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Equity-Based Compensation (Details) (LTIP, USD $)
In Millions, except Share data, unless otherwise specified
3 Months Ended
Mar. 31, 2015
May 31, 2011
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Pool of shares of common stock for issuance in connection with share-based awards before additional approval   3,400,000cld_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAuthorizedBeforeAmendment
Pool of shares of common stock for issuance in connection with share-based awards   5,500,000us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAuthorized
Number of shares available for grant 1,600,000us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant  
Award vesting period 3 years  
Vesting Period Change in Control Termination Without Cause 2 years  
Restricted Stock and Restricted Stock Units
   
Restricted Stock and Restricted Stock Units    
Share Based Compensation Arrangements By Share Based Payment Award Options Expiration Term After Retirement Or Termination Without Cause 1 year  
Restricted stock and performance based share unit award activity    
Non-vested shares at the beginning of the period, number (in shares) 407,000us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockMember
/ us-gaap_PlanNameAxis
= cld_LongTermIncentivePlan2009Member
 
Granted, number (in shares) 460,000us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockMember
/ us-gaap_PlanNameAxis
= cld_LongTermIncentivePlan2009Member
 
Forfeited, number (in shares) (24,000)us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockMember
/ us-gaap_PlanNameAxis
= cld_LongTermIncentivePlan2009Member
 
Vested, number (in shares) (102,000)us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockMember
/ us-gaap_PlanNameAxis
= cld_LongTermIncentivePlan2009Member
 
Non-vested shares at the end of the period, number (in shares) 741,000us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockMember
/ us-gaap_PlanNameAxis
= cld_LongTermIncentivePlan2009Member
 
Weighted Average Grant-Date Fair Value (per share)    
Non-vested shares at the beginning of the period (in dollars per share) 18.14us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockMember
/ us-gaap_PlanNameAxis
= cld_LongTermIncentivePlan2009Member
 
Granted (in dollars per share) 8.05us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockMember
/ us-gaap_PlanNameAxis
= cld_LongTermIncentivePlan2009Member
 
Forfeited (in dollars per share) 18.34us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeituresWeightedAverageGrantDateFairValue
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockMember
/ us-gaap_PlanNameAxis
= cld_LongTermIncentivePlan2009Member
 
Vested (in dollars per share) 17us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodWeightedAverageGrantDateFairValue
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockMember
/ us-gaap_PlanNameAxis
= cld_LongTermIncentivePlan2009Member
 
Non-vested shares at the end of the period (in dollars per share) 12.03us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockMember
/ us-gaap_PlanNameAxis
= cld_LongTermIncentivePlan2009Member
 
Unrecognized compensation cost 5.4us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognized
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockMember
/ us-gaap_PlanNameAxis
= cld_LongTermIncentivePlan2009Member
 
Weighted-average period over which unrecognized compensation cost recognized prior to vesting 2 years 4 months 24 days  
Performance Based Share Units
   
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Award vesting period 3 years  
Restricted stock and performance based share unit award activity    
Non-vested shares at the beginning of the period, number (in shares) 497,000us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber
/ us-gaap_AwardTypeAxis
= cld_PerformanceBasedShareUnitsMember
/ us-gaap_PlanNameAxis
= cld_LongTermIncentivePlan2009Member
 
Granted, number (in shares) 601,000us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod
/ us-gaap_AwardTypeAxis
= cld_PerformanceBasedShareUnitsMember
/ us-gaap_PlanNameAxis
= cld_LongTermIncentivePlan2009Member
 
Forfeited, number (in shares) (47,000)us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod
/ us-gaap_AwardTypeAxis
= cld_PerformanceBasedShareUnitsMember
/ us-gaap_PlanNameAxis
= cld_LongTermIncentivePlan2009Member
 
Vested, number (in shares) (97,000)us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod
/ us-gaap_AwardTypeAxis
= cld_PerformanceBasedShareUnitsMember
/ us-gaap_PlanNameAxis
= cld_LongTermIncentivePlan2009Member
 
Non-vested shares at the end of the period, number (in shares) 954,000us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber
/ us-gaap_AwardTypeAxis
= cld_PerformanceBasedShareUnitsMember
/ us-gaap_PlanNameAxis
= cld_LongTermIncentivePlan2009Member
 
Weighted Average Grant-Date Fair Value (per share)    
Non-vested shares at the beginning of the period (in dollars per share) 21.84us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue
/ us-gaap_AwardTypeAxis
= cld_PerformanceBasedShareUnitsMember
/ us-gaap_PlanNameAxis
= cld_LongTermIncentivePlan2009Member
 
Granted (in dollars per share) 9.66us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue
/ us-gaap_AwardTypeAxis
= cld_PerformanceBasedShareUnitsMember
/ us-gaap_PlanNameAxis
= cld_LongTermIncentivePlan2009Member
 
Forfeited (in dollars per share) 22.73us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeituresWeightedAverageGrantDateFairValue
/ us-gaap_AwardTypeAxis
= cld_PerformanceBasedShareUnitsMember
/ us-gaap_PlanNameAxis
= cld_LongTermIncentivePlan2009Member
 
Vested (in dollars per share) 17.61us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodWeightedAverageGrantDateFairValue
/ us-gaap_AwardTypeAxis
= cld_PerformanceBasedShareUnitsMember
/ us-gaap_PlanNameAxis
= cld_LongTermIncentivePlan2009Member
 
Non-vested shares at the end of the period (in dollars per share) 14.56us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue
/ us-gaap_AwardTypeAxis
= cld_PerformanceBasedShareUnitsMember
/ us-gaap_PlanNameAxis
= cld_LongTermIncentivePlan2009Member
 
Assumptions:    
Risk-free interest rate (as a percent) 1.00%us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRate
/ us-gaap_AwardTypeAxis
= cld_PerformanceBasedShareUnitsMember
/ us-gaap_PlanNameAxis
= cld_LongTermIncentivePlan2009Member
 
Expected volatility (as a percent) 37.70%us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedVolatilityRate
/ us-gaap_AwardTypeAxis
= cld_PerformanceBasedShareUnitsMember
/ us-gaap_PlanNameAxis
= cld_LongTermIncentivePlan2009Member
 
Expected option life 2 years 9 months 18 days  
Fair value (in dollars per share) 9.66us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageGrantDateFairValue
/ us-gaap_AwardTypeAxis
= cld_PerformanceBasedShareUnitsMember
/ us-gaap_PlanNameAxis
= cld_LongTermIncentivePlan2009Member
 
Performance Based Share Units | Maximum
   
Performance-Based Share Units    
Percentage of target amount, high end of range 200.00%cld_PercentageOfTargetAmountHighEndOfRange
/ us-gaap_AwardTypeAxis
= cld_PerformanceBasedShareUnitsMember
/ us-gaap_PlanNameAxis
= cld_LongTermIncentivePlan2009Member
/ us-gaap_RangeAxis
= us-gaap_MaximumMember
 
Performance Based Share Units | Minimum
   
Performance-Based Share Units    
Percentage of target amount, low end of range 0.00%cld_PercentageOfTargetAmountLowEndOfRange
/ us-gaap_AwardTypeAxis
= cld_PerformanceBasedShareUnitsMember
/ us-gaap_PlanNameAxis
= cld_LongTermIncentivePlan2009Member
/ us-gaap_RangeAxis
= us-gaap_MinimumMember