Fair Value Measurements - Financial Assets and Liabilities that are Measured at Fair Value on a Recurring Basis (Details) - USD ($) $ in Millions	Jun. 30, 2020	Sep. 30, 2019	Jun. 30, 2019
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Derivative instruments, Assets	$ 116.6	$ 101.1	$ 83.9
Derivative instruments, Liabilities	(237.9)	(191.6)	(120.2)
Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Non-qualified supplemental postretirement grantor trust investments	41.4	39.7	40.8
Fair Value, Measurements, Recurring | Commodity contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Derivative instruments, Assets	67.6	42.1	54.4
Derivative instruments, Liabilities	(171.9)	(175.0)	(107.2)
Fair Value, Measurements, Recurring | Foreign currency contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Derivative instruments, Assets	49.0	59.0	29.5
Derivative instruments, Liabilities	(7.0)	(4.3)	(5.2)
Fair Value, Measurements, Recurring | Interest rate contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Derivative instruments, Liabilities	(59.0)	(12.3)	(7.8)
Fair Value, Measurements, Recurring | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Non-qualified supplemental postretirement grantor trust investments	41.4	39.7	40.8
Fair Value, Measurements, Recurring | Level 1 | Commodity contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Derivative instruments, Assets	38.3	32.0	30.1
Derivative instruments, Liabilities	(57.7)	(62.3)	(44.8)
Fair Value, Measurements, Recurring | Level 1 | Foreign currency contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
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
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
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 instruments, Assets	29.3	10.1	24.3
Derivative instruments, Liabilities	(114.2)	(112.7)	(62.4)
Fair Value, Measurements, Recurring | Level 2 | Foreign currency contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Derivative instruments, Assets	49.0	59.0	29.5
Derivative instruments, Liabilities	(7.0)	(4.3)	(5.2)
Fair Value, Measurements, Recurring | Level 2 | Interest rate contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Derivative instruments, Liabilities	(59.0)	(12.3)	(7.8)
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 instruments, Assets	0.0	0.0	0.0
Derivative 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 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
Derivative instruments, Liabilities	$ 0.0	$ 0.0	$ 0.0