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Fair Value of Financial Instruments - Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) (USD $) In Thousands, unless otherwise specified | Dec. 31, 2014 | Apr. 30, 2014 | Apr. 30, 2013
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Assets: |  |  |
Restricted assets | $ 6,632 us-gaap_RestrictedCashAndInvestmentsNoncurrent | $ 681 us-gaap_RestrictedCashAndInvestmentsNoncurrent | $ 545 us-gaap_RestrictedCashAndInvestmentsNoncurrent
Capital Projects [Member] |  |  |
Assets: |  |  |
Restricted assets | 5,819 us-gaap_RestrictedCashAndInvestmentsNoncurrent / us-gaap_PropertyPlantAndEquipmentByTypeAxis = cwst_CapitalProjectsMember |  |
Landfills [Member] |  |  |
Assets: |  |  |
Restricted assets | 813 us-gaap_RestrictedCashAndInvestmentsNoncurrent / us-gaap_PropertyPlantAndEquipmentByTypeAxis = us-gaap_LandfillMember | 681 us-gaap_RestrictedCashAndInvestmentsNoncurrent / us-gaap_PropertyPlantAndEquipmentByTypeAxis = us-gaap_LandfillMember | 545 us-gaap_RestrictedCashAndInvestmentsNoncurrent / us-gaap_PropertyPlantAndEquipmentByTypeAxis = us-gaap_LandfillMember
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Fair Value, Measurements, Recurring [Member] |  |  |
Assets: |  |  |
Restricted assets | 6,632 us-gaap_RestrictedCashAndInvestmentsNoncurrent / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember | 681 us-gaap_RestrictedCashAndInvestmentsNoncurrent / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember | 545 us-gaap_RestrictedCashAndInvestmentsNoncurrent / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Fair Value, Measurements, Recurring [Member] | Capital Projects [Member] |  |  |
Assets: |  |  |
Restricted assets | 5,819 us-gaap_RestrictedCashAndInvestmentsNoncurrent / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember / us-gaap_PropertyPlantAndEquipmentByTypeAxis = cwst_CapitalProjectsMember |  |
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Fair Value, Measurements, Recurring [Member] | Landfills [Member] |  |  |
Assets: |  |  |
Restricted assets | 813 us-gaap_RestrictedCashAndInvestmentsNoncurrent / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember / us-gaap_PropertyPlantAndEquipmentByTypeAxis = us-gaap_LandfillMember |  |
Significant Other Observable Inputs (Level 2) [Member] | Fair Value, Measurements, Recurring [Member] |  |  |
Liabilities: |  |  |
Interest rate derivatives | $ 1,668 us-gaap_DerivativeInstrumentsNotDesignatedAsHedgingInstrumentsLiabilityAtFairValue / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember | $ 2,770 us-gaap_DerivativeInstrumentsNotDesignatedAsHedgingInstrumentsLiabilityAtFairValue / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember | $ 4,229 us-gaap_DerivativeInstrumentsNotDesignatedAsHedgingInstrumentsLiabilityAtFairValue / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsRecurringMember

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- Definition Fair value as of the balance sheet date of derivative instrument not designated as hedging instrument and classified as a liability. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 4B -Subparagraph (c)(2) -URI http://asc.fasb.org/extlink&oid=35706170&loc=SL5624163-113959 + Details
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- Definition The noncurrent cash, cash equivalents and investments that is restricted as to withdrawal or usage. Restrictions may include legally restricted deposits held as compensating balances against borrowing arrangements, contracts entered into with others, or entity statements of intention with regard to particular deposits classified as long-term; that is not expected to be released from such existing restrictions within one year of the balance sheet date or operating cycle, whichever is longer. Excludes compensating balance arrangements that are not agreements which legally restrict the use of cash amounts shown on the balance sheet. Includes noncurrent cash equivalents and investments that are similarly restricted as to withdrawal, usage or disposal. + References No definition available. + Details
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