Fair Value Measurements - Financial Assets and Liabilities that are Measured at Fair Value on a Recurring Basis (Details) - USD ($) $ in Millions	Jun. 30, 2018	Sep. 30, 2017	Jun. 30, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Derivative financial instruments, assets	$ 147.6	$ 116.3	$ 51.1
Derivative financial instruments, liabilities	(53.6)	(82.5)	(70.3)
Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Non-qualified supplemental postretirement grantor trust investments	38.4	35.6	35.8
Fair Value, Measurements, Recurring | Commodity contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Derivative financial instruments, assets	128.5	104.1	39.8
Derivative financial instruments, liabilities	(34.2)	(39.1)	(42.9)
Fair Value, Measurements, Recurring | Foreign currency contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Derivative financial instruments, assets	18.4	12.2	11.3
Derivative financial instruments, liabilities	(17.9)	(38.2)	(24.3)
Fair Value, Measurements, Recurring | Interest rate contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Derivative financial instruments, liabilities	(1.5)	(2.3)	(2.2)
Fair Value, Measurements, Recurring | Cross-currency contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Derivative financial instruments, assets	0.7
Derivative financial instruments, liabilities		(2.9)	(0.9)
Fair Value, Measurements, Recurring | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Non-qualified supplemental postretirement grantor trust investments	38.4	35.6	35.8
Fair Value, Measurements, Recurring | Level 1 | Commodity contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Derivative financial instruments, assets	58.5	27.2	29.3
Derivative financial instruments, liabilities	(25.3)	(27.7)	(25.6)
Fair Value, Measurements, Recurring | Level 1 | Foreign currency contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Derivative financial instruments, assets	0.0	0.0	0.0
Derivative financial instruments, liabilities	0.0	0.0	0.0
Fair Value, Measurements, Recurring | Level 1 | Interest rate contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Derivative financial instruments, liabilities	0.0	0.0	0.0
Fair Value, Measurements, Recurring | Level 1 | Cross-currency contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Derivative financial instruments, assets	0.0
Derivative financial instruments, liabilities		0.0	0.0
Fair Value, Measurements, Recurring | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Non-qualified supplemental postretirement grantor trust investments	0.0	0.0	0.0
Fair Value, Measurements, Recurring | Level 2 | Commodity contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Derivative financial instruments, assets	70.0	76.9	10.5
Derivative financial instruments, liabilities	(8.9)	(11.4)	(17.3)
Fair Value, Measurements, Recurring | Level 2 | Foreign currency contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Derivative financial instruments, assets	18.4	12.2	11.3
Derivative financial instruments, liabilities	(17.9)	(38.2)	(24.3)
Fair Value, Measurements, Recurring | Level 2 | Interest rate contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Derivative financial instruments, liabilities	(1.5)	(2.3)	(2.2)
Fair Value, Measurements, Recurring | Level 2 | Cross-currency contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Derivative financial instruments, assets	0.7
Derivative financial instruments, liabilities		(2.9)	(0.9)
Fair Value, Measurements, Recurring | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Non-qualified supplemental postretirement grantor trust investments	0.0	0.0	0.0
Fair Value, Measurements, Recurring | Level 3 | Commodity contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Derivative financial instruments, assets	0.0	0.0	0.0
Derivative financial instruments, liabilities	0.0	0.0	0.0
Fair Value, Measurements, Recurring | Level 3 | Foreign currency contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Derivative financial instruments, assets	0.0	0.0	0.0
Derivative financial instruments, liabilities	0.0	0.0	0.0
Fair Value, Measurements, Recurring | Level 3 | Interest rate contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Derivative financial instruments, liabilities	0.0	0.0	0.0
Fair Value, Measurements, Recurring | Level 3 | Cross-currency contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Derivative financial instruments, assets	$ 0.0
Derivative financial instruments, liabilities		$ 0.0	$ 0.0