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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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EquityAndEarningsPerShareLineItems

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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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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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Equity And Earnings Per Share (Narrative) (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended		36 Months Ended	96 Months Ended	143 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2010	Dec. 31, 2018	Mar. 31, 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
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	$ 134			$ 140	$ 134
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
Tax savings owed to IBG Holdings LLC	426				426
Tax savings retained by IBG Inc.	$ 75				75
Tax savings paid to IBG Holdings LLC					$ 159
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				$ 504
Shares Issued				15,396,082
Thomas Peterffy and Affiliates Ownership	89.60%				89.60%	84.60%
Distribution from IBG LLC	$ 73
Cash distribution to IBG, Inc.	$ 13
Dividend per share	$ 0.10
Dividends paid to common shareholders	$ 8	$ 7
Declaration Date	Apr. 16, 2019
Payment Date	Jun. 14, 2019
Record Date	May 31, 2019
IBG LLC [Member]
Equity And Earnings Per Share [Line Items]
IBG Inc. ownership % of IBG LLC	18.10%				18.10%	10.00%
IBG LLC [Member] \| Holdings [Member]
Equity And Earnings Per Share [Line Items]
IBG Holdings ownership % of IBG LLC	81.90%				81.90%	90.00%
Common Class A
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	75,231,305			75,230,400	75,231,305
Common stock, shares outstanding	75,101,857			75,100,952	75,101,857
Common Class B
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