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		6 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Mar. 31, 2021	Mar. 31, 2020
Defined Benefit Plan Disclosure
Service cost	$ 3	$ 3	$ 6	$ 6
Interest cost	6	6	11	12
Expected return on assets	(10)	(9)	(20)	(19)
Curtailment gain	0	(1)	0	(1)
Amortization of:
Actuarial loss	3	3	7	7
Net cost	$ 2	$ 2	$ 4	$ 5