Business Segment Information - Summary of Segment Information (Details) $ in Millions	3 Months Ended		9 Months Ended
	Jun. 30, 2020 USD ($)	Jun. 30, 2019 USD ($)	Jun. 30, 2020 USD ($) segment	Jun. 30, 2019 USD ($)
Segment Reporting [Abstract]
Number of reportable segments | segment			2
Segment Reporting Information [Line Items]
Sales	$ 514	$ 1,166	$ 2,286	$ 3,360
External Sales
Segment Reporting Information [Line Items]
Sales	514	1,166	2,286	3,360
Intersegment Sales
Segment Reporting Information [Line Items]
Sales	0	0	0	0
Commercial Truck
Segment Reporting Information [Line Items]
Sales	315	890	1,544	2,564
Aftermarket and Industrial
Segment Reporting Information [Line Items]
Sales	199	276	742	796
Reportable Segments | Commercial Truck
Segment Reporting Information [Line Items]
Sales	336	925	1,630	2,678
Reportable Segments | Commercial Truck | External Sales
Segment Reporting Information [Line Items]
Sales	315	890	1,544	2,564
Reportable Segments | Commercial Truck | Intersegment Sales
Segment Reporting Information [Line Items]
Sales	21	35	86	114
Reportable Segments | Aftermarket and Industrial
Segment Reporting Information [Line Items]
Sales	203	282	755	811
Reportable Segments | Aftermarket and Industrial | External Sales
Segment Reporting Information [Line Items]
Sales	199	276	742	796
Reportable Segments | Aftermarket and Industrial | Intersegment Sales
Segment Reporting Information [Line Items]
Sales	4	6	13	15
Eliminations
Segment Reporting Information [Line Items]
Sales	(25)	(41)	(99)	(129)
Eliminations | External Sales
Segment Reporting Information [Line Items]
Sales	0	0	0	0
Eliminations | Intersegment Sales
Segment Reporting Information [Line Items]
Sales	$ (25)	$ (41)	$ (99)	$ (129)