Assets and Liabilities, at Fair Value - Available for Sale Securities (Maturities) (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2014	Sep. 30, 2013
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Debt Maturities, Next Twelve Months, Fair Value	$ 806.6	$ 568.5
Available-for-sale Securities, Debt Maturities, after Ten Years, Fair Value	4.7	5.3
Available-for-sale Securities, Debt Securities	811.3	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	806.6	568.5
Available-for-sale Securities, Debt Maturities, after Ten Years, Fair Value	0	0
Available-for-sale Securities, Debt Securities	806.6	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	4.7	5.3
Available-for-sale Securities, Debt Securities	$ 4.7	$ 5.3