Acquisitions and Disposals (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended								12 Months Ended
	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2014	Jan. 01, 2015
Business Acquisition [Line Items]
Gross Profit	$ 178.6	$ 175.0	$ 166.1	$ 151.3	$ 178.7	$ 151.6	$ 156.5	$ 137.5	$ 671.0	$ 624.3	$ 490.9
Net income	$ 16.8	$ 14.6	$ 14.5	$ 8.8	$ 21.1	$ 12.2	$ 13.0	$ 9.4	$ 54.7	$ 55.7	$ 19.3
Net basic earnings per share	$ 0.91	$ 0.79	$ 0.77	$ 0.47	$ 1.12	$ 0.64	$ 0.68	$ 0.50	$ 2.94	$ 2.94	$ 1.01
Net diluted earnings per share	$ 0.90	$ 0.78	$ 0.76	$ 0.46	$ 1.09	$ 0.62	$ 0.67	$ 0.49	$ 2.90	$ 2.87	$ 0.98
Business Combination, Consideration Transferred									$ 45.0
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Cash and Equivalents	$ 24.9								24.9
Business Combination, Acquired Receivables, Fair Value	151.6								151.6
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Deferred Tax Assets Noncurrent	5.7								5.7
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Assets, Prepaid Expense and Other Assets	4.8								4.8
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Liabilities	136.0								136.0
Gain on acquisition	6.2	$ 0.0	$ 0.0	$ 0.0					6.2	$ 0.0	$ 0.0
Business Combination, Pro Forma Information, Revenue of Acquiree since Acquisition Date, Actual									8.6
Business Combination, Pro Forma Information, Earnings or Loss of Acquiree since Acquisition Date, Actual									0.1
Goodwill	47.5				$ 47.5				47.5	47.5
Deferred consideration payable related to acquisitions									0.0	$ 5.0	$ 0.0
G.X. Clarke (INTL FCStone Partners) [Member]
Business Acquisition [Line Items]
Present Value of Estimated Total Purchase Price	27.5								27.5
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Net	25.9								25.9
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Intangible Assets, Other than Goodwill	1.5								1.5
Business Combination, Contingent Consideration Arrangements, Range of Outcomes, Value, High	1.5								1.5			$ 0.1
Forward Insight Commodities Acquisition [Member]
Business Acquisition [Line Items]
Present Value of Estimated Total Purchase Price	0.5								0.5
Customer Relationships [Member] | Forward Insight Commodities [Member]
Business Acquisition [Line Items]
Finite-lived Intangible Assets Acquired									0.5
G.X. Clarke (INTL FCStone Partners) [Member]
Business Acquisition [Line Items]
Escrow Deposit	10.0								10.0
Adjustment Escrow	5.0								5.0
Indemnity Escrow	5.0								5.0
Deferred consideration payable related to acquisitions									5.0
Business Acquisition, Contingent Consideration, Potential Cash Payment, Year One	0.5								0.5
After-tax net income over year one									5.0
Business Acquisition, Contingent Consideration, Potential Cash Payment Year Three	$ 1.0								1.0
After-tax net income over earn-out period									$ 30.0