Document And Entity Information	3 Months Ended
	Mar. 31, 2015	Apr. 20, 2015
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, 2015
Amendment Flag	false
Current Fiscal Year End Date	--12-31
Document Fiscal Year Focus	2015
Document Fiscal Period Focus	Q1
Entity Filer Category	Large Accelerated Filer
Entity Current Reporting Status	Yes
Entity Common Stock, Shares Outstanding		212,662,307