Receivables from customers and notes receivable, net - Notes Receivable (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	Jun. 30, 2011 FCStone LLC [Member]	Sep. 30, 2011 FCStone LLC [Member]	Sep. 30, 2010 FCStone LLC [Member]	Sep. 30, 2012 FCStone LLC [Member]	Sep. 30, 2009 FCStone LLC [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for Doubtful Accounts Receivable, Charge-offs	$ (11.2)	$ (113.3)	$ (5.8)			$ 4.2
Allowance for Doubtful Accounts Receivable, Recoveries	0	(3.7)	(0.7)	0.7		15.5
Remaining Acquired Notes Receivable							1.2
Acquired notes receivable						133.7
Notes Receivable								16.7
Notes receivable recovered during the year					15.6
Notes receivable charge-off during the year					111.5
Financing Receivable, Net	$ 92.5	$ 24.3