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Fair Value of Financial Instruments - Assets and Liabilities Measured at Fair Value on Non-Recurring Basis (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Apr. 30, 2014	Apr. 30, 2013
Assets:
Cost method investment - Recycle Rewards	$ 14,432us-gaap_CostMethodInvestments	$ 16,752us-gaap_CostMethodInvestments	$ 16,486us-gaap_CostMethodInvestments
Asset group - BioFuels		359us-gaap_AssetsOfDisposalGroupIncludingDiscontinuedOperationCurrent	61us-gaap_AssetsOfDisposalGroupIncludingDiscontinuedOperationCurrent
Recycle Rewards Inc [Member]
Assets:
Cost method investment - Recycle Rewards	2,160us-gaap_CostMethodInvestments / dei_LegalEntityAxis = cwst_RecycleRewardsIncMember
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Fair Value, Measurements, Nonrecurring [Member] | Recycle Rewards Inc [Member]
Assets:
Cost method investment - Recycle Rewards
Significant Other Observable Inputs (Level 2) [Member] | Fair Value, Measurements, Nonrecurring [Member] | Recycle Rewards Inc [Member]
Assets:
Cost method investment - Recycle Rewards
Significant Unobservable Inputs (Level 3) [Member] | Fair Value, Measurements, Nonrecurring [Member]
Liabilities:
Guaranty			2,073us-gaap_GuarantyLiabilities / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsNonrecurringMember
Significant Unobservable Inputs (Level 3) [Member] | Fair Value, Measurements, Nonrecurring [Member] | Casella-Altela Regional Environmental Services, LLC [Member]
Assets:
Asset group - CARES		650us-gaap_PropertyPlantAndEquipmentFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsNonrecurringMember / dei_LegalEntityAxis = cwst_CasellaAltelaRegionalEnvironmentalServicesLlcMember
Significant Unobservable Inputs (Level 3) [Member] | Fair Value, Measurements, Nonrecurring [Member] | BioFuels Assets [Member]
Assets:
Asset group - BioFuels			61us-gaap_AssetsOfDisposalGroupIncludingDiscontinuedOperationCurrent / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsNonrecurringMember / dei_LegalEntityAxis = cwst_BioFuelsAssetsMember
Significant Unobservable Inputs (Level 3) [Member] | Fair Value, Measurements, Nonrecurring [Member] | Recycle Rewards Inc [Member]
Assets:
Cost method investment - Recycle Rewards	$ 2,160us-gaap_CostMethodInvestments / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsNonrecurringMember / dei_LegalEntityAxis = cwst_RecycleRewardsIncMember

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Amount classified as assets attributable to disposal group held for sale or disposed of, expected to be disposed of within one year or the normal operating cycle, if longer.

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Fair value portion of physical assets used in the normal conduct of business to produce goods and services and not intended for resale. Examples include, but are not limited to, land, buildings, machinery and equipment, office equipment, and furniture and fixtures.

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