Business Segment Information - Segment Income Attributable to Parent (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]
Interest expense, net	$ (17)	$ (16)	$ (45)	$ (30)
Provision for income taxes	(22)	(73)	(29)	(86)
Depreciation and amortization			(52)	(50)
Noncontrolling interests	(3)	(1)	(4)	(3)
Restructuring			(8)	(15)
Income from WABCO distribution termination	0	265	0	265
Income from continuing operations attributable to Meritor, Inc.	63	240	95	279
Reportable Segments
Segment Reporting Information [Line Items]
Segment adjusted EBITDA	107	104	205	200
Reportable Segments | Commercial Truck
Segment Reporting Information [Line Items]
Segment adjusted EBITDA	73	58	136	115
Reportable Segments | Aftermarket & Industrial
Segment Reporting Information [Line Items]
Segment adjusted EBITDA	34	46	69	85
Segment Reconciling Items
Segment Reporting Information [Line Items]
Unallocated legacy and corporate expense, net	4	3	8	5
Interest expense, net	(17)	(16)	(45)	(30)
Provision for income taxes	(22)	(73)	(29)	(86)
Depreciation and amortization	(25)	(26)	(52)	(50)
Noncontrolling interests	(3)	(1)	(4)	(3)
Loss on sale of receivables	(1)	(1)	(2)	(2)
Restructuring	(2)	(10)	(8)	(15)
Brazil VAT	22	0	22	0
Transaction costs	0	(5)	0	(5)
Income from WABCO distribution termination	$ 0	$ 265	$ 0	$ 265