Assets and Liabilities, at Fair Value - Available for Sale Securities (Maturities) (Details) (USD $) In Millions, unless otherwise specified	Jun. 30, 2014	Sep. 30, 2013
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Debt Maturities, Next Twelve Months, Fair Value	$ 593.2	$ 568.5
Available-for-sale Securities, Debt Maturities, after Ten Years, Fair Value	204.4	5.3
Available-for-sale Securities, Debt Securities	797.6	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	593.2	568.5
Available-for-sale Securities, Debt Maturities, after Ten Years, Fair Value	200.0	0
Available-for-sale Securities, Debt Securities	793.2	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	0
Available-for-sale Securities, Debt Maturities, after Ten Years, Fair Value	4.4	5.3
Available-for-sale Securities, Debt Securities	$ 4.4	$ 5.3