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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 | $ 120 cwst_DivestitureAcquisitionFinancingCosts | $ 144 cwst_DivestitureAcquisitionFinancingCosts | $ 1,410 cwst_DivestitureAcquisitionFinancingCosts |  |
Impairment charge |  |  | 7,455 us-gaap_AssetImpairmentCharges |  | 40,746 us-gaap_AssetImpairmentCharges |
Development project charge |  |  | 1,394 cwst_AbandonedDevelopmentProjectExpense |  | 131 cwst_AbandonedDevelopmentProjectExpense |
Deferred project costs | 0 us-gaap_OtherDeferredCostsGross |  | 0 us-gaap_OtherDeferredCostsGross | 1,644 us-gaap_OtherDeferredCostsGross |  |
Severance and reorganization charge |  | 161 us-gaap_SeveranceCosts1 | 586 us-gaap_SeveranceCosts1 | 3,709 us-gaap_SeveranceCosts1 |  |
Liability associated with severance and reorganization | 0 cwst_SeveranceAndReorganizationLiability |  | 478 cwst_SeveranceAndReorganizationLiability | 680 cwst_SeveranceAndReorganizationLiability |  |
Legal settlement expenses |  |  |  |  | 1,359 us-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 |  |  |  | 1,410 cwst_DivestitureAcquisitionFinancingCosts / us-gaap_InvestmentTypeAxis = cwst_BbiWasteServicesMember |  |
Western Region [Member] | Cares [Member] |  |  |  |  |  |
Asset Retirement Obligation [Line Items] |  |  |  |  |  |
Impairment charge |  |  | 7,455 us-gaap_AssetImpairmentCharges / dei_EntityByLocationAxis = cwst_WesternRegionMember / us-gaap_InvestmentTypeAxis = cwst_CaresMember |  |  |
BioFuels Assets [Member] |  |  |  |  |  |
Asset Retirement Obligation [Line Items] |  |  |  |  |  |
Gain from divestiture of business | 553 us-gaap_GainLossOnSaleOfBusiness / us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis = cwst_BioFuelsAssetsMember |  |  |  |  |
New York [Member] |  |  |  |  |  |
Asset Retirement Obligation [Line Items] |  |  |  |  |  |
Legal settlement expenses |  |  |  |  | 359 us-gaap_LitigationSettlementAmount / us-gaap_StatementGeographicalAxis = stpr_NY |
Vermont [Member] |  |  |  |  |  |
Asset Retirement Obligation [Line Items] |  |  |  |  |  |
Legal settlement expenses |  |  |  |  | 1,000 us-gaap_LitigationSettlementAmount / us-gaap_StatementGeographicalAxis = stpr_VT |
Maine Energy Facility [Member] |  |  |  |  |  |
Asset Retirement Obligation [Line Items] |  |  |  |  |  |
Legal costs for the acquisition |  |  |  | 505 cwst_DivestitureAcquisitionFinancingCosts / us-gaap_SignificantAcquisitionsAndDisposalsByTransactionAxis = cwst_MaineEnergyFacilityMember |  |
Development project charge |  |  | 1,394 cwst_AbandonedDevelopmentProjectExpense / us-gaap_SignificantAcquisitionsAndDisposalsByTransactionAxis = cwst_MaineEnergyFacilityMember |  | 131 cwst_AbandonedDevelopmentProjectExpense / us-gaap_SignificantAcquisitionsAndDisposalsByTransactionAxis = cwst_MaineEnergyFacilityMember |
Maine Energy Facility [Member] | Eastern Region [Member] |  |  |  |  |  |
Asset Retirement Obligation [Line Items] |  |  |  |  |  |
Impairment charge |  |  |  |  |  | 40,746 us-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 |  |  | 144 cwst_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 |  |  |  | 602 cwst_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 |  |  |  | $ 303 cwst_DivestitureAcquisitionFinancingCosts / us-gaap_InvestmentTypeAxis = cwst_BbiWasteServicesMember / us-gaap_SignificantAcquisitionsAndDisposalsByTransactionAxis = cwst_SecondLienNotesMember |  |

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