Document And Entity Information	6 Months Ended
	Jun. 30, 2011	Aug. 04, 2011
Document and Entity Information
Entity Registrant Name	ARCH COAL INC
Entity Central Index Key	0001037676
Document Type	10-Q
Document Period End Date	Jun. 30, 2011
Amendment Flag	false
Current Fiscal Year End Date	--12-31
Document Fiscal Year Focus	2011
Document Fiscal Period Focus	Q2
Entity Filer Category	Large Accelerated Filer
Entity Common Stock, Shares Outstanding		213,081,699