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Commitments and Contingencies - Schedule of Environmental Liability Associated with Postdam (Detail) (USD $) In Thousands, unless otherwise specified | 8 Months Ended | 12 Months Ended
--- | --- | ---
Dec. 31, 2014 | Apr. 30, 2014 | Apr. 30, 2013
Site Contingency [Line Items] |  |  |
Ending balance | $ 5,142 us-gaap_AccrualForEnvironmentalLossContingencies |  |
Potsdam Environmental Remediation Liability [Member] |  |  |
Site Contingency [Line Items] |  |  |
Beginning balance | 5,320 us-gaap_AccrualForEnvironmentalLossContingencies / us-gaap_EnvironmentalRemediationSiteAxis = cwst_PotsdamEnvironmentalLiabilityMember | 5,297 us-gaap_AccrualForEnvironmentalLossContingencies / us-gaap_EnvironmentalRemediationSiteAxis = cwst_PotsdamEnvironmentalLiabilityMember | 5,210 us-gaap_AccrualForEnvironmentalLossContingencies / us-gaap_EnvironmentalRemediationSiteAxis = cwst_PotsdamEnvironmentalLiabilityMember
Revisions in estimates |  | (118)us-gaap_AccrualForEnvironmentalLossContingenciesIncreaseDecreaseForRevisionInEstimates / us-gaap_EnvironmentalRemediationSiteAxis = cwst_PotsdamEnvironmentalLiabilityMember |
Payments | (270)us-gaap_AccrualForEnvironmentalLossContingenciesPayments / us-gaap_EnvironmentalRemediationSiteAxis = cwst_PotsdamEnvironmentalLiabilityMember | (28)us-gaap_AccrualForEnvironmentalLossContingenciesPayments / us-gaap_EnvironmentalRemediationSiteAxis = cwst_PotsdamEnvironmentalLiabilityMember | (51)us-gaap_AccrualForEnvironmentalLossContingenciesPayments / us-gaap_EnvironmentalRemediationSiteAxis = cwst_PotsdamEnvironmentalLiabilityMember
Ending balance | 5,142 us-gaap_AccrualForEnvironmentalLossContingencies / us-gaap_EnvironmentalRemediationSiteAxis = cwst_PotsdamEnvironmentalLiabilityMember | 5,320 us-gaap_AccrualForEnvironmentalLossContingencies / us-gaap_EnvironmentalRemediationSiteAxis = cwst_PotsdamEnvironmentalLiabilityMember | 5,297 us-gaap_AccrualForEnvironmentalLossContingencies / us-gaap_EnvironmentalRemediationSiteAxis = cwst_PotsdamEnvironmentalLiabilityMember
Potsdam Environmental Remediation Liability [Member] | Obligations Incurred [Member] |  |  |
Site Contingency [Line Items] |  |  |
Obligations incurred / Accretion expense |  | 31 us-gaap_AccrualForEnvironmentalLossContingenciesProvisionForNewLosses / us-gaap_EnvironmentalRemediationSiteAxis = cwst_PotsdamEnvironmentalLiabilityMember / us-gaap_NatureOfExpenseAxis = cwst_EnvironmentalRemediationMember |
Potsdam Environmental Remediation Liability [Member] | Accretion Expense [Member] |  |  |
Site Contingency [Line Items] |  |  |
Obligations incurred / Accretion expense | $ 92 us-gaap_AccrualForEnvironmentalLossContingenciesProvisionForNewLosses / us-gaap_EnvironmentalRemediationSiteAxis = cwst_PotsdamEnvironmentalLiabilityMember / us-gaap_NatureOfExpenseAxis = cwst_EnvironmentalAccretionExpenseMember | $ 138 us-gaap_AccrualForEnvironmentalLossContingenciesProvisionForNewLosses / us-gaap_EnvironmentalRemediationSiteAxis = cwst_PotsdamEnvironmentalLiabilityMember / us-gaap_NatureOfExpenseAxis = cwst_EnvironmentalAccretionExpenseMember | $ 138 us-gaap_AccrualForEnvironmentalLossContingenciesProvisionForNewLosses / us-gaap_EnvironmentalRemediationSiteAxis = cwst_PotsdamEnvironmentalLiabilityMember / us-gaap_NatureOfExpenseAxis = cwst_EnvironmentalAccretionExpenseMember

X
- Definition Total costs accrued as of the balance sheet date for environmental loss contingencies. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 410 -SubTopic 30 -Section 50 -Paragraph 7 -URI http://asc.fasb.org/extlink&oid=6393242&loc=d3e13207-110859 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 450 -SubTopic 20 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=25496072&loc=d3e14326-108349 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 450 -SubTopic 20 -Section 25 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=6395460&loc=d3e13647-108346 + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Definition The Increase or Decrease in the accrual for environmental loss in the period from revisions in estimates to existing obligations. The amount does not relate to changes in the environmental accrual related to business acquisitions or divestitures, or to currency movements. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 450 -SubTopic 20 -Section S99 -Paragraph 1 -Subparagraph (SAB TOPIC 5.Y.Q2) -URI http://asc.fasb.org/extlink&oid=27011672&loc=d3e149879-122751 + Details
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Balance Type: | debit
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- Definition The aggregate amount disbursed during the reporting period to settle environmental loss contingencies. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 450 -SubTopic 20 -Section S99 -Paragraph 1 -Subparagraph (SAB TOPIC 5.Y.Q2) -URI http://asc.fasb.org/extlink&oid=27011672&loc=d3e149879-122751 + Details
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- Definition The amount added to the accrual for newly incurred environmental loss contingencies. These additions include remediation expense and interest accretion. These additions do not relate to changes in the environmental accrual related to a business acquisition or divestiture, to currency movements, or revisions in previously existing estimates. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 450 -SubTopic 20 -Section S99 -Paragraph 1 -Subparagraph (SAB TOPIC 5.Y.Q2) -URI http://asc.fasb.org/extlink&oid=27011672&loc=d3e149879-122751 + Details
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- Details
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Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Details
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Balance Type: |
Period Type: |