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Equity And Earnings Per Share (Narrative) (Details) - USD ($) $ / shares in Units, $ in Millions |  | 12 Months Ended | 36 Months Ended | 96 Months Ended | 152 Months Ended |  |
--- | --- | --- | --- | --- | --- | --- | ---
Jul. 26, 2019 | Dec. 31, 2019 | Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2010 | Dec. 31, 2018 | Dec. 31, 2019 | Oct. 07, 2019 | May 03, 2007
Equity And Earnings Per Share [Line Items] |  |  |  |  |  |  |  |  |
IBG Holdings ownership % of IBG LLC |  | 81.50% |  |  |  |  | 81.50% |  | 90.00%
IBG Holdings Redemption of IBG LLC |  |  |  |  |  |  |  |  | 10.00%
IBG Holdings LLC Ownership Percentage of Class B Common Stock |  |  |  |  |  |  |  |  | 100.00%
Common stock, par value |  | $ 0.01 | $ 0.01 |  |  | $ 0.01 | $ 0.01 |  |
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 |  | $ 116 | $ 140 |  |  | $ 140 | $ 116 |  |
Percent of tax savings owed to IBG Holdings LLC |  | 85.00% |  |  |  |  |  |  |
Percentage of tax savings retained by IBG Inc. |  | 15.00% |  |  |  |  |  |  |
Gain on remeasurement of Tax Receivable Agreement liability |  |  | 3 | $ 93 |  |  |  |  |
Deferred tax asset from common stock offerings |  | $ 499 |  |  |  |  | 499 |  |
Tax savings owed to IBG Holdings LLC |  |  |  |  |  |  | 424 |  |
Tax savings retained by IBG Inc. |  |  |  |  |  |  | 75 |  |
Tax savings paid to IBG Holdings LLC |  |  |  |  |  |  | $ 188 |  |
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 |  |  | 505 |  |  | $ 505 |  |  |
Shares Issued | 21,075 |  |  |  |  | 15,396,082 |  |  |
Shares issued, value | $ 1 |  |  |  |  |  |  |  |
Thomas Peterffy and Affiliates Ownership |  | 89.60% |  |  |  |  | 89.60% |  | 84.60%
Distribution from IBG LLC |  | $ 438 | 426 | 328 |  |  |  |  |
Cash distribution to IBG, Inc. |  | $ 81 | 76 | $ 56 |  |  |  |  |
Dividend per share |  | $ 0.10 |  |  |  |  |  |  |
Tax Receivable Agreement remeasured, gain recognized |  |  | $ 93 |  |  |  |  |  |
U.S. Statutory Tax Rate |  | 21.00% | 21.00% | 35.00% |  |  |  |  |
Dividends paid to common shareholders |  | $ 31 | $ 29 | $ 28 |  |  |  |  |
Declaration Date |  | Jan. 21, 2020 |  |  |  |  |  |  |
Payment Date |  | Mar. 13, 2020 |  |  |  |  |  |  |
Record Date |  | Feb. 28, 2020 |  |  |  |  |  |  |
Maximum [Member] |  |  |  |  |  |  |  |  |
Equity And Earnings Per Share [Line Items] |  |  |  |  |  |  |  |  |
Common stock, shares issued |  |  |  |  |  |  |  | 1,000,000 |
IBG LLC [Member] |  |  |  |  |  |  |  |  |
Equity And Earnings Per Share [Line Items] |  |  |  |  |  |  |  |  |
IBG Inc. ownership % of IBG LLC |  | 18.50% |  |  |  |  | 18.50% |  | 10.00%
IBG LLC [Member] | Assuming All Shares Are Issued [Member] | Maximum [Member] |  |  |  |  |  |  |  |  |
Equity And Earnings Per Share [Line Items] |  |  |  |  |  |  |  |  |
IBG Inc. ownership % of IBG LLC |  |  |  |  |  |  |  | 18.70% |
IBG LLC [Member] | Assuming All Shares Are Issued [Member] | Minimum [Member] |  |  |  |  |  |  |  |  |
Equity And Earnings Per Share [Line Items] |  |  |  |  |  |  |  |  |
IBG Inc. ownership % of IBG LLC |  |  |  |  |  |  |  | 18.50% |
Common Class A |  |  |  |  |  |  |  |  |
Equity And Earnings Per Share [Line Items] |  |  |  |  |  |  |  |  |
Common stock, par value |  | $ 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 |  | 76,889,040 | 75,230,400 |  |  | 75,230,400 | 76,889,040 |  |
Common stock, shares outstanding |  | 76,750,110 | 75,100,952 |  |  | 75,100,952 | 76,750,110 |  |
Common Class B |  |  |  |  |  |  |  |  |
Equity And Earnings Per Share [Line Items] |  |  |  |  |  |  |  |  |
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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