Regulatory Requirements and Subsidiary Dividend Restrictions - Customer Secured Funds (Details) - USD ($) $ in Millions	Sep. 30, 2015	Sep. 30, 2014
Cash and Securities Segregated under Commodity Exchange Act [Abstract]
Cash Segregated under Commodity Exchange Act Regulation	$ 126.9	$ 314.0
Securities Segregated under Commodity Exchange Act Regulation	492.5	0.0
Securities Held for Customers in Lieu of Cash, At Banks, Segregated under Commodity Exchange Act Regulation	0.9	0.5
Deposits with and Receivables From Clearing Organizations, Segregated under Commodity Exchange Act Regulation	1,237.8	1,476.5
Securities Held for Customers in Lieu of Cash, Segregated under Commodity Exchange Act Regulation	22.1	14.8
Cash and Securities Segregated under Commodity Exchange Act Regulation		1,805.8
Cash and Securities Segregated under Commodity Exchange Act Regulation, Amount Required to be Segregated		1,769.3
Cash and Securities Segregated under Commodity Exchange Act Regulation, Excess Funds in Segregation	49.3	36.5
Cash and Securities Segregated under Other Regulations [Abstract]
Cash Segregated under Other Regulations	64.7	46.8
Securities Segregated under Other Regulations	0.0	0.0
Equities with Futures Commission Merchants Segregated under Other Regulations	2.6	11.9
Amounts Held by Clearing Organizations Segregated under Other Regulations	0.0	0.0
Amounts Held by Members of Foreign Boards of Trade Segregated under Other Regulations	18.3	14.5
Secured funds		73.2
Secured Funds Required Under Commodity Exchange Act		53.3
Cash and Securities Segregated under Other Regulations	$ 20.4	$ 19.9