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)

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.

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))
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Reference 2: http://www.xbrl.org/2003/role/presentationRef
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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
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-SubTopic 10
-Section 50
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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
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-SubTopic 20
-Section 45
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Amount of right to receive cash and securities on deposit with clearing organizations. Clearing organizations perform post trade processing and trade comparisons among numerous broker-dealers and act as settlement agents between buying and selling broker-dealers.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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-SubTopic 10
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