Fair Value Measurements - Financial Assets and Liabilities that are Measured at Fair Value on a Recurring Basis (Details) (Fair Value, Measurements, Recurring, USD $) In Millions, unless otherwise specified	Mar. 31, 2015		Sep. 30, 2014		Mar. 31, 2014
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Non-qualified supplemental postretirement grantor trust investments	$ 31.8	[1]	$ 30.0	[1]	$ 29.8	[1],[2]
Commodity Contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Derivative financial instruments, assets	22.2		30.4		31.5	[2]
Derivative financial instruments, liabilities	(168.3)		(54.1)		(29.7)	[2]
Foreign Currency Contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Derivative financial instruments, assets	35.8		12.8		0.1	[2]
Derivative financial instruments, liabilities			(0.1)		(5.6)	[2]
Interest Rate Contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Derivative financial instruments, assets	0.1		0.1
Derivative financial instruments, liabilities	(12.5)		(21.0)		(27.8)	[2]
Cross-Currency Swaps
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Derivative financial instruments, assets	9.7		2.1
Derivative financial instruments, liabilities					(2.2)	[2]
Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Non-qualified supplemental postretirement grantor trust investments	31.8	[1]	30.0	[1]	29.8	[1],[2]
Level 1 | Commodity Contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Derivative financial instruments, assets	13.9		10.6		17.5	[2]
Derivative financial instruments, liabilities	(64.0)		(21.2)		(10.0)	[2]
Level 1 | Foreign Currency Contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Derivative financial instruments, assets	0		0		0	[2]
Derivative financial instruments, liabilities			0		0	[2]
Level 1 | Interest Rate Contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Derivative financial instruments, assets	0		0
Derivative financial instruments, liabilities	0		0		0	[2]
Level 1 | Cross-Currency Swaps
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Derivative financial instruments, assets	0		0
Derivative financial instruments, liabilities					0	[2]
Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Non-qualified supplemental postretirement grantor trust investments	0	[1]	0	[1]	0	[1],[2]
Level 2 | Commodity Contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Derivative financial instruments, assets	8.3		19.8		14.0	[2]
Derivative financial instruments, liabilities	(104.3)		(32.9)		(19.7)	[2]
Level 2 | Foreign Currency Contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Derivative financial instruments, assets	35.8		12.8		0.1	[2]
Derivative financial instruments, liabilities			(0.1)		(5.6)	[2]
Level 2 | Interest Rate Contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Derivative financial instruments, assets	0.1		0.1
Derivative financial instruments, liabilities	(12.5)		(21.0)		(27.8)	[2]
Level 2 | Cross-Currency Swaps
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Derivative financial instruments, assets	9.7		2.1
Derivative financial instruments, liabilities					(2.2)	[2]
Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Non-qualified supplemental postretirement grantor trust investments	0	[1]	0	[1]	0	[1],[2]
Level 3 | Commodity Contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Derivative financial instruments, assets	0		0		0	[2]
Derivative financial instruments, liabilities	0		0		0	[2]
Level 3 | Foreign Currency Contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Derivative financial instruments, assets	0		0		0	[2]
Derivative financial instruments, liabilities			0		0	[2]
Level 3 | Interest Rate Contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Derivative financial instruments, assets	0		0
Derivative financial instruments, liabilities	0		0		0	[2]
Level 3 | Cross-Currency Swaps
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Derivative financial instruments, assets	0		0
Derivative financial instruments, liabilities					$ 0	[2]

[1] Consists primarily of mutual fund investments held in grantor trusts associated with non-qualified supplemental retirement plans.

[2] Certain immaterial amounts have been revised to correct the classification of derivatives.