Total value of shares redeemed by IBG Holdings from IBG LLC funded by using cash on hand at IBG LLC.

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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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Percentage of tax savings retained by the Company according to the "Tax Receivable Agreement"

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'Cumulative amount of tax savings realized by the Company, from the IPO and follow-on offerings, that has been paid to IBG Holdings, LLC according to the "Tax Receivable Agreement".

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Cumulative amount of tax savings realized by the Company, from the IPO and follow-on offerings, that is payable to IBG Holdings, LLC according to the "Tax Receivable Agreement".

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Percent of tax savings realized by the Company that is payable to Holdings according to the "Tax Receivable Agreement"

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Cumulative amount of tax savings realized by the Company, from the IPO and follow-on offerings, that is retained by the Company according to the "Tax Receivable Agreement".

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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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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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The unamortized balance of the cumulative deferred tax asset arising from Common Stock offerings.

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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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Aggregate number of common shares reserved for future issuance.

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Face amount or stated value per share of common stock.

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The maximum number of common shares permitted to be issued by an entity's charter and bylaws.

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Total number of common shares of an entity that have been sold or granted to shareholders (includes common shares that were issued, repurchased and remain in the treasury). These shares represent capital invested by the firm's shareholders and owners, and may be all or only a portion of the number of shares authorized. Shares issued include shares outstanding and shares held in the treasury.

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Number of shares of common stock outstanding. Common stock represent the ownership interest in a corporation.

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Amount, before allocation of a valuation allowances, of deferred tax assets attributable to deductible tax credit carryforwards including, but not limited to, research, foreign, general business, alternative minimum tax, and other deductible tax credit carryforwards.

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

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Date the holder must own the stock to be entitled to the dividend, in CCYY-MM-DD format.

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The equity interest of noncontrolling shareholders, partners or other equity holders in consolidated entity.

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The parent entity's interest in net assets of the subsidiary, expressed as a percentage.

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Cash outflow in the form of capital distributions and dividends to common shareholders, preferred shareholders and noncontrolling interests.

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The maximum number of nonredeemable preferred shares (or preferred stock redeemable solely at the option of the issuer) permitted to be issued by an entity's charter and bylaws.

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Equity And Earnings Per Share (Narrative) (Details) - USD ($) $ / shares in Units, $ in Millions		1 Months Ended		6 Months Ended		36 Months Ended	96 Months Ended	146 Months Ended
	Jul. 26, 2019	Jun. 30, 2019	Mar. 31, 2019	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2010	Dec. 31, 2018	Jun. 30, 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		$ 129		$ 129			$ 140	$ 129
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								$ 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%				89.60%	84.60%
Distribution from IBG LLC				$ 119
Cash distribution to IBG, Inc.				$ 21
Dividend per share		$ 0.10	$ 0.10	$ 0.10
Dividends paid to common shareholders		$ 7	$ 8	$ 15	$ 14
Declaration Date				Jul. 16, 2019
Payment Date				Sep. 13, 2019
Record Date				Aug. 30, 2019
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%
Subsequent Event [Member]
Equity And Earnings Per Share [Line Items]
Shares Issued	21,075
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		76,856,784		76,856,784			75,230,400	76,856,784
Common stock, shares outstanding		76,727,336		76,727,336			75,100,952	76,727,336
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