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Equity And Earnings Per Share (Narrative) (Details) - USD ($) $ / shares in Units, $ in Millions			3 Months Ended		36 Months Ended	51 Months Ended	144 Months Ended	191 Months Ended
	May 05, 2023	Jul. 27, 2020	Mar. 31, 2023	Mar. 31, 2022	Dec. 31, 2010	Mar. 31, 2023	Dec. 31, 2022	Mar. 31, 2023	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			$ 189			$ 189	$ 193	$ 189
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			$ 654			$ 654		654
Tax savings owed to IBG Holdings LLC								556
Tax savings retained by IBG Inc.								98
Tax savings paid to IBG Holdings LLC								$ 243
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							$ 1,700
Stock registered, number of common stock		990,000
Shares Issued						320,000	37,478,697
Distribution from IBG LLC			158
Cash distribution to IBG, Inc.			$ 39
Dividend per share			$ 0.10	$ 0.10
Dividends paid to common shareholders			$ 10	$ 9
Declaration Date			Apr. 18, 2023
Payment Date			Jun. 14, 2023
Record Date			Jun. 01, 2023
Mr. Thomas Peterffy [Member]
Equity And Earnings Per Share [Line Items]
Majority stakeholder percentage			90.50%			90.50%		90.50%	84.60%
Subsequent Event [Member]
Equity And Earnings Per Share [Line Items]
Shares Issued	50,000
IBG LLC [Member]
Equity And Earnings Per Share [Line Items]
IBG Inc. ownership % of IBG LLC			24.50%			24.50%		24.50%	10.00%
IBG Holdings ownership % of IBG LLC			75.50%			75.50%		75.50%
IBG LLC [Member] \| Holdings [Member]
Equity And Earnings Per Share [Line Items]
IBG Holdings ownership % of IBG LLC									90.00%
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			103,128,710			103,128,710	103,057,148	103,128,710
Common stock, shares outstanding			102,984,753			102,984,753	102,887,728	102,984,753
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