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Divestiture Transactions and Discontinued Operations - Additional Information (Detail) (USD $) In Thousands, unless otherwise specified | 3 Months Ended | 8 Months Ended | 12 Months Ended | 3 Months Ended
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Jul. 31, 2013 | Dec. 31, 2013 | Apr. 30, 2014 | Apr. 30, 2012 | Jul. 31, 2012
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |
Loss on disposal of discontinued operations, net of tax | $ (378)us-gaap_DiscontinuedOperationGainLossOnDisposalOfDiscontinuedOperationNetOfTax | $ (378)us-gaap_DiscontinuedOperationGainLossOnDisposalOfDiscontinuedOperationNetOfTax | $ (378)us-gaap_DiscontinuedOperationGainLossOnDisposalOfDiscontinuedOperationNetOfTax | $ 725 us-gaap_DiscontinuedOperationGainLossOnDisposalOfDiscontinuedOperationNetOfTax |
BioFuels Assets [Member] |  |  |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |
Undiscounted purchase consideration | 2,000 us-gaap_SignificantAcquisitionsAndDisposalsAcquisitionCostsOrSaleProceeds / us-gaap_SignificantAcquisitionsAndDisposalsByTransactionAxis = cwst_BioFuelsAssetsMember |  |  |  |
Undiscounted purchase consideration paid in installments | 5 years |  |  |  |
Maine Energy Facility [Member] |  |  |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |
Undiscounted purchase consideration |  |  |  |  | 6,650 us-gaap_SignificantAcquisitionsAndDisposalsAcquisitionCostsOrSaleProceeds / us-gaap_SignificantAcquisitionsAndDisposalsByTransactionAxis = cwst_MaineEnergyFacilityMember
Undiscounted purchase consideration paid in installments |  |  |  |  | 21 years
Maine Energy Facility [Member] | Maximum [Member] |  |  |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |
Gain on divestiture |  |  |  |  | $ 1,000 us-gaap_GainLossOnSaleOfBusiness / us-gaap_RangeAxis = us-gaap_MaximumMember / us-gaap_SignificantAcquisitionsAndDisposalsByTransactionAxis = cwst_MaineEnergyFacilityMember

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- Definition Period over which consideration for sale of disposal group assets to be received. + References No definition available. + Details
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- Definition Amount of gain (loss), after tax expense or benefit and not previously recognized, resulting from the sale of a business component. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 205 -SubTopic 20 -Section 45 -Paragraph 3 -URI http://asc.fasb.org/extlink&oid=6892542&loc=d3e957-107759 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 205 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=6360339&loc=d3e1361-107760 + Details
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- Definition Gain (loss) from sale or disposal of an organization or integrated set of activities (for example, but not limited to, a partnership or corporation) engaged in providing a product or service in a commercial, industrial, or professional environment. + References No definition available. + Details
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- Definition The value of all consideration given or received by the Entity in the significant acquisition or disposal. + References No definition available. + Details
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