DEBT - Credit Facility Narrative (Details)	6 Months Ended
Jun. 30, 2021 USD ($)
Debt Instrument [Line Items]
Additional borrowing capacity	$ 125,000,000
Line of Credit
Debt Instrument [Line Items]
Debt instrument, term	5 years
Net leverage ratio	2.25
Line of credit facility, remaining borrowing capacity	$ 173,575,000
Line of Credit \| LIBOR \| Maximum
Debt Instrument [Line Items]
Floor interest rate received	1.50%
Line of Credit \| LIBOR \| Minimum
Debt Instrument [Line Items]
Floor interest rate received	1.25%
Secured Debt \| Term Loan Facility \| Line of Credit
Debt Instrument [Line Items]
Aggregate principal amount	$ 350,000,000
Revolving Credit Facility \| Revolving Credit Facility Due 2023 \| Line of Credit
Debt Instrument [Line Items]
Credit facility maximum	200,000,000
Letters of credit outstanding	$ 26,425,000

Debt Instrument, Covenant Terms, Net Leverage Ratio

No definition available.

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Balance Type:	na
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- Definition

Amount of additional optional borrowing capacity under the credit facility.

+ References

No definition available.

+ Details

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Namespace Prefix:	cwst_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Face (par) amount of debt instrument at time of issuance.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 470
-SubTopic 20
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+ Details

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Balance Type:	credit
Period Type:	instant

- 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

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- Definition

Period of time between issuance and maturity of debt instrument, in PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

No definition available.

+ Details

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Period Type:	duration

- Definition

The percentage points added to the reference rate to compute the variable rate on the interest rate derivative.

+ References

No definition available.

+ Details

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- Definition

The total amount of the contingent obligation under letters of credit outstanding as of the reporting date.

+ References

No definition available.

+ Details

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- Definition

Maximum borrowing capacity under the credit facility without consideration of any current restrictions on the amount that could be borrowed or the amounts currently outstanding under the facility.

+ References

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-Publisher FASB
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-SubTopic 10
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-Paragraph 1
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- Definition

Amount of borrowing capacity currently available under the credit facility (current borrowing capacity less the amount of borrowings outstanding).

+ References

+ Details

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