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Equity And Earnings Per Share (Narrative) (Details) - USD ($) $ / shares in Units, $ in Millions			3 Months Ended			9 Months Ended		36 Months Ended	120 Months Ended	173 Months Ended
	Jul. 30, 2021	Jul. 27, 2020	Sep. 30, 2021	Jun. 30, 2021	Mar. 31, 2021	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2010	Dec. 31, 2020	Sep. 30, 2021	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
Preferred stock shares issued			0			0			0	0
Preferred stock shares outstanding			0			0			0	0
Amortization period DTA (years)						15 years
Unamortized deferred tax asset arising from equity offerings			$ 216			$ 216			$ 190	$ 216
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			634			$ 634				634
Tax savings owed to IBG Holdings LLC										539
Tax savings retained by IBG Inc.										95
Tax savings paid to IBG Holdings LLC										$ 205
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									$ 1,100
Stock registered, number of common stock		990,000
Shares Issued	6,079,542					100,000			28,127,765
Shares issued, value	$ 376
Distribution from IBG LLC						$ 297
Cash distribution to IBG, Inc.						$ 67
Dividend per share						$ 0.10
Dividends paid to common shareholders			$ 10	$ 9	$ 9	$ 28	$ 23
Declaration Date						Oct. 19, 2021
Payment Date						Dec. 14, 2021
Record Date						Dec. 01, 2021
Mr. Thomas Peterffy [Member]
Equity And Earnings Per Share [Line Items]
Majority stakeholder percentage			90.60%			90.60%				90.60%	84.60%
IBG LLC [Member]
Equity And Earnings Per Share [Line Items]
IBG Inc. ownership % of IBG LLC			23.50%			23.50%				23.50%	10.00%
IBG Holdings ownership % of IBG LLC			76.50%			76.50%				76.50%
IBG LLC [Member] \| Holdings [Member]
Equity And Earnings Per Share [Line Items]
IBG Holdings ownership % of IBG LLC											90.00%
Common Class A [Member]
Equity And Earnings Per Share [Line Items]
Common stock, par value			$ 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
Common stock, shares issued			98,305,396			98,305,396			90,909,889	98,305,396
Common stock, shares outstanding			98,162,756			98,162,756			90,773,105	98,162,756
Common Class B [Member]
Equity And Earnings Per Share [Line Items]
Common stock, par value			$ 0.01			$ 0.01			$ 0.01	$ 0.01
Shares authorized			100			100			100	100
Common stock, shares issued			100			100			100	100
Common stock, shares outstanding			100			100			100	100