Segment And Geographic Information (Segment Table) (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015	Dec. 31, 2015
Segment Reporting Information [Line Items]
Total net revenues	$ 345	$ 359	$ 1,203	$ 918
Income before income taxes	183	202	733	331
Assets	55,064		55,064		$ 48,734
Electronic Brokerage [Member]
Segment Reporting Information [Line Items]
Total net revenues	288	300	945	878
Income before income taxes	162	184	588	423
Assets	50,307		50,307		44,421
Market Making [Member]
Segment Reporting Information [Line Items]
Total net revenues	43	90	145	229
Income before income taxes	7	46	32	103
Assets	11,777		11,777		10,825
Corporate And Eliminations [Member]
Segment Reporting Information [Line Items]
Total net revenues	14	(31)	113	(189)
Income before income taxes	14	$ (28)	113	$ (195)
Assets	$ (7,020)		$ (7,020)		$ (6,512)

This element represents the income or loss from continuing operations attributable to the economic entity which may also be defined as revenue less expenses from ongoing operations, after income or loss from equity method investments, but before income taxes, extraordinary items, and noncontrolling interest.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 940
-SubTopic 20
-Section 25
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=51673353&loc=d3e41242-110953

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 235
-SubTopic 10
-Section S99
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e23780-122690

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Regulation S-X (SX)
-Number 210
-Section 03
-Paragraph 10
-Article 5

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