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Cover Page - USD ($)
$ in Billions
 12 Months Ended
Dec. 31, 2019
Jan. 31, 2020
Jun. 30, 2019
Cover page.      
Document Type 10-K    
Document Annual Report true    
Document Period End Date Dec. 31, 2019    
Document Transition Report false    
Entity File Number 1-13105    
Entity Registrant Name Arch Coal, Inc.    
Entity Incorporation, State or Country Code DE    
Entity Tax Identification Number 43-0921172    
Entity Central Index Key 0001037676    
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 Large Accelerated Filer    
Entity Small Business Company false    
Entity Emerging Growth Company false    
Entity Shell Company false    
Entity Public Float     $ 1.5
Entity Common Stock, Shares Outstanding (in shares)   15,131,573  
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 2020 annual stockholders’ meeting are incorporated by reference into Part III of this Form 10-K.
    
Amendment Flag false    
Current Fiscal Year End Date --12-31    
Document Fiscal Year Focus 2019    
Document Fiscal Period Focus FY