Employee Benefit Plans - Schedule of Amortization of Prior Service Cost (Credit) Recognized in AOCI (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2019
Pension benefits
Defined Benefit Plan Disclosure [Line Items]
Amortization of prior service credits recognized in Accumulated other comprehensive income	$ 0	$ 0	$ 0
Net periodic benefit income	(113)	(47)	64
Total pre-tax amount recognized in comprehensive income	(113)	(47)	64
Other postretirement benefits
Defined Benefit Plan Disclosure [Line Items]
Amortization of prior service credits recognized in Accumulated other comprehensive income	12	12	12
Net periodic benefit income	(11)	(9)	(2)
Total pre-tax amount recognized in comprehensive income	$ 1	$ 3	$ 10