New Accounting Standards (Policies)	9 Months Ended
New Accounting Standards (Policies)	Jun. 30, 2020
Accounting Policies [Abstract]
New Accounting Standards	New Accounting Standards Accounting standards implemented during fiscal year 2020 On October 1, 2019, the company implemented Accounting Standards Update ("ASU") 2016-02, Leases (Topic 842). The company elected the practical expedient package which allowed the company to not reassess whether existing contracts contain a lease and to not reassess classification of existing leases. The company also adopted ASU 2018-11, Leases (Topic 842) Targeted Improvements, electing to not separate lease and non-lease components in contracts that contain both and electing to not restate comparative periods when adopting ASU 2016-02. As a result, the company recognized a right-of-use asset and lease liability as a lessee for substantially all existing operating leases and has included new and expanded disclosures. (See Note 5) Accounting standards to be implemented The following represent the standards that may result in a significant change in practice and/or have a significant financial impact on the company. In June 2016, the Financial Accounting Standards Board ("FASB") issued ASU 2016-13, Financial Instruments—Credit Losses (Topic 326): Measurement of Credit Losses on Financial Instruments. The ASU introduces an approach based on expected losses to estimate credit losses on certain types of financial instruments, including accounts receivable. The ASU also modifies the impairment model for available-for-sale debt securities and provides for a simplified accounting model for purchased financial assets with credit deterioration since their origination. The amendments in this update are required to be adopted by public business entities in fiscal years beginning after December 15, 2019, including interim periods within those fiscal years. The company is currently evaluating the potential impact of this guidance on its accounting policies and its Condensed Consolidated Financial Statements. In August 2018, the FASB issued ASU 2018-13, Fair Value Measurement (Topic 820): Disclosure Framework—Changes to the Disclosure Requirements for Fair Value Measurement. The amendments in this ASU add, modify, and eliminate certain disclosure requirements on fair value measurements in Topic 820. The amendments in this update are effective for all entities for fiscal years beginning after December 15, 2019, including interim periods within those fiscal years. Certain amendments should be applied prospectively for only the most recent interim or annual period presented in the initial fiscal year of adoption. Others should be applied retrospectively. Early adoption is permitted. An entity is permitted to early adopt any removed or modified disclosures upon issuance of this ASU and delay adoption of the additional disclosures until their effective date. The company is currently evaluating the potential impact of this guidance on its accounting policies and its Condensed Consolidated Financial Statements.
Inventories	Inventories Inventories are stated at the lower of cost (using FIFO or average methods) or market (determined on the basis of estimated realizable values)
Fair Value	Fair Value FASB guidance provides a fair value hierarchy that prioritizes the inputs to valuation techniques used to measure fair value. The hierarchy gives the highest priority to unadjusted quoted prices in active markets for identical instruments (Level 1) and the lowest priority to unobservable inputs (Level 3). The three levels of the fair value hierarchy are described below: •Level 1 inputs use quoted prices in active markets for identical instruments. •Level 2 inputs use other inputs that are observable, either directly or indirectly. These Level 2 inputs include quoted prices for similar instruments in active markets and other inputs such as interest rates and yield curves that are observable at commonly quoted intervals. •Level 3 inputs are unobservable inputs, including inputs that are available in situations where there is little, if any, market activity for the related instrument. In instances where inputs used to measure fair value fall into different levels in the above fair value hierarchy, fair value measurements in their entirety are categorized based on the lowest priority level input that is significant to the valuation. The company's assessment of the significance of particular inputs to these fair value measurements requires judgment and considers factors specific to each asset or liability.
Environmental	Environmental Federal, state and local requirements relating to the discharge of substances into the environment, the disposal of hazardous wastes and other activities affecting the environment have, and will continue to have, an impact on the operations of the company. The process of estimating environmental liabilities is complex and dependent upon evolving physical and scientific data at the sites, uncertainties as to remedies and technologies to be used and the outcome of discussions with regulatory agencies. The company records liabilities for environmental issues in the accounting period in which they are considered to be probable and the cost can be reasonably estimated. At environmental sites in which more than one potentially responsible party has been identified, the company records a liability for its allocable share of costs related to its involvement with the site, as well as an allocable share of costs related to insolvent parties or unidentified shares. At environmental sites in which Meritor is the only potentially responsible party, the company records a liability for the total probable and estimable costs of remediation before consideration of recovery from insurers or other third parties.

Disclosure of inventory accounting policy for inventory classes, including, but not limited to, basis for determining inventory amounts, methods by which amounts are added and removed from inventory classes, loss recognition on impairment of inventories, and situations in which inventories are stated above cost.