Assets and Liabilities, at Fair Value Assets and Liabilities, at Fair Value - Available for Sale Securities - Dep and Rec (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012	Sep. 30, 2012
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis	$ 660.5	$ 1,305.7
Available-for-sale Securities, Gross Unrealized Gains	0.1	0.1
Available-for-sale Securities, Gross Unrealized Losses	0	0
Available-for-sale Securities, Debt Securities	660.6	1,305.8
US Government Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis	654.1	1,298.9
Available-for-sale Securities, Gross Unrealized Gains	0	0
Available-for-sale Securities, Gross Unrealized Losses	0	0
Available-for-sale Securities, Debt Securities	654.1	1,298.9
Collateralized Mortgage Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis	6.4	6.8
Available-for-sale Securities, Gross Unrealized Gains	0.1	0.1
Available-for-sale Securities, Gross Unrealized Losses	0	0
Available-for-sale Securities, Debt Securities	6.5	6.9
Deposits and Receivables from Exchange Clearing Organizations [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Gross Unrealized Gain (Loss)	0.1	0.1
Available-for-sale Securities, Debt Securities	$ 660.6	$ 1,305.8

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

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Reference 2: http://www.xbrl.org/2003/role/presentationRef

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-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

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Reference 4: http://www.xbrl.org/2003/role/presentationRef

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-Name Statement of Financial Accounting Standard (FAS)

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-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 5: http://www.xbrl.org/2003/role/presentationRef

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Reference 9: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

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-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 10: http://www.xbrl.org/2003/role/presentationRef

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-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

+ Details

- Definition

This item represents the gross unrealized gains for securities, at a point in time, which are categorized neither as held-to-maturity nor trading securities.

+ References+ Details