Fair Value Measurements - Financial Assets and Liabilities that are Measured at Fair Value on a Recurring Basis (Details) - USD ($) $ in Millions	Jun. 30, 2021	Sep. 30, 2020	Jun. 30, 2020
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Derivative instruments, Assets	$ 661	$ 139	$ 117
Derivative instruments, Liabilities	(193)	(187)	(238)
Commodity contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Derivative instruments, Assets	14	7	3
Derivative instruments, Liabilities	(6)	0	(2)
Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Non-qualified supplemental postretirement grantor trust investments	42	42	41
Recurring | Commodity contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Derivative instruments, Assets	637	107	68
Derivative instruments, Liabilities	(145)	(118)	(172)
Recurring | Foreign currency contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Derivative instruments, Assets	24	32	49
Derivative instruments, Liabilities	(13)	(14)	(7)
Recurring | Interest rate contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Derivative instruments, Liabilities	(35)	(55)	(59)
Recurring | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Non-qualified supplemental postretirement grantor trust investments	42	42	41
Recurring | Level 1 | Commodity contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Derivative instruments, Assets	241	68	39
Derivative instruments, Liabilities	(136)	(54)	(58)
Recurring | Level 1 | Foreign currency contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Derivative instruments, Assets	0	0	0
Derivative instruments, Liabilities	0	0	0
Recurring | Level 1 | Interest rate contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Derivative instruments, Liabilities	0	0	0
Recurring | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Non-qualified supplemental postretirement grantor trust investments	0	0	0
Recurring | Level 2 | Commodity contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Derivative instruments, Assets	396	39	29
Derivative instruments, Liabilities	(9)	(64)	(114)
Recurring | Level 2 | Foreign currency contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Derivative instruments, Assets	24	32	49
Derivative instruments, Liabilities	(13)	(14)	(7)
Recurring | Level 2 | Interest rate contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Derivative instruments, Liabilities	(35)	(55)	(59)
Recurring | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Non-qualified supplemental postretirement grantor trust investments	0	0	0
Recurring | Level 3 | Commodity contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Derivative instruments, Assets	0	0	0
Derivative instruments, Liabilities	0	0	0
Recurring | Level 3 | Foreign currency contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Derivative instruments, Assets	0	0	0
Derivative instruments, Liabilities	0	0	0
Recurring | Level 3 | Interest rate contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Derivative instruments, Liabilities	$ 0	$ 0	$ 0