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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	9 Months Ended		36 Months Ended	69 Months Ended	156 Months Ended	209 Months Ended
	Oct. 15, 2024	Jul. 25, 2024	Jul. 26, 2023	Oct. 07, 2019	Oct. 31, 2024	Jun. 30, 2024	Mar. 31, 2024	Sep. 30, 2024	Jun. 30, 2024	Mar. 31, 2024	Sep. 30, 2023	Jun. 30, 2023	Mar. 31, 2023	Sep. 30, 2024	Sep. 30, 2024	Sep. 30, 2023	Dec. 31, 2010	Sep. 30, 2024	Dec. 31, 2023	Sep. 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								$ 190						$ 190	$ 190			$ 190	$ 197	$ 190
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		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																			$ 1,900
Stock registered, number of common stock			630,000	1,000,000
Shares Issued		333,000																	40,111,445
Shares issued, value		$ 39
Distribution from IBG LLC															774
Cash distribution to IBG, Inc.															198
Dividend per share						$ 0.25	$ 0.10	$ 0.25	$ 0.25	$ 0.10	$ 0.10	$ 0.10	$ 0.10
Dividends paid to common shareholders							$ 11							$ 54	$ 65	$ 31
O 2024 Q4 Dividends [Member]
Equity And Earnings Per Share [Line Items]
Declaration Date															Oct. 15, 2024
Payment Date															Dec. 13, 2024
Record Date															Nov. 29, 2024
Mr. Thomas Peterffy [Member]
Equity And Earnings Per Share [Line Items]
Majority stakeholder percentage		25.80%						25.70%						25.70%	25.70%			25.70%		25.70%
Subsequent Event [Member]
Equity And Earnings Per Share [Line Items]
Dividend per share	$ 0.25
IBG LLC [Member]
Equity And Earnings Per Share [Line Items]
IBG Inc. ownership % of IBG LLC								25.80%						25.80%	25.80%			25.80%		25.80%	10.00%
Shares Issued																		570,000
IBG LLC [Member] \| Thomas Peterffy And Affiliates [Member]
Equity And Earnings Per Share [Line Items]
Majority stakeholder percentage		91.40%
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]
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								109,009,224						109,009,224	109,009,224			109,009,224	107,178,928	109,009,224
Common stock, shares outstanding								108,867,197						108,867,197	108,867,197			108,867,197	107,045,894	108,867,197
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