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Cover Page - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2020
Jan. 31, 2021
Jun. 30, 2020
Cover page.      
Document Type 10-K    
Document Annual Report true    
Document Period End Date Dec. 31, 2020    
Document Transition Report false    
Entity File Number 1-13105    
Entity Registrant Name Arch Resources, Inc.    
Entity Incorporation, State or Country Code DE    
Entity Tax Identification Number 43-0921172    
Entity Address, Address Line One One CityPlace Drive    
Entity Address, Address Line Two Suite 300    
Entity Address, City or Town St. Louis    
Entity Address, State or Province MO    
Entity Address, Postal Zip Code 63141    
City Area Code 314    
Local Phone Number 994-2700    
Title of 12(b) Security Common Stock, $.01 par value    
Trading Symbol ARCH    
Security Exchange Name NYSE    
Entity Well Known Seasoned Issuer Yes    
Entity Voluntary Filers No    
Entity Current Reporting Status Yes    
Entity Interactive Data Current Yes    
Entity Filer Category Accelerated Filer    
Entity Small Business Company false    
Entity Emerging Growth Company false    
Entity Shell Company false    
ICFR Auditor Attestation Flag true    
Entity Public Float     $ 430.3
Entity Common Stock, Shares Outstanding (in shares)   15,234,899  
Documents Incorporated by Reference

Portions of the registrant’s definitive proxy statement to be filed with the Securities and Exchange Commission in connection with the 2021 annual stockholders’ meeting are incorporated by reference into Part III of this Form 10-K.

    
Amendment Flag false    
Entity Central Index Key 0001037676    
Current Fiscal Year End Date --12-31    
Document Fiscal Year Focus 2020    
Document Fiscal Period Focus FY