Employee Benefit Plans - Schedule of Amortization of Prior Service Cost (Credit) Recognized in AOCI (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2017
Pension benefits
Defined Benefit Plan Disclosure [Line Items]
Amortization of prior service credit recognized in accumulated other comprehensive income	$ 0	$ 0
Net periodic benefit costs (income)	64	12	$ (120)
Total amount recognized in net periodic benefit costs and accumulated other comprehensive income	64	12
Other postretirement benefits
Defined Benefit Plan Disclosure [Line Items]
Amortization of prior service credit recognized in accumulated other comprehensive income	12	12
Net periodic benefit costs (income)	(2)	(10)	$ (16)
Total amount recognized in net periodic benefit costs and accumulated other comprehensive income	$ 10	$ 2