Name:	cld_CommitmentAndContingencyLineItems
Namespace Prefix:	cld_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- References

No definition available.

Name:	cld_DaysAllowedToOSMForEnvironmentalAnalysis
Namespace Prefix:	cld_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

Represents the estimated quantity of mineral rights under challenge from a third party

No definition available.

Name:	cld_EstimatedQuantityOfMineralRightsUnderChallenge
Namespace Prefix:	cld_
Data Type:	num:massItemType
Balance Type:	na
Period Type:	instant

Number of appeals consolidated

No definition available.

Name:	cld_NumberOfAppealsConsolidated
Namespace Prefix:	cld_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

Represents the number of challenged approvals issued to subsidiaries for which claims were filed during the period.

No definition available.

Name:	cld_NumberOfChallengedApprovalsIssuedToSubsidiaries
Namespace Prefix:	cld_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

Represents the number of challenged approvals transferred to other federal district courts for the states in which the non-Colorado mines were located during the period.

No definition available.

Name:	cld_NumberOfChallengedApprovalsTransferredToOtherFederalDistrictCourtsForNonColoradoMines
Namespace Prefix:	cld_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

Represents the number of different coal mines located in four different states for which approvals are challenged during the period.

No definition available.

Name:	cld_NumberOfDifferentCoalMinesLocatedInFourDifferentStatesForWhichApprovalsAreChallenged
Namespace Prefix:	cld_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

Represents the number of different states where seven different coal mines are located for which approvals are challenged during the period.

No definition available.

Name:	cld_NumberOfDifferentStatedWhereSevenCoalMinesLocatedForWhichApprovalsAreChallenged
Namespace Prefix:	cld_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

Number of subsidiaries included in OSM motion to sever complaint and transfer to district court

No definition available.

Name:	cld_NumberOfSubsidiariesIncludedInOSMMotionToSeverComplaintAndTransfer
Namespace Prefix:	cld_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

Number of subsidiary mine plans challenged

No definition available.

Name:	cld_SubsidiaryMinePlansChallenged
Namespace Prefix:	cld_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

Name:	us-gaap_LossContingenciesByNatureOfContingencyAxis=cld_WildEarthGuardiansRegulatoryChallengeToOfficeOfSurfaceMiningsApprovalProcessForMinePlansLitigationMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	us-gaap_LossContingenciesByNatureOfContingencyAxis=cld_NorthernPlainsResourceCouncilAndWildEarthGuardiansCombinedComplaintsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	us-gaap_LossContingenciesByNatureOfContingencyAxis=cld_MontanaDistrictCourtMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	us-gaap_LossContingenciesByNatureOfContingencyAxis=cld_MontanaMagistrateJudgeMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	us-gaap_PropertyPlantAndEquipmentByTypeAxis=cld_CorderoRojoMineMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	cld_AllStatesAndProvincesAxis=stpr_WY
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	us-gaap_PropertyPlantAndEquipmentByTypeAxis=cld_SpringCreekMineMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	cld_AllStatesAndProvincesAxis=stpr_MT
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: