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Assets and Liabilities, at Fair Value - Details of Level 3 Assets and Liabilities (Details) - USD ($) $ in Millions | Mar. 31, 2018 | Sep. 30, 2017
--- | --- | ---
Payables to Broker-Dealers and Clearing Organizations | $ 79.7 | $ 125.7
Assets | 7,203.7 | 6,243.4
Fair Value, Measurements, Recurring [Member] |  |
Receivables from Clearing Organizations | 649.5 | 204.7
Assets, Fair Value Disclosure | 3,345.6 | 2,068.0
Fair Value, Measurements, Recurring [Member] | US Government Debt Securities [Member] |  |
Receivables from Clearing Organizations |  | 244.7
Fair Value, Measurements, Recurring [Member] | Derivative [Member] |  |
Payables to Broker-Dealers and Clearing Organizations |  | 55.2
Counterparty And Cash Collateral Netting Adjustment [Member] | Fair Value, Measurements, Recurring [Member] |  |
Receivables from Clearing Organizations | (3,729.8) | (2,952.9)
Assets, Fair Value Disclosure | $ (4,381.5) | (4,344.2)
Counterparty And Cash Collateral Netting Adjustment [Member] | Fair Value, Measurements, Recurring [Member] | US Government Debt Securities [Member] |  |
Receivables from Clearing Organizations |  | 0.0
Counterparty And Cash Collateral Netting Adjustment [Member] | Fair Value, Measurements, Recurring [Member] | Collateralized Mortgage Backed Securities [Member] |  |
Receivables from Clearing Organizations |  | 0.0
Counterparty And Cash Collateral Netting Adjustment [Member] | Fair Value, Measurements, Recurring [Member] | Foreign Government Debt Securities [Member] |  |
Receivables from Clearing Organizations |  | 0.0
Counterparty And Cash Collateral Netting Adjustment [Member] | Fair Value, Measurements, Recurring [Member] | Derivative [Member] |  |
Receivables from Clearing Organizations |  | $ (2,952.9)

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- Definition 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 -Paragraph 1 -Subparagraph (SX 210.7-03(a)(12)) -URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03(11)) -URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878 + Details
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- Definition Fair value portion of 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 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=66048111&loc=d3e19207-110258 + Details
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- Definition Amount payable to broker-dealers and clearing organizations, including, but not limited to, securities failed-to-receive, certain deposits received for securities loaned, open transactions, and floor-brokerage payables. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 940 -SubTopic 20 -Section 45 -Paragraph 3 -URI http://asc.fasb.org/extlink&oid=35710130&loc=d3e41645-110958 + Details
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