Recent Accounting Pronouncements - Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2017	Oct. 01, 2019
Significant Accounting Policies [Line Items]
Net sales and other operating revenues	$ 827	$ 845	$ 844	$ 821	$ 850	$ 854	$ 818	$ 720		$ 3,337	$ 3,242	$ 2,717
Cost of sales										$ 2,652	$ 2,470	$ 2,060
Subsequent Event [Member] | Minimum [Member]
Significant Accounting Policies [Line Items]
Right of use asset													$ 100
Lease payable obligation													100
Subsequent Event [Member] | Maximum [Member]
Significant Accounting Policies [Line Items]
Right of use asset													120
Lease payable obligation													$ 120
Difference between Revenue Guidance in Effect before and after Topic 606
Significant Accounting Policies [Line Items]
Net sales and other operating revenues									$ (76)
Cost of sales									$ (76)