Investments - Summary of Selected Financial Information for KCG (Details) - KCG Holdings, Inc [Member] - USD ($) $ in Millions	Dec. 31, 2016	Sep. 30, 2016
Schedule of Equity Method Investments [Line Items]
Total assets	$ 6,261.3	$ 6,750.6
Total liabilities	4,904.0	5,311.0
Total equity	$ 1,357.3	$ 1,439.6

Amount of equity, excluding noncontrolling interest, attributable to the equity method investment of the entity.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 323
-SubTopic 10
-Section 50
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-URI http://asc.fasb.org/extlink&oid=66011266&loc=d3e33918-111571

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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Reference 4: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 323
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-Section 50
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-URI http://asc.fasb.org/extlink&oid=66011266&loc=d3e33912-111571

Reference 5: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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