Receivables from customers and notes receivable, net - Bad Debt Expense and Recoveries (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended							3 Months Ended	12 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2010	Sep. 30, 2012 C&RM Segment [Member]	Sep. 30, 2011 C&RM Segment [Member]	Sep. 30, 2012 CES Segment [Member]	Sep. 30, 2011 CES Segment [Member]	Jun. 30, 2011 FCStone LLC [Member]	Sep. 30, 2010 FCStone LLC [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Provision for Doubtful Accounts				$ 0.7	$ 4.5				$ 3.5
Allowance for Doubtful Accounts Receivable, Charge-offs	(11.2)	(113.3)	(5.8)	0.8	8.2				4.2
Allowance for Doubtful Accounts Receivable, Period Increase (Decrease)	0.4	7.2	2.3
Allowance for Doubtful Accounts Receivable, Recoveries	0	(3.7)	(0.7)	0.1	3.7			0.7	15.5
FCM give-up bad debt expense									2.3
Consigned gold bad debt expense			2.5
CR&M customer deficit charge-off							3.7
Consigned gold charge-off						8.5
CES customer deficits charge-off						$ 2.7