Document And Entity Information - shares
6 Months Ended
Jun. 30, 2017
Jul. 21, 2017
Document and Entity Information [Abstract]    
Entity Registrant Name ARCH COAL INC  
Entity Central Index Key 0001037676  
Document Type 10-Q  
Document Period End Date Jun. 30, 2017  
Amendment Flag false  
Current Fiscal Year End Date --12-31  
Document Fiscal Year Focus 2017  
Document Fiscal Period Focus Q2  
Entity Filer Category Non-accelerated Filer  
Entity Common Stock, Shares Outstanding   24,310,578