BUSINESS SEGMENT INFORMATION - Summary of Segment Information (Details) - 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, 2019	Sep. 30, 2018	Sep. 30, 2017
Segment Reporting Information [Line Items]
Total sales	$ 1,028	$ 1,166	$ 1,156	$ 1,038	$ 1,080	$ 1,129	$ 1,066	$ 903	$ 4,388	$ 4,178	$ 3,347
Commercial Truck
Segment Reporting Information [Line Items]
Total sales									3,110	3,037	2,347
Aftermarket, Industrial and Trailer
Segment Reporting Information [Line Items]
Total sales									1,278	1,141	1,000
Operating Segments
Segment Reporting Information [Line Items]
Total sales									4,388	4,178	3,347
Operating Segments | Commercial Truck
Segment Reporting Information [Line Items]
Total sales									3,252	3,172	2,469
Operating Segments | Aftermarket, Industrial and Trailer
Segment Reporting Information [Line Items]
Total sales									1,313	1,176	1,032
Elims
Segment Reporting Information [Line Items]
Total sales									(177)	(170)	(154)
Elims | Commercial Truck
Segment Reporting Information [Line Items]
Total sales									142	135	122
Elims | Aftermarket, Industrial and Trailer
Segment Reporting Information [Line Items]
Total sales									$ 35	$ 35	$ 32