Assets and Liabilities, at Fair Value - Available for Sale Securities (Maturities) (Details) (USD $) In Millions, unless otherwise specified	Sep. 30, 2012	Sep. 30, 2011
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Debt Maturities, Next Twelve Months, Fair Value	$ 1,298.9	$ 446.2
Available-for-sale Securities, Debt Maturities, after Ten Years, Fair Value	6.9	8.5
Available-for-sale Securities, Debt Securities	1,305.8	454.7
US Government Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Debt Maturities, Next Twelve Months, Fair Value	1,298.9	441.2
Available-for-sale Securities, Debt Maturities, after Ten Years, Fair Value	0	0
Available-for-sale Securities, Debt Securities	1,298.9	441.2
Corporate Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Debt Maturities, Next Twelve Months, Fair Value		5.0
Available-for-sale Securities, Debt Maturities, after Ten Years, Fair Value		0
Available-for-sale Securities, Debt Securities		5.0
Collateralized Mortgage Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Debt Maturities, Next Twelve Months, Fair Value	0	0
Available-for-sale Securities, Debt Maturities, after Ten Years, Fair Value	6.9	8.5
Available-for-sale Securities, Debt Securities	$ 6.9	$ 8.5

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

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

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