Document and Entity Information - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Feb. 13, 2016	Jun. 30, 2015
Document and Entity Information [Abstract]
Entity Registrant Name	ARCH COAL INC
Entity Central Index Key	0001037676
Document Type	10-K
Document Period End Date	Dec. 31, 2015
Amendment Flag	false
Current Fiscal Year End Date	--12-31
Document Fiscal Year Focus	2015
Document Fiscal Period Focus	FY
Entity Filer Category	Accelerated Filer
Entity Common Stock, Shares Outstanding		21,446,233
Entity Well known seasoned issuer	Yes
Entity Voluntary Filers	No
Entity Current reporting status	Yes
Entity Public Float			$ 72.4