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Useful life of deferred tax asset related to common stock offerings. This deferred tax asset is amortized as additional deferred income tax expense from the IPO date and from the additional redemption dates.

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Percentage of tax savings retained by the Company according to the "Tax Receivable Agreement"

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'Cumulative amount of tax savings realized by the Company, from the IPO and follow-on offerings, that has been paid to IBG Holdings, LLC according to the "Tax Receivable Agreement".

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Cumulative amount of tax savings realized by the Company, from the IPO and follow-on offerings, that is payable to IBG Holdings, LLC according to the "Tax Receivable Agreement".

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Percent of tax savings realized by the Company that is payable to Holdings according to the "Tax Receivable Agreement"

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Cumulative amount of tax savings realized by the Company, from the IPO and follow-on offerings, that is retained by the Company according to the "Tax Receivable Agreement".

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Distributions made by IBG LLC to controlling and noncontrolling interests.

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EquityAndEarningsPerShareLineItems

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Fair Value of Issued Shares in Exchange for Membership Interests.

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IBG Holdings LLC's ownership percentage in Class B common stock.

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Percent ownership of IBG LLC redeemed by IBG Holdings from its members in connection with the consummation of the IPO.

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Amount of membership interests redeemed by IBG Holdings, LLC from IBG LLC which were funded using cash on hand at IBG LLC.

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The unamortized balance of the cumulative deferred tax asset arising from Common Stock offerings.

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This element represents disclosure of the total aggregate cash dividends paid to the entity by consolidated subsidiaries, by unconsolidated subsidiaries, and by 50 percent or less owned persons accounted for using the equity method.

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Aggregate dividends paid during the period for each share of common stock outstanding.

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Face amount or stated value per share of common stock.

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Amount, before allocation of a valuation allowances, of deferred tax assets attributable to deductible tax credit carryforwards including, but not limited to, research, foreign, general business, alternative minimum tax, and other deductible tax credit carryforwards.

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Date the declared dividend will be paid, in YYYY-MM-DD format.

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Date the holder must own the stock to be entitled to the dividend, in YYYY-MM-DD format.

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The equity interest of noncontrolling shareholders, partners or other equity holders in consolidated entity.

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The parent entity's interest in net assets of the subsidiary, expressed as a percentage.

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Amount of cash outflow in the form of ordinary dividends to common shareholders of the parent entity.

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Equity And Earnings Per Share (Narrative) (Details) - USD ($) $ / shares in Units, $ in Millions				1 Months Ended	3 Months Ended		36 Months Ended	75 Months Ended	168 Months Ended	215 Months Ended
	Apr. 15, 2025	Jul. 26, 2023	Oct. 07, 2019	Mar. 31, 2025	Mar. 31, 2025	Mar. 31, 2024	Dec. 31, 2010	Mar. 31, 2025	Dec. 31, 2024	Mar. 31, 2025	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%
Amortization period DTA (years)					15 years
Unamortized deferred tax asset arising from equity offerings				$ 192	$ 192			$ 192	$ 196	$ 192
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				$ 688	$ 688			$ 688		688
Tax savings owed to IBG Holdings LLC										585
Tax savings retained by IBG Inc.										103
Tax savings paid to IBG Holdings LLC										$ 293
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									$ 2,000
Stock registered, number of common stock		630,000	1,000,000
Shares Issued									40,444,445
Distribution from IBG LLC					263
Cash distribution to IBG, Inc.					$ 68
Dividend per share				$ 0.25	$ 0.25	$ 0.10
Dividends paid to common shareholders				$ 27	$ 27	$ 11
Subsequent Event [Member]
Equity And Earnings Per Share [Line Items]
Dividend per share	$ 0.32
Subsequent Event [Member] \| O 2025 Q2 Dividends [Member]
Equity And Earnings Per Share [Line Items]
Declaration Date	Apr. 15, 2025
Payment Date	Jun. 13, 2025
Record Date	May 30, 2025
IBG LLC [Member]
Equity And Earnings Per Share [Line Items]
IBG Inc. ownership % of IBG LLC				25.80%	25.80%			25.80%		25.80%	10.00%
IBG Holdings ownership % of IBG LLC				74.20%	74.20%			74.20%		74.20%	90.00%
Shares Issued								670,000
IBG LLC [Member] \| Thomas Peterffy And Affiliates [Member]
Equity And Earnings Per Share [Line Items]
Majority stakeholder percentage				91.40%	91.40%			91.40%		91.40%	84.60%
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
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