |  |  |  |  |  |  |  |  | 76 |  |  |
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 |  |  |  |  |  |  |  |  |  | $ 506 |  |  |  |
Stock registered, number of common stock |  |  | 990,000 |  |  |  |  |  |  |  |  |  |  |
Shares Issued |  | 1,000,000 |  |  |  |  |  |  |  | 15,417,157 |  |  |  |
Shares issued, value |  | $ 52 |  |  |  |  |  |  |  |  |  |  |  |
Thomas Peterffy and Affiliates Ownership |  |  |  | 89.60% |  |  | 89.60% |  |  |  | 89.60% |  |  | 84.60%
Distribution from IBG LLC |  |  |  |  |  |  | $ 304 |  |  |  |  |  |  |
Cash distribution to IBG, Inc. |  |  |  |  |  |  | $ 57 |  |  |  |  |  |  |
Dividend per share |  |  |  | $ 0.10 | $ 0.10 | $ 0.10 | $ 0.10 |  |  |  |  |  |  |
Dividends paid to common shareholders |  |  |  | $ 8 | $ 7 | $ 8 | $ 23 | $ 23 |  |  |  |  |  |
Payment Date |  |  |  |  |  |  | Dec. 14, 2020 |  |  |  |  |  |  |
Record Date |  |  |  |  |  |  | Dec. 01, 2020 |  |  |  |  |  |  |
Subsequent Event [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Equity And Earnings Per Share [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Shares Issued | 11,710,608 |  |  |  |  |  |  |  |  |  |  |  |  |
Shares issued, value | $ 557 |  |  |  |  |  |  |  |  |  |  |  |  |
Thomas Peterffy and Affiliates Ownership |  |  |  |  |  |  |  |  |  |  |  | 89.80% |  |
IBG LLC [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Equity And Earnings Per Share [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
IBG Inc. ownership % of IBG LLC |  |  |  | 19.00% |  |  | 19.00% |  |  |  | 19.00% |  |  | 10.00%
IBG LLC [Member] | Holdings [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Equity And Earnings Per Share [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
IBG Holdings ownership % of IBG LLC |  |  |  | 81.00% |  |  | 81.00% |  |  |  | 81.00% |  |  | 90.00%
IBG LLC [Member] | Subsequent Event [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Equity And Earnings Per Share [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
IBG Inc. ownership % of IBG LLC | 21.80% |  |  |  |  |  |  |  |  |  |  |  | 19.00% |
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 |  |  |  | 79,187,067 |  |  | 79,187,067 |  |  | 76,889,040 | 79,187,067 |  |  |
Common stock, shares outstanding |  |  |  | 79,054,435 |  |  | 79,054,435 |  |  | 76,750,110 | 79,054,435 |  |  |
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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