Document and Entity Information - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Feb. 16, 2017	Jun. 30, 2016
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, 2016
Amendment Flag	false
Current Fiscal Year End Date	--12-31
Document Fiscal Year Focus	2016
Document Fiscal Period Focus	FY
Entity Filer Category	Non-accelerated Filer
Entity Common Stock, Shares Outstanding		25,001,819
Entity Well known seasoned issuer	Yes
Entity Voluntary Filers	No
Entity Current reporting status	Yes
Entity Public Float			$ 6.4