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Cover Page - USD ($)
 12 Months Ended
Sep. 30, 2021
Oct. 31, 2021
Mar. 31, 2021
Cover [Abstract]      
Document Type 10-K    
Document Annual Report true    
Document Period End Date Sep. 30, 2021    
Current Fiscal Year End Date --09-30    
Document Transition Report false    
Entity File Number 000-30205    
Entity Registrant Name CMC Materials, Inc.    
Entity Incorporation, State or Country Code DE    
Entity Tax Identification Number 36-4324765    
Entity Address, Address Line One 870 North Commons Drive    
Entity Address, City or Town Aurora    
Entity Address, State or Province IL    
Entity Address, Postal Zip Code 60504    
City Area Code 630    
Local Phone Number 375-6631    
Title of 12(b) Security Common Stock, $0.001 par value    
Trading Symbol CCMP    
Security Exchange Name NASDAQ    
Entity Well-known Seasoned Issuer Yes    
Entity Voluntary Filers No    
Entity Current Reporting Status Yes    
Entity Interactive Data Current Yes    
Entity Filer Category Large Accelerated Filer    
Entity Small Business false    
Entity Emerging Growth Company false    
ICFR Auditor Attestation Flag true    
Entity Shell Company false    
Entity Public Float     $ 5,125,185,387
Entity Common Stock, Shares Outstanding   28,425,485  
Documents Incorporated by Reference
Portions of the registrant’s definitive Proxy Statement for the Annual Meeting of Stockholders to be held March 9, 2022, are incorporated by reference in Part III of this Annual Report on Form 10-K to the extent stated herein.
This Annual Report on Form 10-K includes statements that constitute “forward-looking statements” within the meaning of federal securities regulations. For more detail regarding “forward-looking statements” see Item 7 of Part II of this Annual Report on Form 10-K.
    
Entity Central Index Key 0001102934    
Amendment Flag false    
Document Fiscal Year Focus 2021    
Document Fiscal Period Focus FY