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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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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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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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Equity And Earnings Per Share (Narrative) (Details) - USD ($) $ / shares in Units, $ in Millions					1 Months Ended	3 Months Ended				6 Months Ended		36 Months Ended	78 Months Ended	168 Months Ended	218 Months Ended
	Jul. 30, 2025	Jul. 17, 2025	Jul. 26, 2023	Oct. 07, 2019	Mar. 31, 2025	Jun. 30, 2025	Mar. 31, 2025	Jun. 30, 2024	Mar. 31, 2024	Jun. 30, 2025	Jun. 30, 2024	Dec. 31, 2010	Jun. 30, 2025	Dec. 31, 2024	Jun. 30, 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						$ 188				$ 188			$ 188	$ 196	$ 188
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															$ 308
Shares reserved for future issuance						1,440,000,000				1,440,000,000			1,440,000,000		1,440,000,000	360,000,000
Shares redeemed by IBG Holdings from IBG LLC												20,053,036
Cash Redemptions IBG Holdings												$ 114
Fair Value of Issued Shares in Exchange for Membership Interests														$ 2,000
Stock registered, number of common stock			2,520,000	4,000,000
Shares Issued														161,777,780
Distribution from IBG LLC										$ 653
Cash distribution to IBG, Inc.										169
Dividend per share					$ 0.0625	$ 0.08	$ 0.0625	$ 0.0625	$ 0.025
Dividends paid to common shareholders					$ 27	$ 35				$ 62	$ 38
Subsequent Event [Member]
Equity And Earnings Per Share [Line Items]
Fair Value of Issued Shares in Exchange for Membership Interests	$ 254
Shares Issued	3,836,000
Dividend per share		$ 0.08
Subsequent Event [Member] \| O 2025 Q3 Dividends [Member]
Equity And Earnings Per Share [Line Items]
Declaration Date		Jul. 17, 2025
Payment Date		Sep. 12, 2025
Record Date		Sep. 01, 2025
IBG LLC [Member]
Equity And Earnings Per Share [Line Items]
IBG Inc. ownership % of IBG LLC						26.00%				26.00%			26.00%		26.00%	10.00%
IBG Holdings ownership % of IBG LLC						74.00%				74.00%			74.00%		74.00%	90.00%
Shares Issued													2,880,000
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