Assets and Liabilities, at Fair Value - Available for Sale Securities - Dep and Rec (Details) (USD $) In Millions, unless otherwise specified	6 Months Ended	12 Months Ended
	Mar. 31, 2015	Sep. 30, 2014
US Government Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis	$ 1,076.7	$ 666.8
Available-for-sale Securities, Gross Unrealized Gains	4.5	0
Available-for-sale Securities, Gross Unrealized Losses	0	0
Available-for-sale Securities, Debt Securities	1,081.2	666.8
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