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Summary of Significant Accounting Policies - Cash and Cash Equivalents - Additional Information (Detail)	12 Months Ended
	Apr. 30, 2014
Accounting Policies [Abstract]
Maximum cash equivalents maturity period	Three months

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- Definition

Maturity for a highly liquid investment to be considered a cash equivalent.

+ References

No definition available.

+ Details

Name:	cwst_CashAndCashEquivalentMaximumMaturity
Namespace Prefix:	cwst_
Data Type:	us-types:durationStringItemType
Balance Type:	na
Period Type:	duration

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- Details

Name:	us-gaap_AccountingPoliciesAbstract
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration