Net quantities of an enterprise's interests in proved undeveloped reserves of either crude oil (including condensate and natural gas liquids), natural gas, synthetic oil and gas, or other renewable natural resource that is intended to be upgraded into synthetic oil and gas as of the beginning and the end of the year. "Net" quantities of reserves include those relating to the enterprise's operating and nonoperating interests in properties. Quantities of reserves relating to royalty interests owned are included in "net" quantities if the necessary information is available to the enterprise. "Net" quantities does not include reserves relating to interests of others in properties owned by the enterprise. The unit of measure for reserve quantities is defined as "barrels" for oil and synthetic oil reserves or "cubic feet" or "cubic meters" for natural gas and synthetic gas reserves.

No definition available.

Name:	ceix_ProvedUndevelopedReserveMass
Namespace Prefix:	ceix_
Data Type:	num:massItemType
Balance Type:	na
Period Type:	instant

Supplemental Coal Data (unaudited) (Narrative) (Details) - T T in Millions	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Assigned Coal Reserves
Reserve Quantities [Line Items]
Coal Reserves	227
Steam [Member]
Reserve Quantities [Line Items]
Proved Undeveloped Reserve (Mass)	2,211
Coal Segment
Reserve Quantities [Line Items]
Proved Undeveloped Reserve (Mass)	2,298	2,361	3,047	3,238	3,032	4,229
metallurgical coal [Member]
Reserve Quantities [Line Items]
Proved Undeveloped Reserve (Mass)	87