Defined Benefit Pension and Other Postretirement Plans - Components of Net Periodic Pension Expense and Other Postretirement Benefit Costs (Details) - Pension Benefits - USD ($) $ in Millions	3 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Defined Benefit Plan Disclosure
Service cost	$ 3	$ 3
Interest cost	5	6
Expected return on assets	(10)	(10)
Amortization of:
Actuarial loss	4	4
Net cost	$ 2	$ 3