Employee Benefit Plans - Schedule of Amortization of Prior Service Cost (Credit) Recognized in AOCI (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2016
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)	12	(120)	$ (24)
Total amount recognized in net periodic benefit cost (income) and accumulated other comprehensive income	12	(120)
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)	(10)	(16)	$ (1)
Total amount recognized in net periodic benefit cost (income) and accumulated other comprehensive income	$ 2	$ (4)