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Distributions made by IBG LLC to controlling and noncontrolling interests.

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Equity And Earnings Per Share (Narrative) (Details) - USD ($) $ / shares in Units, $ in Millions					1 Months Ended		3 Months Ended		9 Months Ended		12 Months Ended			36 Months Ended	84 Months Ended	168 Months Ended	224 Months Ended
	Jan. 20, 2026	Jul. 30, 2025	Jul. 26, 2023	Oct. 07, 2019	Apr. 30, 2025	Apr. 30, 2024	Mar. 31, 2025	Mar. 31, 2024	Dec. 31, 2025	Dec. 31, 2024	Dec. 31, 2025	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2010	Dec. 31, 2025	Dec. 31, 2024	Dec. 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									$ 222	$ 196	$ 222	$ 196			$ 222	$ 196	$ 222
Deferred tax asset from common stock offerings																	727
Percent of tax savings owed to IBG Holdings LLC											85.00%
Percentage of tax savings retained by IBG Inc.											15.00%
Tax savings owed to IBG Holdings LLC																	618
Tax savings retained by IBG Inc.																	109
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		$ 254														$ 2,000
Stock registered, number of common stock			2,520,000	4,000,000
Shares Issued		3,836,000														161,777,780
Distribution from IBG LLC											$ 1,224	961	$ 741
Cash distribution to IBG, Inc.											319	246	$ 185
Dividend per share							$ 0.0625	$ 0.025	$ 0.08	$ 0.0625			$ 0.025
Dividends paid to common shareholders											$ 134	$ 92	$ 42
Minimum [Member]
Equity And Earnings Per Share [Line Items]
Dividend per share					$ 0.0625	$ 0.025
Maximum [Member]
Equity And Earnings Per Share [Line Items]
Dividend per share					$ 0.08	$ 0.0625
Subsequent Event [Member]
Equity And Earnings Per Share [Line Items]
Dividend per share	$ 0.08
Subsequent Event [Member] \| O2025 Q1 Dividends [Member]
Equity And Earnings Per Share [Line Items]
Declaration Date	Jan. 20, 2026
Payment Date	Mar. 13, 2026
Record Date	Feb. 27, 2026
IBG LLC [Member]
Equity And Earnings Per Share [Line Items]
IBG Inc. ownership % of IBG LLC									26.30%		26.30%				26.30%		26.30%	10.00%
IBG Holdings ownership % of IBG LLC									73.70%		73.70%				73.70%		73.70%	90.00%
Shares Issued															3,080,000
IBG LLC [Member] \| Thomas Peterffy And Affiliates [Member]
Equity And Earnings Per Share [Line Items]
Majority stakeholder percentage									91.60%		91.60%				91.60%		91.60%	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	$ 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	$ 0.01	$ 0.01