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Income Taxes - Additional Information (Detail) (USD $)
In Thousands, unless otherwise specified
 8 Months Ended 12 Months Ended
Dec. 31, 2014
Apr. 30, 2014
Apr. 30, 2013
Jan. 18, 2011
Income Taxes [Line Items]        
Income tax settlement amount     $ 800cwst_IncomeTaxSettlement  
Tax paid for income tax settlement     430cwst_IncomeTaxSettlementTaxPaid  
Interest paid for income tax settlement     370cwst_IncomeTaxSettlementInterestPaid  
Aggregate finance asserting liability total       3,852cwst_AssertedLiabilityNet
Taxation payable       2,220cwst_AssertedTaxLiability
Penalties and interest payable       1,632cwst_AssertedTaxLiabilityPenaltiesAndInterest
Potential cumulative liability     8.00%cwst_PercentageOfPotentialCumulativeLiability  
Net operating loss carryforwards 83,132us-gaap_DeferredTaxAssetsOperatingLossCarryforwardsDomestic      
Net operating loss carryforwards expiry period 2024 through 2034      
State net operating loss carryforwards expiry period 100,363us-gaap_DeferredTaxAssetsOperatingLossCarryforwardsStateAndLocal      
Net operating loss carryforwards benefit recorded additional paid-in capital 383us-gaap_DeferredTaxAssetsOperatingLossCarryforwardsNotSubjectToExpiration      
Alternative minimum tax credit carryforwards 3,457us-gaap_DeferredTaxAssetsTaxCreditCarryforwardsAlternativeMinimumTax 3,330us-gaap_DeferredTaxAssetsTaxCreditCarryforwardsAlternativeMinimumTax 3,330us-gaap_DeferredTaxAssetsTaxCreditCarryforwardsAlternativeMinimumTax  
General business tax credit carryforwards 2,921us-gaap_DeferredTaxAssetsTaxCreditCarryforwardsGeneralBusiness 2,666us-gaap_DeferredTaxAssetsTaxCreditCarryforwardsGeneralBusiness 2,095us-gaap_DeferredTaxAssetsTaxCreditCarryforwardsGeneralBusiness  
General business tax credit carryforwards expiration date 2023 through 2034      
Valuation allowance 14,188us-gaap_ValuationAllowanceDeferredTaxAssetChangeInAmount 2,581us-gaap_ValuationAllowanceDeferredTaxAssetChangeInAmount    
Settlements     (430)us-gaap_UnrecognizedTaxBenefitsDecreasesResultingFromSettlementsWithTaxingAuthorities  
Unrecognized tax benefit net of federal benefit on state issue 0us-gaap_UnrecognizedTaxBenefitsThatWouldImpactEffectiveTaxRate 0us-gaap_UnrecognizedTaxBenefitsThatWouldImpactEffectiveTaxRate 0us-gaap_UnrecognizedTaxBenefitsThatWouldImpactEffectiveTaxRate  
Unrecognized tax benefits 178us-gaap_SignificantChangeInUnrecognizedTaxBenefitsIsReasonablyPossibleAmountOfUnrecordedBenefit      
Uncertain tax positions, accrued interest 143us-gaap_UnrecognizedTaxBenefitsInterestOnIncomeTaxesAccrued 116us-gaap_UnrecognizedTaxBenefitsInterestOnIncomeTaxesAccrued 76us-gaap_UnrecognizedTaxBenefitsInterestOnIncomeTaxesAccrued  
Uncertain tax positions, penalties 8us-gaap_UnrecognizedTaxBenefitsIncomeTaxPenaltiesAccrued 8us-gaap_UnrecognizedTaxBenefitsIncomeTaxPenaltiesAccrued 9us-gaap_UnrecognizedTaxBenefitsIncomeTaxPenaltiesAccrued  
Accrued income tax expense 26us-gaap_UnrecognizedTaxBenefitsIncomeTaxPenaltiesAndInterestAccrued 40us-gaap_UnrecognizedTaxBenefitsIncomeTaxPenaltiesAndInterestAccrued 41us-gaap_UnrecognizedTaxBenefitsIncomeTaxPenaltiesAndInterestAccrued  
Business Acquisition [Member]        
Income Taxes [Line Items]        
Increase (decrease) in valuation allowance   $ 5,084us-gaap_IncomeTaxReconciliationChangeInDeferredTaxAssetsValuationAllowance
/ us-gaap_BusinessAcquisitionAxis
= cwst_BusinessAcquisitionMember		 $ (5,084)us-gaap_IncomeTaxReconciliationChangeInDeferredTaxAssetsValuationAllowance
/ us-gaap_BusinessAcquisitionAxis
= cwst_BusinessAcquisitionMember		  
State [Member]        
Income Taxes [Line Items]        
Net operating loss carryforwards expiry period 2015 through 2034