Assets and Liabilities, at Fair Value - Available for Sale Securities (Maturities) (Details) (US Government Debt Securities [Member], USD $) In Millions, unless otherwise specified	Mar. 31, 2015	Sep. 30, 2014
US Government Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Debt Maturities, Next Twelve Months, Fair Value	$ 398.5	$ 287.6
Available-for-sale Securities, Debt Maturities, after Ten Years, Fair Value	682.7	379.2
Available-for-sale Securities, Debt Securities	$ 1,081.2	$ 666.8

Amount of debt securities categorized neither as held-to-maturity nor trading.

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

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-Paragraph 1

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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

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant