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Operating Asset Impairments and Other Items - Additional Information (Detail) (USD $)
In Thousands, unless otherwise specified
8 Months Ended 12 Months Ended 3 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Apr. 30, 2014
Apr. 30, 2013
Apr. 30, 2012
Apr. 30, 2012
Asset Retirement Obligation [Line Items]            
Legal costs for the acquisition $ (553)cwst_DivestitureAcquisitionFinancingCosts $ 120cwst_DivestitureAcquisitionFinancingCosts $ 144cwst_DivestitureAcquisitionFinancingCosts $ 1,410cwst_DivestitureAcquisitionFinancingCosts    
Impairment charge     7,455us-gaap_AssetImpairmentCharges   40,746us-gaap_AssetImpairmentCharges  
Development project charge     1,394cwst_AbandonedDevelopmentProjectExpense   131cwst_AbandonedDevelopmentProjectExpense  
Deferred project costs 0us-gaap_OtherDeferredCostsGross   0us-gaap_OtherDeferredCostsGross 1,644us-gaap_OtherDeferredCostsGross    
Severance and reorganization charge   161us-gaap_SeveranceCosts1 586us-gaap_SeveranceCosts1 3,709us-gaap_SeveranceCosts1    
Liability associated with severance and reorganization 0cwst_SeveranceAndReorganizationLiability   478cwst_SeveranceAndReorganizationLiability 680cwst_SeveranceAndReorganizationLiability    
Legal settlement expenses         1,359us-gaap_LitigationSettlementAmount  
Tompkins [Member]            
Asset Retirement Obligation [Line Items]            
Equity method investment membership interest     50.00%us-gaap_EquityMethodInvestmentOwnershipPercentage
/ us-gaap_InvestmentTypeAxis
= cwst_TompkinsMember
     
BBI Acquisition [Member]            
Asset Retirement Obligation [Line Items]            
Legal costs for the acquisition       1,410cwst_DivestitureAcquisitionFinancingCosts
/ us-gaap_InvestmentTypeAxis
= cwst_BbiWasteServicesMember
   
Western Region [Member] | Cares [Member]            
Asset Retirement Obligation [Line Items]            
Impairment charge     7,455us-gaap_AssetImpairmentCharges
/ dei_EntityByLocationAxis
= cwst_WesternRegionMember
/ us-gaap_InvestmentTypeAxis
= cwst_CaresMember
     
BioFuels Assets [Member]            
Asset Retirement Obligation [Line Items]            
Gain from divestiture of business 553us-gaap_GainLossOnSaleOfBusiness
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= cwst_BioFuelsAssetsMember
         
New York [Member]            
Asset Retirement Obligation [Line Items]            
Legal settlement expenses         359us-gaap_LitigationSettlementAmount
/ us-gaap_StatementGeographicalAxis
= stpr_NY
 
Vermont [Member]            
Asset Retirement Obligation [Line Items]            
Legal settlement expenses         1,000us-gaap_LitigationSettlementAmount
/ us-gaap_StatementGeographicalAxis
= stpr_VT
 
Maine Energy Facility [Member]            
Asset Retirement Obligation [Line Items]            
Legal costs for the acquisition       505cwst_DivestitureAcquisitionFinancingCosts
/ us-gaap_SignificantAcquisitionsAndDisposalsByTransactionAxis
= cwst_MaineEnergyFacilityMember
   
Development project charge     1,394cwst_AbandonedDevelopmentProjectExpense
/ us-gaap_SignificantAcquisitionsAndDisposalsByTransactionAxis
= cwst_MaineEnergyFacilityMember
  131cwst_AbandonedDevelopmentProjectExpense
/ us-gaap_SignificantAcquisitionsAndDisposalsByTransactionAxis
= cwst_MaineEnergyFacilityMember
 
Maine Energy Facility [Member] | Eastern Region [Member]            
Asset Retirement Obligation [Line Items]            
Impairment charge           40,746us-gaap_AssetImpairmentCharges
/ dei_EntityByLocationAxis
= cwst_EasternRegionMember
/ us-gaap_SignificantAcquisitionsAndDisposalsByTransactionAxis
= cwst_MaineEnergyFacilityMember
Discount rate           3.50%us-gaap_FairValueInputsDiscountRate
/ dei_EntityByLocationAxis
= cwst_EasternRegionMember
/ us-gaap_SignificantAcquisitionsAndDisposalsByTransactionAxis
= cwst_MaineEnergyFacilityMember
Legal Costs [Member] | Tompkins [Member]            
Asset Retirement Obligation [Line Items]            
Legal costs for the acquisition     144cwst_DivestitureAcquisitionFinancingCosts
/ us-gaap_InvestmentTypeAxis
= cwst_TompkinsMember
/ us-gaap_SignificantAcquisitionsAndDisposalsByTransactionAxis
= cwst_LegalRelatedCostsMember
     
Legal Costs [Member] | BBI Acquisition [Member]            
Asset Retirement Obligation [Line Items]            
Legal costs for the acquisition       602cwst_DivestitureAcquisitionFinancingCosts
/ us-gaap_InvestmentTypeAxis
= cwst_BbiWasteServicesMember
/ us-gaap_SignificantAcquisitionsAndDisposalsByTransactionAxis
= cwst_LegalRelatedCostsMember
   
Second Lien Notes [Member] | BBI Acquisition [Member]            
Asset Retirement Obligation [Line Items]            
Legal costs for the acquisition       $ 303cwst_DivestitureAcquisitionFinancingCosts
/ us-gaap_InvestmentTypeAxis
= cwst_BbiWasteServicesMember
/ us-gaap_SignificantAcquisitionsAndDisposalsByTransactionAxis
= cwst_SecondLienNotesMember