Document And Entity Information	3 Months Ended
	Mar. 31, 2013	May 01, 2013
Document and Entity Information [Abstract]
Entity Registrant Name	ARCH COAL INC
Entity Central Index Key	0001037676
Document Type	10-Q
Document Period End Date	Mar. 31, 2013
Amendment Flag	false
Current Fiscal Year End Date	--12-31
Document Fiscal Year Focus	2013
Document Fiscal Period Focus	Q1
Entity Filer Category	Large Accelerated Filer
Entity Common Stock, Shares Outstanding		212,246,799