Fair Value Measurements - Financial Assets and Liabilities that are Measured at Fair Value on a Recurring Basis (Details) - USD ($) $ in Millions	Mar. 31, 2020	Sep. 30, 2019	Mar. 31, 2019
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative instruments, Assets	$ 106.3	$ 101.1	$ 90.8
Derivative instruments, Liabilities	(374.7)	(191.6)	(83.3)
Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Non-qualified supplemental postretirement grantor trust investments	38.6	39.7	40.2
Fair Value, Measurements, Recurring | Commodity contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative instruments, Assets	48.3	42.1	61.0
Derivative instruments, Liabilities	(313.8)	(175.0)	(72.7)
Fair Value, Measurements, Recurring | Forward foreign currency exchange contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative instruments, Assets	58.0	59.0	29.8
Derivative instruments, Liabilities	(6.8)	(4.3)	(4.9)
Fair Value, Measurements, Recurring | Interest rate contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative instruments, Liabilities	(54.1)	(12.3)	(5.7)
Fair Value, Measurements, Recurring | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Non-qualified supplemental postretirement grantor trust investments	38.6	39.7	40.2
Fair Value, Measurements, Recurring | Level 1 | Commodity contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative instruments, Assets	36.4	32.0	33.3
Derivative instruments, Liabilities	(92.2)	(62.3)	(17.0)
Fair Value, Measurements, Recurring | Level 1 | Forward foreign currency exchange contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative instruments, Assets	0.0	0.0	0.0
Derivative 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 [Line Items]
Derivative instruments, Liabilities	0.0	0.0	0.0
Fair Value, Measurements, Recurring | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
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 [Line Items]
Derivative instruments, Assets	11.9	10.1	27.7
Derivative instruments, Liabilities	(221.6)	(112.7)	(55.7)
Fair Value, Measurements, Recurring | Level 2 | Forward foreign currency exchange contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative instruments, Assets	58.0	59.0	29.8
Derivative instruments, Liabilities	(6.8)	(4.3)	(4.9)
Fair Value, Measurements, Recurring | Level 2 | Interest rate contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative instruments, Liabilities	(54.1)	(12.3)	(5.7)
Fair Value, Measurements, Recurring | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
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 [Line Items]
Derivative instruments, Assets	0.0	0.0	0.0
Derivative instruments, Liabilities	0.0	0.0	0.0
Fair Value, Measurements, Recurring | Level 3 | Forward foreign currency exchange contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative instruments, Assets	0.0	0.0	0.0
Derivative 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 [Line Items]
Derivative instruments, Liabilities	$ 0.0	$ 0.0	$ 0.0