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Segment And Geographic Information (Segment Table) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Segment Reporting Information [Line Items]
Total net revenues	$ 527	$ 374
Income before income taxes	340	213
Assets	59,093		$ 61,162
Electronic Brokerage [Member]
Segment Reporting Information [Line Items]
Total net revenues	465	314
Income before income taxes	291	185
Assets	57,316		58,787
Market Making [Member]
Segment Reporting Information [Line Items]
Total net revenues	21	8
Income before income taxes	9	(22)
Assets	3,618		8,469
Corporate [Member]
Segment Reporting Information [Line Items]
Total net revenues	41	52
Income before income taxes	40	$ 50
Assets	$ (1,841)		$ (6,094)

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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-SubTopic 210
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Reference 1: http://www.xbrl.org/2003/role/presentationRef
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