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Equity And Earnings Per Share (Narrative) (Details) - USD ($) $ / shares in Units, $ in Millions									3 Months Ended				6 Months Ended		36 Months Ended	81 Months Ended	180 Months Ended	230 Months Ended
	Sep. 14, 2026	Sep. 01, 2026	Jul. 31, 2026	Jul. 21, 2026	Jun. 30, 2026	Apr. 30, 2026	Mar. 31, 2026	Oct. 07, 2019	Jun. 30, 2026	Mar. 31, 2026	Jun. 30, 2025	Mar. 31, 2025	Jun. 30, 2026	Jun. 30, 2025	Dec. 31, 2010	Jun. 30, 2026	Dec. 31, 2025	Jun. 30, 2026	Apr. 15, 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%
Common stock, par value					$ 0.01				$ 0.01				$ 0.01			$ 0.01	$ 0.01	$ 0.01
Amortization period DTA (years)													15 years
Unamortized deferred tax asset arising from equity offerings					$ 213				$ 213				$ 213			$ 213	$ 222	$ 213
Percent of tax savings owed to IBG Holdings LLC													85.00%
Percentage of tax savings retained by IBG Inc.													15.00%
Deferred Tax Asset Arising From Equity Offerings																		727
Tax savings owed to IBG Holdings LLC																		618
Tax savings retained by IBG Inc.																		109
Tax savings paid to IBG Holdings LLC																		$ 321
Shares reserved for future issuance																			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			$ 224														$ 2,200
Stock registered, number of common stock			920,000					4,000,000
Shares Issued			2,499,567														165,613,780
Distribution from IBG LLC													$ 810
Distribution to IBG, Inc													213
Dividend per share					$ 0.0875		$ 0.08		$ 0.0875	$ 0.08	$ 0.08	$ 0.0625
Dividends paid to common shareholders					$ 39		$ 36						$ 75	$ 62
Subsequent Event [Member] \| O 2026 Q3 Dividends [Member]
Equity And Earnings Per Share [Line Items]
Payment Date	Sep. 14, 2026
Record Date		Sep. 01, 2026
IBG LLC [Member]
Equity And Earnings Per Share [Line Items]
IBG Inc. ownership % of IBG LLC					26.50%				26.50%				26.50%			26.50%		26.50%		10.00%
IBG Holdings ownership % of IBG LLC			73.40%																	90.00%
Shares Issued																3,080,000
IBG LLC [Member] \| Thomas Peterffy And Affiliates [Member]
Equity And Earnings Per Share [Line Items]
IBG Inc. ownership % of IBG LLC			26.60%
Majority stakeholder percentage			91.80%																	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
Minimum [Member]
Equity And Earnings Per Share [Line Items]
Dividend per share						$ 0.08
Minimum [Member] \| Subsequent Event [Member]
Equity And Earnings Per Share [Line Items]
Dividend per share				$ 0.0875
Maximum [Member]
Equity And Earnings Per Share [Line Items]
Dividend per share						$ 0.0875