Assets and Liabilities, at Fair Value Assets and Liabilities, at Fair Value - Available for Sale Securities (Maturities) (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012	Sep. 30, 2012
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Debt Maturities, Next Twelve Months, Fair Value	$ 654.1	$ 1,298.9
Available-for-sale Securities, Debt Maturities, after Ten Years, Fair Value	6.5	6.9
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 Securities, Debt Maturities, Next Twelve Months, Fair Value	654.1	1,298.9
Available-for-sale Securities, Debt Maturities, after Ten Years, Fair Value	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 Securities, Debt Maturities, Next Twelve Months, Fair Value	0	0
Available-for-sale Securities, Debt Maturities, after Ten Years, Fair Value	6.5	6.9
Available-for-sale Securities, Debt Securities	$ 6.5	$ 6.9

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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