Assets and Liabilities, at Fair Value - Available for Sale Securities - Dep and Rec (Details) - USD ($) $ in Millions	Jun. 30, 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	$ 929.7	$ 666.8
Available-for-sale Debt Securities, Accumulated Gross Unrealized Gain, before Tax	3.5	0.0
Available-for-sale Debt Securities, Accumulated Gross Unrealized Loss, before Tax	0.0	0.0
Available-for-sale Securities, Debt Securities	$ 933.2	666.8
Deposits and Receivables from Exchange Clearing Organizations [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Debt Securities, Accumulated Gross Unrealized Gain, before Tax		$ 0.1