| Document and Entity Information - USD ($) | 12 Months Ended | ||
| Sep. 30, 2021 | Jan. 07, 2022 | Mar. 31, 2021 | |
| Cover [Abstract] | |||
| Document Type | 10-K/A | ||
| Amendment Flag | false | ||
| Document Period End Date | Sep. 30, 2021 | ||
| Current Fiscal Year End Date | --09-30 | ||
| Document Transition Report | false | ||
| Entity Registrant Name | CMC Materials, Inc. | ||
| Entity Incorporation, State or Country Code | DE | ||
| Entity File Number | 000-30205 | ||
| 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 | ||
| Entity Well-known Seasoned Issuer | Yes | ||
| Entity Voluntary Filers | No | ||
| Entity Filer Category | Large Accelerated Filer | ||
| Entity Current Reporting Status | Yes | ||
| Entity Interactive Data Current | Yes | ||
| Entity Emerging Growth Company | false | ||
| Entity Shell Company | false | ||
| Entity Small Business | false | ||
| Entity Public Float | $ 5,125,185,387 | ||
| Entity Common Stock, Shares Outstanding | 28,568,967 | ||
| Document Fiscal Year Focus | 2021 | ||
| Document Fiscal Period Focus | FY | ||
| Entity Central Index Key | 0001102934 | ||
| Document Annual Report | true | ||
| ICFR Auditor Attestation Flag | true | ||
| Title of 12(b) Security | Common Stock, $0.001 par value | ||
| Trading Symbol | CCMP | ||
| Security Exchange Name | NASDAQ |
