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