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Stockholders' Equity - Summary of Restricted Stock, Restricted Stock Unit and Performance Stock Unit Activity (Parenthetical) (Detail) (Performance Stock Units [Member]) In Thousands, unless otherwise specified | 6 Months Ended
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Oct. 31, 2013
Performance Stock Units [Member] |
Schedule Of Restricted Stock Restricted Stock Unit And Performance Stock Unit Activity [Line Items] |
Performance stock units, percentage of attainment level | 100.00%
Common stock additional issue | 354

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- Definition Additional shares of Class A Common Stock that could be issued if maximum attainment levels are met. + References No definition available. + Details
Name: | cwst_AdditionalSharesIssuedMaximumAttainmentLevel
Namespace Prefix: | cwst_
Data Type: | xbrli:sharesItemType
Balance Type: | na
Period Type: | duration

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- Details
Name: | cwst_ScheduleOfRestrictedStockRestrictedStockUnitAndPerformanceStockUnitActivityLineItems
Namespace Prefix: | cwst_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition The attainment level for performance stock units included in the roll forward. + References No definition available. + Details
Name: | cwst_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardTargetPerformanceRate
Namespace Prefix: | cwst_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant