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Equity And Earnings Per Share (Narrative) (Details) - USD ($) $ / shares in Units, $ in Millions				1 Months Ended			9 Months Ended		36 Months Ended	108 Months Ended	161 Months Ended
	Oct. 30, 2020	Aug. 20, 2020	Jul. 27, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2010	Dec. 31, 2019	Sep. 30, 2020	Oct. 31, 2020	Oct. 29, 2020	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				$ 107			$ 107			$ 116	$ 107
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				$ 506			$ 506				506
Tax savings owed to IBG Holdings LLC											430
Tax savings retained by IBG Inc.											76
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										$ 506
Stock registered, number of common stock			990,000
Shares Issued		1,000,000								15,417,157
Shares issued, value		$ 52
Thomas Peterffy and Affiliates Ownership				89.60%			89.60%				89.60%			84.60%
Distribution from IBG LLC							$ 304
Cash distribution to IBG, Inc.							$ 57
Dividend per share				$ 0.10	$ 0.10	$ 0.10	$ 0.10
Dividends paid to common shareholders				$ 8	$ 7	$ 8	$ 23	$ 23
Payment Date							Dec. 14, 2020
Record Date							Dec. 01, 2020
Subsequent Event [Member]
Equity And Earnings Per Share [Line Items]
Shares Issued	11,710,608
Shares issued, value	$ 557
Thomas Peterffy and Affiliates Ownership												89.80%
IBG LLC [Member]
Equity And Earnings Per Share [Line Items]
IBG Inc. ownership % of IBG LLC				19.00%			19.00%				19.00%			10.00%
IBG LLC [Member] \| Holdings [Member]
Equity And Earnings Per Share [Line Items]
IBG Holdings ownership % of IBG LLC				81.00%			81.00%				81.00%			90.00%
IBG LLC [Member] \| Subsequent Event [Member]
Equity And Earnings Per Share [Line Items]
IBG Inc. ownership % of IBG LLC	21.80%												19.00%
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				79,187,067			79,187,067			76,889,040	79,187,067
Common stock, shares outstanding				79,054,435			79,054,435			76,750,110	79,054,435
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