Assets and Liabilities, at Fair Value - Available for Sale Securities - Dep and Rec (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Sep. 30, 2014	Sep. 30, 2013
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis		$ 573.7
Available-for-sale Securities, Gross Unrealized Gains		0.1
Available-for-sale Securities, Gross Unrealized Losses		0
Available-for-sale Securities, Debt Securities		573.8
US Government Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis	666.8	568.5
Available-for-sale Securities, Gross Unrealized Gains	0	0
Available-for-sale Securities, Gross Unrealized Losses	0	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 Debt Securities, Amortized Cost Basis		5.2
Available-for-sale Securities, Gross Unrealized Gains		0.1
Available-for-sale Securities, Gross Unrealized Losses		0
Available-for-sale Securities, Debt Securities		5.3
Deposits and Receivables from Exchange Clearing Organizations [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Gross Unrealized Gain (Loss)	0.1	0.1
Available-for-sale Securities, Debt Securities		$ 573.8