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Financial Instruments with Off-Balance Sheet Risk and Concentrations of Credit Risk (Tables) | 12 Months Ended
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Sep. 30, 2018
Financial Instruments with Off-Balance Sheet Risk and Concentrations of Credit Risk [Abstract] |
Schedule of Derivative Instruments Included in Trading Activities [Table Text Block] | As of September 30, 2018, TBA and forward settling securities recorded within deposits with and receivables from broker-dealers, clearing organizations, and counterparties are summarized as follows (in millions):
|  | Gain / (Loss) | Notional Amounts
Unrealized gain on TBA securities purchased |  | $ | 1.2 |  | $ | 721.5 |
Unrealized loss on TBA securities purchased |  | $ | (0.6 | ) | $ | 293.2 |
Unrealized gain on TBA securities sold |  | $ | 3.2 |  | $ | (1,099.5 | )
Unrealized loss on TBA securities sold |  | $ | (1.5 | ) | $ | (812.7 | )
Unrealized loss on forward settling securities purchased |  | $ | 0.5 |  | $ | 614.3 |
Unrealized gain on forward settling securities sold |  | $ | 0.1 |  | $ | (427.2 | )
The notional amounts of these instruments reflect the extent of the Company’s involvement in TBA and forward settling securities and do not represent counterparty exposure.
Schedule of Derivative Instruments [Table Text Block] | Listed below are the fair values of the Company’s derivative assets and liabilities as of September 30, 2018 and 2017. Assets represent net unrealized gains and liabilities represent net unrealized losses.
| September 30, 2018 |  | September 30, 2017
(in millions) | Assets (1) |  | Liabilities (1) |  | Assets (1) |  | Liabilities (1)
Derivative contracts not accounted for as hedges: |  |  |  |  |  |  |
Exchange-traded commodity derivatives | $ | 2,455.7 |  |  | $ | 2,499.3 |  |  | $ | 2,094.2 |  |  | $ | 1,975.0 |
OTC commodity derivatives | 207.0 |  |  | 369.9 |  |  | 1,084.0 |  |  | 1,110.3 |
Exchange-traded foreign exchange derivatives | 49.8 |  |  | 37.2 |  |  | 66.0 |  |  | 52.0 |
OTC foreign exchange derivatives | 302.5 |  |  | 303.9 |  |  | 618.5 |  |  | 609.8 |
Exchange-traded interest rate derivatives | 449.3 |  |  | 478.7 |  |  | 228.4 |  |  | 203.6 |
OTC interest rate derivatives | 24.8 |  |  | 25.9 |  |  | — |  |  | — |
Exchange-traded equity index derivatives | 4,541.8 |  |  | 4,794.0 |  |  | 221.3 |  |  | 245.4 |
TBA and forward settling securities | 5.0 |  |  | 2.1 |  |  | 8.8 |  |  | 4.9 |
Gross fair value of derivative contracts | 8,035.9 |  |  | 8,511.0 |  |  | 4,321.2 |  |  | 4,201.0 |
Impact of netting and collateral | (8,118.5 | ) |  | (8,317.6 | ) |  | (4,205.5 | ) |  | (3,879.2 | )
Total fair value included in ‘Deposits with and receivables from broker-dealers, clearing organizations and counterparties’ | $ | (268.7 | ) |  |  |  | $ | (46.4 | ) |  |
Total fair value included in ‘Financial instruments owned, at fair value’ | $ | 186.1 |  |  |  |  | $ | 162.1 |  |  |
Total fair value included in ‘Payables to broker-dealers, clearing organizations and counterparties |  |  | $ | — |  |  |  |  | $ | 4.8 |
Fair value included in ‘Financial instruments sold, not yet purchased, at fair value’ |  |  | $ | 193.4 |  |  |  |  | $ | 317.0 |
(1) | As of September 30, 2018 and 2017, the Company’s derivative contract volume for open positions was approximately 10.6 million and 6.1 million contracts, respectively.
Schedule of Derivative Instruments, Gain (Loss) in Statement of Financial Performance [Table Text Block] | The following table sets forth the Company’s net gains (losses) related to derivative financial instruments for the fiscal years ended September 30, 2018, 2017, and 2016, in accordance with the Derivatives and Hedging Topic of the ASC. The net gains (losses) set forth below are included in ‘trading gains, net’ and ‘cost of sales of physical commodities’ in the consolidated income statements.
| Year Ended September 30,
(in millions) | 2018 |  | 2017 |  | 2016
Commodities | $ | 94.0 |  |  | $ | 47.3 |  |  | $ | 41.8 |
Foreign exchange | 9.2 |  |  | 8.7 |  |  | 9.7 |
Interest rate and equity | 1.0 |  |  | (0.1 | ) |  | 0.8 |
TBA and forward settling securities | 14.5 |  |  | (2.5 | ) |  | (14.4 | )
Net gains from derivative contracts | $ | 118.7 |  |  | $ | 53.4 |  |  | $ | 37.9 |

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- Definition Financial Instruments with Off-Balance Sheet Risk and Concentrations of Credit Risk [Abstract] + References No definition available. + Details
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- Definition Tabular disclosure of the location and amount of derivative instruments and nonderivative instruments designated as hedging instruments reported before netting adjustments, and the amount of gain (loss) on derivative instruments and nonderivative instruments designated and qualified as hedging instruments. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 4C -URI http://asc.fasb.org/extlink&oid=84234895&loc=SL5624171-113959 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 4A -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=84234895&loc=SL5618551-113959 + Details
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- Definition Tabular disclosure of gains and losses on derivative instruments included in an entity's trading activities. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 55 -Paragraph 184 -URI http://asc.fasb.org/extlink&oid=95459573&loc=SL5634876-113961 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 4F -Subparagraph (a),(b) -URI http://asc.fasb.org/extlink&oid=84234895&loc=SL5624186-113959 + Details
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- Definition Tabular disclosure of pertinent information about a derivative or group of derivatives on a disaggregated basis, such as for individual instruments, or small groups of similar instruments. May include a combination of the type of instrument, risks being hedged, notional amount, hedge designation, related hedged item, inception date, maturity date, or other relevant item. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 5 -URI http://asc.fasb.org/extlink&oid=84234895&loc=d3e41641-113959 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 4B -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 815 -SubTopic 10 -Section 50 -Paragraph 8 -URI http://asc.fasb.org/extlink&oid=84234895&loc=d3e41678-113959 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 4 -URI http://asc.fasb.org/extlink&oid=84234895&loc=d3e41638-113959 Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 1B -URI http://asc.fasb.org/extlink&oid=84234895&loc=SL5580258-113959 Reference 6: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 1A -URI http://asc.fasb.org/extlink&oid=84234895&loc=SL5579245-113959 Reference 7: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 4C -URI http://asc.fasb.org/extlink&oid=84234895&loc=SL5624171-113959 Reference 8: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=84234895&loc=SL5579240-113959 Reference 9: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=84234895&loc=d3e41620-113959 Reference 10: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 235 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.4-08.(n)(2)) -URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e23780-122690 + Details
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