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Useful life of deferred tax asset related to common stock offerings. This deferred tax asset is amortized as additional deferred income tax expense from the IPO date and from the additional redemption dates.

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Distributions made by IBG LLC to controlling and noncontrolling interests.

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IBG Holdings LLC's ownership percentage in Class B common stock.

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This element represents disclosure of the total aggregate cash dividends paid to the entity by consolidated subsidiaries, by unconsolidated subsidiaries, and by 50% or less owned persons accounted for using the equity method.

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Equity And Earnings Per Share (Narrative) (Details) - USD ($) $ / shares in Units, $ in Millions		3 Months Ended		36 Months Ended	120 Months Ended	167 Months Ended
	Jul. 27, 2020	Mar. 31, 2021	Mar. 31, 2020	Dec. 31, 2010	Dec. 31, 2020	Mar. 31, 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
Preferred stock shares issued		0			0	0
Preferred stock shares outstanding		0			0	0
Amortization period DTA (years)		15 years
Unamortized deferred tax asset arising from equity offerings		$ 183			$ 190	$ 183
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		$ 585				585
Tax savings owed to IBG Holdings LLC						497
Tax savings retained by IBG Inc.						88
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		50,000			28,127,765
Distribution from IBG LLC		$ 110
Cash distribution to IBG, Inc.		$ 24
Dividend per share		$ 0.10
Dividends paid to common shareholders		$ 9	$ 8
Declaration Date		Apr. 20, 2021
Payment Date		Jun. 14, 2021
Record Date		Jun. 01, 2021
Mr. Thomas Peterffy [Member]
Equity And Earnings Per Share [Line Items]
Majority stakeholder percentage		89.60%				89.60%	84.60%
IBG LLC [Member]
Equity And Earnings Per Share [Line Items]
IBG Inc. ownership % of IBG LLC		21.80%				21.80%	10.00%
IBG Holdings ownership % of IBG LLC		78.20%				78.20%
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
Shares authorized		1,000,000,000			1,000,000,000	1,000,000,000
Common stock, shares issued		90,959,889			90,909,889	90,959,889
Common stock, shares outstanding		90,807,181			90,773,105	90,807,181
Common Class B [Member]
Equity And Earnings Per Share [Line Items]
Common stock, par value		$ 0.01			$ 0.01	$ 0.01
Shares authorized		100			100	100
Common stock, shares issued		100			100	100
Common stock, shares outstanding		100			100	100