Assets and Liabilities, at Fair Value - Available for Sale Securities (Maturities) (Details) (USD $) In Millions, unless otherwise specified	Sep. 30, 2014	Sep. 30, 2013
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Debt Maturities, Next Twelve Months, Fair Value		$ 568.5
Available-for-sale Securities, Debt Maturities, after Ten Years, Fair Value		5.3
Available-for-sale Securities, Debt Securities		573.8
US Government Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Debt Maturities, Next Twelve Months, Fair Value	287.6	568.5
Available-for-sale Securities, Debt Maturities, after Ten Years, Fair Value	379.2	0
Available-for-sale Securities, Debt Securities	666.8	568.5
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
Available-for-sale Securities, Debt Maturities, after Ten Years, Fair Value		5.3
Available-for-sale Securities, Debt Securities		$ 5.3