Business Segment Information - Summary of Segment Information (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Mar. 31, 2021	Mar. 31, 2020
Segment Reporting Information [Line Items]
Sales	$ 983	$ 871	$ 1,872	$ 1,772
External Sales
Segment Reporting Information [Line Items]
Sales	983	871	1,872	1,772
Commercial Truck
Segment Reporting Information [Line Items]
Sales	742	598	1,402	1,229
Aftermarket & Industrial
Segment Reporting Information [Line Items]
Sales	241	273	470	543
Reportable Segments | Commercial Truck
Segment Reporting Information [Line Items]
Sales	777	631	1,468	1,294
Reportable Segments | Commercial Truck | External Sales
Segment Reporting Information [Line Items]
Sales	742	598	1,402	1,229
Reportable Segments | Aftermarket & Industrial
Segment Reporting Information [Line Items]
Sales	247	277	481	552
Reportable Segments | Aftermarket & Industrial | External Sales
Segment Reporting Information [Line Items]
Sales	241	273	470	543
Eliminations
Segment Reporting Information [Line Items]
Sales	41	37	77	74
Eliminations | Commercial Truck
Segment Reporting Information [Line Items]
Sales	35	33	66	65
Eliminations | Aftermarket & Industrial
Segment Reporting Information [Line Items]
Sales	$ 6	$ 4	$ 11	$ 9