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Income Taxes (Narrative) (Details) - USD ($) $ in Billions | Sep. 30, 2019 | Dec. 31, 2018
--- | --- | ---
Income Taxes [Abstract] |  |
Undistributed accumulated earnings of foreign subsidiaries | $ 1.2 | $ 1.1
Accumulated earnings of foreign pass through subsidiaries | 0.2 | 0.2
Accumulated earnings subject to additional foreign tax | $ 0.2 | $ 0.2

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- Definition Accumulated earnings attributable to our foreign subsidiaries that are considered pass-through entities for U.S. income tax purposes. + References No definition available. + Details
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Namespace Prefix: | ibkr_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Accumulated earnings of certain pass through subsidiaries where repatriation would result in additional foreign taxes in the form of dividend withholding tax imposed on the recipient of the distribution or dividend distribution tax imposed on the payor of the distribution. + References No definition available. + Details
Name: | ibkr_AccumulatedEarningsSubjectToAdditionalForeignTax
Namespace Prefix: | ibkr_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Amount of undistributed earnings of foreign subsidiaries. + References No definition available. + Details
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Namespace Prefix: | ibkr_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- References No definition available. + Details
Name: | us-gaap_IncomeTaxDisclosureAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration