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Business Combinations - Schedule of Purchase Price Paid for Acquisitions (Parenthetical) (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Apr. 30, 2014
Business Combinations [Abstract]
Gain on settlement of acquisition related contingent consideration	$ 1,058

X

- Definition

The gain or loss recorded as a result of the settlement of acquisition related contingent consideration.

+ References

No definition available.

+ Details

Name:	cwst_GainLossOnSettlementOfContingentConsideration
Namespace Prefix:	cwst_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

X

- Details

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