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Equity And Earnings Per Share (Narrative) (Details) - USD ($) $ / shares in Units, $ in Millions		12 Months Ended			36 Months Ended	96 Months Ended	152 Months Ended
	Jul. 26, 2019	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2010	Dec. 31, 2018	Dec. 31, 2019	Oct. 07, 2019	May 03, 2007
Equity And Earnings Per Share [Line Items]
IBG Holdings ownership % of IBG LLC		81.50%					81.50%		90.00%
IBG Holdings Redemption of IBG LLC									10.00%
IBG Holdings LLC Ownership Percentage of Class B Common Stock									100.00%
Common stock, par value		$ 0.01	$ 0.01			$ 0.01	$ 0.01
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		$ 116	$ 140			$ 140	$ 116
Percent of tax savings owed to IBG Holdings LLC		85.00%
Percentage of tax savings retained by IBG Inc.		15.00%
Gain on remeasurement of Tax Receivable Agreement liability			3	$ 93
Deferred tax asset from common stock offerings		$ 499					499
Tax savings owed to IBG Holdings LLC							424
Tax savings retained by IBG Inc.							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			505			$ 505
Shares Issued	21,075					15,396,082
Shares issued, value	$ 1
Thomas Peterffy and Affiliates Ownership		89.60%					89.60%		84.60%
Distribution from IBG LLC		$ 438	426	328
Cash distribution to IBG, Inc.		$ 81	76	$ 56
Dividend per share		$ 0.10
Tax Receivable Agreement remeasured, gain recognized			$ 93
U.S. Statutory Tax Rate		21.00%	21.00%	35.00%
Dividends paid to common shareholders		$ 31	$ 29	$ 28
Declaration Date		Jan. 21, 2020
Payment Date		Mar. 13, 2020
Record Date		Feb. 28, 2020
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%		10.00%
IBG LLC [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] \| 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
Shares authorized		1,000,000,000	1,000,000,000			1,000,000,000	1,000,000,000
Common stock, shares issued		76,889,040	75,230,400			75,230,400	76,889,040
Common stock, shares outstanding		76,750,110	75,100,952			75,100,952	76,750,110
Common Class B
Equity And Earnings Per Share [Line Items]
Shares authorized		100	100			100	100
Common stock, shares issued		100	100			100	100
Common stock, shares outstanding		100	100			100	100