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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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Fair Value of Issued Shares in Exchange for Membership Interests.

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

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Percent ownership of IBG LLC redeemed by IBG Holdings from its members in connection with the consummation of the IPO.

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Percentage of IBG Holdings LLC interest held by Thomas Peterffy and his affiliates.

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Amount of membership interests redeemed by IBG Holdings, LLC from IBG LLC which were funded using cash on hand at IBG LLC.

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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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Date the declared dividend will be paid, in CCYY-MM-DD format.

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Equity And Earnings Per Share (Narrative) (Details) - USD ($) $ / shares in Units, $ in Millions		1 Months Ended		3 Months Ended	9 Months Ended		36 Months Ended	96 Months Ended	149 Months Ended
	Jul. 26, 2019	Jun. 30, 2019	Mar. 31, 2019	Sep. 30, 2019	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2010	Dec. 31, 2018	Sep. 30, 2019	Oct. 07, 2019	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				$ 124	$ 124			$ 140	$ 124
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				501	$ 501				501
Tax savings owed to IBG Holdings LLC				426	426				426
Tax savings retained by IBG Inc.				$ 75	$ 75				75
Tax savings paid to IBG Holdings LLC									$ 188
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							$ 505
Shares Issued	21,075							15,396,082
Thomas Peterffy and Affiliates Ownership				89.60%	89.60%				89.60%		84.60%
Distribution from IBG LLC					$ 371
Cash distribution to IBG, Inc.					$ 68
Dividend per share		$ 0.10	$ 0.10	$ 0.10	$ 0.10
Dividends paid to common shareholders		$ 7	$ 8	$ 8	$ 23	$ 22
Declaration Date					Oct. 15, 2019
Payment Date					Dec. 13, 2019
Record Date					Nov. 29, 2019
Subsequent Event [Member] \| Maximum [Member]
Equity And Earnings Per Share [Line Items]
Common stock, shares issued										1,000,000
IBG LLC [Member]
Equity And Earnings Per Share [Line Items]
IBG Inc. ownership % of IBG LLC				18.50%	18.50%				18.50%		10.00%
IBG LLC [Member] \| Holdings [Member]
Equity And Earnings Per Share [Line Items]
IBG Holdings ownership % of IBG LLC				81.50%	81.50%				81.50%		90.00%
IBG LLC [Member] \| Subsequent Event [Member] \| Assuming All Shares Are Issued [Member] \| Maximum [Member]
Equity And Earnings Per Share [Line Items]
IBG Inc. ownership % of IBG LLC										18.70%
IBG LLC [Member] \| Subsequent Event [Member] \| Assuming All Shares Are Issued [Member] \| Minimum [Member]
Equity And Earnings Per Share [Line Items]
IBG Inc. ownership % of IBG LLC										18.50%
Common Class A
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				76,878,300	76,878,300			75,230,400	76,878,300
Common stock, shares outstanding				76,748,852	76,748,852			75,100,952	76,748,852
Common Class B
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