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Segment And Geographic Information (Segment Table) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Segment Reporting Information [Line Items]
Total net revenues	$ 439	$ 426	$ 1,411	$ 1,187
Income before income taxes	276	268	887	685
Assets	62,062		62,062		$ 61,162
Electronic Brokerage [Member]
Segment Reporting Information [Line Items]
Total net revenues	444	367	1,352	1,015
Income before income taxes	292	225	866	608
Assets	60,152		60,152		58,787
Market Making [Member]
Segment Reporting Information [Line Items]
Total net revenues	16	30	59	61
Income before income taxes	7	11	25	(35)
Assets	2,577		2,577		8,469
Corporate [Member]
Segment Reporting Information [Line Items]
Total net revenues	(21)	29		111
Income before income taxes	(23)	$ 32	(4)	$ 112
Assets	$ (667)		$ (667)		$ (6,094)

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Sum of the carrying amounts as of the balance sheet date of all assets that are recognized. Assets are probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
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-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
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-URI http://asc.fasb.org/extlink&oid=68072869&loc=d3e41242-110953

Reference 2: http://www.xbrl.org/2003/role/presentationRef
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