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Namespace Prefix:	cld_
Data Type:	xbrli:stringItemType
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Represents the estimated quantity of mineral rights under challenge from a third party

+ References

No definition available.

+ Details

Name:	cld_EstimatedQuantityOfMineralRightsUnderChallenge
Namespace Prefix:	cld_
Data Type:	num:massItemType
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- Definition

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

+ References

No definition available.

+ Details

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

- Definition

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.

+ References

No definition available.

+ Details

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

- Definition

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

+ References

No definition available.

+ Details

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- Definition

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

+ References

No definition available.

+ Details

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Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

- Definition

Amount of the unrecorded obligation to transfer funds in the future for fixed or minimum amounts or quantities of goods or services at fixed or minimum prices (for example, as in take-or-pay contracts or throughput contracts).

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 440

-SubTopic 10

-Section 50

-Paragraph 4

-Subparagraph (b)

-URI http://asc.fasb.org/extlink&oid=6394976&loc=d3e25336-109308

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Details

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

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Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_PropertyPlantAndEquipmentByTypeAxis=cld_PurchaseCommitmentForEquipmentMember
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Data Type:	na
Balance Type:
Period Type:

- Details

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

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Commitments and Contingencies (Details 1) (USD $) In Thousands, unless otherwise specified	Sep. 05, 2014 t	Feb. 27, 2013 Item	Mar. 31, 2015	Dec. 31, 2014
Wild Earth Guardians Regulatory Challenge to Office of Surface Minings Approval Process for Mine Plans Litigation [Member]
Commitment and Contingency [Line Items]
Number of different coal mines located in four different states for which approvals are challenged		7
Number of different states where seven different coal mines are located for which approvals are challenged		4
Estimated Quantity of Mineral Rights Under Challenge	15,800,000
Wild Earth Guardians Regulatory Challenge to Office of Surface Minings Approval Process for Mine Plans Litigation [Member] \| WYOMING
Commitment and Contingency [Line Items]
Number of challenged approvals issued to subsidiaries		1
Number of challenged approvals transferred to U.S. District Court in Wyoming or Montana relating to other OSM approvals		1
Wild Earth Guardians Regulatory Challenge to Office of Surface Minings Approval Process for Mine Plans Litigation [Member] \| MONTANA
Commitment and Contingency [Line Items]
Number of challenged approvals issued to subsidiaries		1
Number of challenged approvals transferred to U.S. District Court in Wyoming or Montana relating to other OSM approvals		1
Equipment
Commitment and Contingency [Line Items]
Outstanding amount of long-term purchase commitment			$ 30,973	$ 11,751
Land
Commitment and Contingency [Line Items]
Outstanding amount of long-term purchase commitment			23,678	24,663
Coal purchase commitments
Commitment and Contingency [Line Items]
Outstanding amount of long-term purchase commitment			2,589	2,592
Transportation agreements
Commitment and Contingency [Line Items]
Outstanding amount of long-term purchase commitment			673,644	691,530
Materials and supplies
Commitment and Contingency [Line Items]
Outstanding amount of long-term purchase commitment			$ 11,237	$ 12,185