Assets and Liabilities, at Fair Value - Available for Sale Securities - Dep and Rec (Details) (USD $) In Millions, unless otherwise specified	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012 US Government Debt Securities [Member]	Sep. 30, 2011 US Government Debt Securities [Member]	Sep. 30, 2012 Collateralized Mortgage Backed Securities [Member]	Sep. 30, 2011 Collateralized Mortgage Backed Securities [Member]	Sep. 30, 2012 Deposits and Receivables from Exchange Clearing Organizations [Member]	Sep. 30, 2011 Deposits and Receivables from Exchange Clearing Organizations [Member]	Sep. 30, 2011 Financial instruments owned [Member]	Sep. 30, 2010 Financial instruments owned [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Gross Unrealized Gain (Loss)							$ 0.1		$ 0.1
Available-for-sale Debt Securities, Amortized Cost Basis	1,305.7	448.9	1,298.9	440.6	6.8	8.3				5.5
Available-for-sale Securities, Gross Unrealized Gains	0.1	0.3	0	0.1	0.1	0.2				0
Available-for-sale Securities, Gross Unrealized Losses	0	0	0	0	0	0				0
Available-for-sale Securities, Debt Securities	$ 1,305.8	$ 454.7	$ 1,298.9	$ 440.7	$ 6.9	$ 8.5	$ 1,305.8	$ 449.2		$ 5.5

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

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

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

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

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-Name Statement of Financial Accounting Standard (FAS)

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

-Publisher FASB

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+ 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