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Financial Instruments with Off-Balance Sheet Risk and Concentrations of Credit Risk - Gross Derivative Assets and Liabilities by Type and Balance Sheet Location (Details) - USD ($) $ in Millions | Mar. 31, 2019 | Sep. 30, 2018
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Derivatives, Fair Value [Line Items] |  |
Derivative Asset, Fair Value, Gross Asset | $ 4,044.2 | $ 8,035.9
Derivative Liability, Fair Value, Gross Liability | 4,019.0 | 8,511.0
Impact of netting and collateral | 3,857.1 | 8,118.5
Impact of netting and collateral | 3,801.8 | 8,317.6
Deposits with and receivables from broker-dealers, clearing organizations, and counterparties [Member] |  |
Derivatives, Fair Value [Line Items] |  |
Derivative, Fair Value, Net | (16.6) | (268.7)
Financial instruments owned [Member] |  |
Derivatives, Fair Value [Line Items] |  |
Derivative, Fair Value, Net | 203.7 | 186.1
Payable to broker-dealers, clearing organizations and counterparties [Member] |  |
Derivatives, Fair Value [Line Items] |  |
Derivative, Fair Value, Net | 15.8 | 0.0
Financial instrument sold, not yet purchased [Member] |  |
Derivatives, Fair Value [Line Items] |  |
Derivative, Fair Value, Net | 201.4 | 193.4
Exchange-traded Commodity Contracts [Member] |  |
Derivatives, Fair Value [Line Items] |  |
Derivative Asset, Fair Value, Gross Asset | 1,360.9 | 2,455.7
Derivative Liability, Fair Value, Gross Liability | 1,312.7 | 2,499.3
Over the Counter (OTC) Commodity Contracts [Member] |  |
Derivatives, Fair Value [Line Items] |  |
Derivative Asset, Fair Value, Gross Asset | 83.1 | 207.0
Derivative Liability, Fair Value, Gross Liability | 123.0 | 369.9
Foreign Exchange Forward [Member] |  |
Derivatives, Fair Value [Line Items] |  |
Derivative Asset, Fair Value, Gross Asset | 62.0 | 49.8
Derivative Liability, Fair Value, Gross Liability | 57.3 | 37.2
Over the Counter (OTC) Foreign Exchange Contracts [Member] |  |
Derivatives, Fair Value [Line Items] |  |
Derivative Asset, Fair Value, Gross Asset | 373.0 | 302.5
Derivative Liability, Fair Value, Gross Liability | 353.5 | 303.9
Exchange-traded interest rate contracts [Member] |  |
Derivatives, Fair Value [Line Items] |  |
Derivative Asset, Fair Value, Gross Asset | 592.5 | 449.3
Derivative Liability, Fair Value, Gross Liability | 693.1 | 478.7
Interest Rate Contract [Member] |  |
Derivatives, Fair Value [Line Items] |  |
Derivative Asset, Fair Value, Gross Asset | 20.9 | 24.8
Derivative Liability, Fair Value, Gross Liability | 22.9 | 25.9
Equity Contract [Member] |  |
Derivatives, Fair Value [Line Items] |  |
Derivative Asset, Fair Value, Gross Asset | 1,539.7 | 4,541.8
Derivative Liability, Fair Value, Gross Liability | 1,439.8 | 4,794.0
TBA and forward settling securities [Member] |  |
Derivatives, Fair Value [Line Items] |  |
Derivative Asset, Fair Value, Gross Asset | 12.1 | 5.0
Derivative Liability, Fair Value, Gross Liability | $ 16.7 | $ 2.1

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- Definition Fair value, before effects of master netting arrangements, of a financial asset or other contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes assets elected not to be offset. Excludes assets not subject to a master netting arrangement. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 20 -Section 50 -Paragraph 3 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=51824906&loc=SL20225862-175312 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 4B -Subparagraph (a),(c) -URI http://asc.fasb.org/extlink&oid=84234895&loc=SL5624163-113959 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 20 -Section 55 -Paragraph 7 -URI http://asc.fasb.org/extlink&oid=82849420&loc=SL20226000-175313 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 860 -SubTopic 20 -Section 50 -Paragraph 4D -Subparagraph (c)(2) -URI http://asc.fasb.org/extlink&oid=82924980&loc=SL51823488-111719 + Details
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- Definition Fair value of the assets less the liabilities of a derivative or group of derivatives. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 45 -Paragraph 6 -URI http://asc.fasb.org/extlink&oid=6945355&loc=d3e41271-113958 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 45 -Paragraph 5 -URI http://asc.fasb.org/extlink&oid=6945355&loc=d3e41228-113958 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 825 -SubTopic 10 -Section 50 -Paragraph 10 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=77997519&loc=d3e13433-108611 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 825 -SubTopic 10 -Section 50 -Paragraph 15 -URI http://asc.fasb.org/extlink&oid=77997519&loc=d3e13495-108611 + Details
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