Receivables from customers and notes receivable, net - Allowance for Bad Debts Rollforward (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2010
Allowance for Doubtful Accounts Receivable [Roll Forward]
Allowance for Doubtful Accounts - Customer and Notes Receivable	$ 11.9	$ 119.2	$ 123.4
Provision for Doubtful Accounts	0.4	7.2	2.3
Closed Derivative Contracts - Deficit Balance	0	2.5	0
Allowance for Doubtful Accounts Receivable, Charge-offs	(11.2)	(113.3)	(5.8)
Allowance for Doubtful Accounts Receivable, Recoveries	0	(3.7)	(0.7)
Allowance for Doubtful Accounts - Customer and Notes Receivable	$ 1.0	$ 11.9	$ 119.2