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Development Project Charge (Narrative) (Detail) (USD $) In Thousands, unless otherwise specified	9 Months Ended
	Jan. 31, 2012	Jan. 31, 2013	Apr. 30, 2012
Collaborative Arrangements Non collaborative Arrangements And Business Acquisitions Transactions [Line Items]
Deferred costs associated with development projects	$ 131
Deferred project costs		$ 1,554	$ 1,163

X

- Definition

Amount recognized as an operating expense or loss during the period to write-off the deferred costs associated with development projects no longer deemed viable.

+ References

No definition available.

+ Details

Name:	cwst_AbandonedDevelopmentProjectExpense
Namespace Prefix:	cwst_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

X

- Details

Name:	cwst_CollaborativeArrangementsNonCollaborativeArrangementsAndBusinessAcquisitionsTransactionsLineItems
Namespace Prefix:	cwst_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

X

- Definition

Capitalized costs related to projects that are ongoing.

+ References

No definition available.

+ Details

Name:	cwst_DeferredProjectCosts
Namespace Prefix:	cwst_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant