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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	66 Months Ended	156 Months Ended	206 Months Ended
	Jul. 25, 2024	Jul. 16, 2024	Jul. 26, 2023	Oct. 07, 2019	Jul. 31, 2024	Jun. 30, 2024	Mar. 31, 2024	Jun. 30, 2024	Mar. 31, 2024	Jun. 30, 2023	Mar. 31, 2023	Jun. 30, 2024	Jun. 30, 2023	Dec. 31, 2010	Jun. 30, 2024	Dec. 31, 2023	Jun. 30, 2024	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%
Preferred stock shares authorized						10,000		10,000				10,000			10,000	10,000	10,000
Preferred stock shares issued						0		0				0			0	0	0
Preferred stock shares outstanding						0		0				0			0	0	0
Amortization period DTA (years)												15 years
Unamortized deferred tax asset arising from equity offerings						$ 189		$ 189				$ 189			$ 189	$ 197	$ 189
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						$ 682		$ 682				$ 682			$ 682		682
Tax savings owed to IBG Holdings LLC																	580
Tax savings retained by IBG Inc.																	102
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																$ 1,900
Stock registered, number of common stock			630,000	1,000,000
Shares Issued																40,111,445
Distribution from IBG LLC												542
Cash distribution to IBG, Inc.												138
Dividend per share						$ 0.25	$ 0.10	$ 0.25	$ 0.10	$ 0.10	$ 0.10
Dividends paid to common shareholders						$ 27	$ 11					$ 38	$ 21
O 2024 Q3 Dividends [Member]
Equity And Earnings Per Share [Line Items]
Declaration Date												Jul. 16, 2024
Payment Date												Sep. 13, 2024
Record Date												Aug. 30, 2024
Mr. Thomas Peterffy [Member]
Equity And Earnings Per Share [Line Items]
Majority stakeholder percentage						25.70%		25.70%				25.70%			25.70%		25.70%
Subsequent Event [Member]
Equity And Earnings Per Share [Line Items]
Shares Issued	333,000
Shares issued, value	$ 39
Dividend per share		$ 0.25
Subsequent Event [Member] \| Mr. Thomas Peterffy [Member]
Equity And Earnings Per Share [Line Items]
Majority stakeholder percentage	25.80%
IBG LLC [Member]
Equity And Earnings Per Share [Line Items]
IBG Inc. ownership % of IBG LLC						25.70%		25.70%				25.70%			25.70%		25.70%	10.00%
Shares Issued															520,000
IBG LLC [Member] \| Holdings [Member]
Equity And Earnings Per Share [Line Items]
IBG Holdings ownership % of IBG LLC																		90.00%
IBG LLC [Member] \| Subsequent Event [Member]
Equity And Earnings Per Share [Line Items]
IBG Inc. ownership % of IBG LLC	91.40%
Shares Issued					50,000
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
Shares authorized						1,000,000,000		1,000,000,000				1,000,000,000			1,000,000,000	1,000,000,000	1,000,000,000
Common stock, shares issued						108,626,224		108,626,224				108,626,224			108,626,224	107,178,928	108,626,224
Common stock, shares outstanding						108,485,969		108,485,969				108,485,969			108,485,969	107,045,894	108,485,969
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
Shares authorized						100		100				100			100	100	100
Common stock, shares issued						100		100				100			100	100	100
Common stock, shares outstanding						100		100				100			100	100	100