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SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Defined Benefit Pension Plan (Details) | 12 Months Ended
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Dec. 31, 2020 plan
Multiemployer Pension Plans | New England Teamsters and Trucking Industry Pension Fund |
Multiemployer Plans [Line Items] |
Number of plans | 1

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- Definition Multiemployer Plans, Number of Plans + References No definition available. + Details
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Namespace Prefix: | cwst_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | duration

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- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + Details
Name: | us-gaap_MultiemployerPlansLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Details
Name: | us-gaap_RetirementPlanTypeAxis=us-gaap_PensionPlansDefinedBenefitMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | us-gaap_RetirementPlanNameAxis=cwst_NewEnglandTeamstersandTruckingIndustryPensionFundMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |