Receivables from customers, net and notes receivable, net - Allowance for Bad Debts Rollforward (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2011
Allowance for Doubtful Accounts Receivable [Roll Forward]
Allowance for Doubtful Accounts - Customer and Notes Receivable	$ 1.0	$ 11.9	$ 119.2
Provision for bad debts	0.2	0.4	7.2
Closed Derivative Contracts - Deficit Balance	0	0	2.5
Allowance for Doubtful Accounts Receivable, Charge-offs	0	(11.2)	(113.3)
Allowance for Doubtful Accounts Receivable, Recoveries	0	(0.1)	(3.7)
Allowance for Doubtful Accounts - Customer and Notes Receivable	$ 1.2	$ 1.0	$ 11.9