Leases (Schedule of Future Minimum Sublease Rentals for Operating Leases) (Details) $ in Thousands	Dec. 31, 2017 USD ($)
Leases [Abstract]
2018	$ 295
2019	0
2020	0
2021	0
2022	0
Thereafter	0
Total	$ 295

Operating Leases, Future Minimum Payments Due, Future Minimum Sublease Rentals, Next Rolling Twelve Months

No definition available.

Name:	ceix_OperatingLeasesFutureMinimumPaymentsDueFutureMinimumSubleaseRentalsNextRollingTwelveMonths
Namespace Prefix:	ceix_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Operating Leases, Future Minimum Payments Due, Future Minimum Sublease Rentals, Year Five

+ References

No definition available.

+ Details

Name:	ceix_OperatingLeasesFutureMinimumPaymentsDueFutureMinimumSubleaseRentalsYearFive
Namespace Prefix:	ceix_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Operating Leases, Future Minimum Payments Due, Future Minimum Sublease Rentals, Year Four

+ References

No definition available.

+ Details

Name:	ceix_OperatingLeasesFutureMinimumPaymentsDueFutureMinimumSubleaseRentalsYearFour
Namespace Prefix:	ceix_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Operating Leases, Future Minimum Payments Due, Future Minimum Sublease Rentals, Year Three

+ References

No definition available.

+ Details

Name:	ceix_OperatingLeasesFutureMinimumPaymentsDueFutureMinimumSubleaseRentalsYearThree
Namespace Prefix:	ceix_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Operating Leases, Future Minimum Payments Due, Future Minimum Sublease Rentals, Year Two

+ References

No definition available.

+ Details

Name:	ceix_OperatingLeasesFutureMinimumPaymentsDueFutureMinimumSubleaseRentalsYearTwo
Namespace Prefix:	ceix_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Operating Leases, Future Minimum Payments Due, Future Minimum Sublease Rentals, after Year Five

+ References

No definition available.

+ Details

Name:	ceix_OperatingLeasesFutureMinimumPaymentsDueFutureMinimumSubleaseRentalsafterYearFive
Namespace Prefix:	ceix_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- References

No definition available.

+ Details

Name:	us-gaap_LeasesAbstract
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Contractually required future rental payments receivable on noncancelable subleasing arrangements.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 840
-SubTopic 20
-Section 50
-Paragraph 2
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=77902758&loc=d3e41502-112717

+ Details

Name:	us-gaap_OperatingLeasesFutureMinimumPaymentsDueFutureMinimumSubleaseRentals
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant