Restructuring Costs - Changes in Restructuring Reserves (Details) $ in Millions	6 Months Ended		12 Months Ended
	Mar. 31, 2021 USD ($)	Mar. 31, 2020 USD ($)	Sep. 30, 2020 USD ($)
Restructuring Reserve [Roll Forward]
Beginning balance	$ 10	$ 8	$ 8
Activity during the period:
Charges	8	15
Cash payments	(8)	(15)
Other	(3)	(1)
Ending balance	7	7	10
Less: non-current restructuring reserves	0	0
Restructuring reserves – current	7	7
Employee Termination Benefits
Restructuring Reserve [Roll Forward]
Beginning balance	10	8	8
Activity during the period:
Charges	6	15
Cash payments	(8)	(15)
Other	(1)	(1)
Ending balance	7	7	10
Less: non-current restructuring reserves	0	0
Restructuring reserves – current	7	7
Plant Shutdown & Other
Restructuring Reserve [Roll Forward]
Beginning balance	0	0	0
Activity during the period:
Charges	2	0
Cash payments	0	0
Other	(2)	0
Ending balance	0	0	$ 0
Less: non-current restructuring reserves	0	0
Restructuring reserves – current	$ 0	$ 0