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SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Defined Benefit Pension Plan (Details)	12 Months Ended
	Dec. 31, 2018 plan
Multiemployer Pension Plans | New England Teamsters and Trucking Industry Pension Fund
Multiemployer Plans [Line Items]
Number of plans	1

X

- Definition

Multiemployer Plans, Number of Plans

+ References

No definition available.

+ Details

Name:	cwst_MultiemployerPlansNumberofPlans
Namespace Prefix:	cwst_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

X

- 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

X

- Details

Name:	us-gaap_MultiemployerPlanTypeAxis=us-gaap_MultiemployerPlansPensionMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	us-gaap_MultiemployerPlanNameAxis=cwst_NewEnglandTeamstersandTruckingIndustryPensionFundMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: