Regulatory Requirements and Subsidiary Dividend Restrictions - Customer Secured Funds (Details) - USD ($) $ in Millions	Sep. 30, 2016	Sep. 30, 2015
Cash and Securities Segregated under Commodity Exchange Act [Abstract]
Cash Segregated under Commodity Exchange Act Regulation	$ 383.6	$ 126.9
Securities Segregated under Commodity Exchange Act Regulation	595.4	492.5
Securities Held for Customers in Lieu of Cash, At Banks, Segregated under Commodity Exchange Act Regulation	1.8	0.9
Deposits with and Receivables From Clearing Organizations, Segregated under Commodity Exchange Act Regulation	1,173.9	1,237.8
Securities Held for Customers in Lieu of Cash, Segregated under Commodity Exchange Act Regulation	23.2	22.1
Cash and Securities Segregated under Commodity Exchange Act Regulation		1,880.2
Cash and Securities Segregated under Commodity Exchange Act Regulation, Amount Required to be Segregated		1,830.9
Cash and Securities Segregated under Commodity Exchange Act Regulation, Excess Funds in Segregation	51.5	49.3
Cash and Securities Segregated under Other Regulations [Abstract]
Cash Segregated under Other Regulations	70.5	64.7
Securities Segregated under Other Regulations	0.0	0.0
Equities with Futures Commission Merchants Segregated under Other Regulations	3.6	2.6
Amounts Held by Clearing Organizations Segregated under Other Regulations	6.7	0.0
Amounts Held by Members of Foreign Boards of Trade Segregated under Other Regulations	26.9	18.3
Secured funds		85.6
Secured Funds Required Under Commodity Exchange Act		65.2
Cash and Securities Segregated under Other Regulations	$ 16.4	$ 20.4