Employee Retirement Plans - Net Periodic Pension Costs (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 11	$ 10	$ 11
Interest cost	23	27	26
Expected return on assets	(38)	(36)	(35)
Curtailment gain	(1)	0	0
Amortization of:
Prior service cost	1		1
Actuarial loss	15	8	13
Net cost	11	9	$ 16
Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Interest cost	$ 1	$ 1