Document And Entity Information	9 Months Ended
Sep. 30, 2011	Nov. 07, 2011
Document and Entity Information [Abstract]
Entity Registrant Name	ARCH COAL INC
Entity Central Index Key	0001037676
Document Type	10-Q
Document Period End Date	Sep. 30, 2011
Amendment Flag	false
Current Fiscal Year End Date	--12-31
Document Fiscal Year Focus	2011
Document Fiscal Period Focus	Q3
Entity Filer Category	Large Accelerated Filer
Entity Common Stock, Shares Outstanding		211,618,197

Document and Entity Information [Abstract]

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