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Commitments and Contingencies - Additional Information (Detail) (USD $) In Thousands, unless otherwise specified	8 Months Ended		12 Months Ended			3 Months Ended
	Dec. 31, 2014 Executives	Dec. 31, 2013	Apr. 30, 2014	Apr. 30, 2013	Apr. 30, 2012	Jul. 31, 2014	Apr. 30, 2011	May 31, 2009	Sep. 30, 2011	Nov. 30, 2011
Contingencies And Commitments [Line Items]
Total rent expense under operating leases charged to operations	$ 4,868us-gaap_OperatingLeasesRentExpenseNet		$ 5,651us-gaap_OperatingLeasesRentExpenseNet	$ 5,372us-gaap_OperatingLeasesRentExpenseNet	$ 5,213us-gaap_OperatingLeasesRentExpenseNet
Litigation settlement					(1,359)us-gaap_LitigationSettlementAmount
Environmental remediation charge	950us-gaap_EnvironmentalRemediationExpense	400us-gaap_EnvironmentalRemediationExpense	400us-gaap_EnvironmentalRemediationExpense
Loss contingency range of possible loss maximum	1,350us-gaap_LossContingencyRangeOfPossibleLossMaximum
Number of executive officers	4cwst_EmploymentContractsExecutiveOfficers
Contract extension period	1 year
Total annual commitments for salaries	1,397cwst_CommitmentAmountUnderEmploymentAgreement
New York Attorney General Matter [Member]
Contingencies And Commitments [Line Items]
Litigation settlement	100us-gaap_LitigationSettlementAmount / us-gaap_LitigationCaseAxis = cwst_NewYorkAttorneyGeneralMatterMember
Greenwood Street Landfill [Member]
Contingencies And Commitments [Line Items]
Litigation settlement	67us-gaap_LitigationSettlementAmount / us-gaap_LitigationCaseAxis = cwst_GreenwoodStreetLandfillMember
Greenwood Street Landfill [Member] | Administrative Penalty [Member]
Contingencies And Commitments [Line Items]
Litigation settlement	201us-gaap_LitigationSettlementAmount / us-gaap_LitigationCaseAxis = cwst_GreenwoodStreetLandfillMember / us-gaap_OtherCommitmentsAxis = cwst_AdministrativePenaltyMember
MAAG [Member]
Contingencies And Commitments [Line Items]
Litigation settlement	200us-gaap_LitigationSettlementAmount / us-gaap_LitigationCaseAxis = cwst_MassachusettsOfficeOfTheAttorneyGeneralMember
Southbridge Landfill [Member]
Contingencies And Commitments [Line Items]
Loss contingency charged against pending satisfactory completion of remediation	50cwst_LossContingencyChargedAgainstPendingSatisfactoryCompletionOfRemediation / us-gaap_LitigationCaseAxis = cwst_SouthbridgeLandfillMember
Charlton [Member]
Contingencies And Commitments [Line Items]
Litigation settlement	20us-gaap_LitigationSettlementAmount / us-gaap_LitigationCaseAxis = cwst_CharltonMember
General and Administration Costs [Member] | New York Attorney General Matter [Member]
Contingencies And Commitments [Line Items]
Litigation settlement charge						100us-gaap_LitigationSettlementExpense / us-gaap_IncomeStatementLocationAxis = us-gaap_GeneralAndAdministrativeExpenseMember / us-gaap_LitigationCaseAxis = cwst_NewYorkAttorneyGeneralMatterMember
General and Administration Costs [Member] | Greenwood Street Landfill [Member]
Contingencies And Commitments [Line Items]
Litigation settlement charge	172us-gaap_LitigationSettlementExpense / us-gaap_IncomeStatementLocationAxis = us-gaap_GeneralAndAdministrativeExpenseMember / us-gaap_LitigationCaseAxis = cwst_GreenwoodStreetLandfillMember
CCI [Member]
Contingencies And Commitments [Line Items]
Proceeds from anticipated remediation	740cwst_RemediationSettlement / us-gaap_RelatedPartyTransactionsByRelatedPartyAxis = cwst_CasellaConstructionIncMember
GCI [Member]
Contingencies And Commitments [Line Items]
Proceeds from anticipated remediation	1,300cwst_RemediationSettlement / us-gaap_RelatedPartyTransactionsByRelatedPartyAxis = cwst_GeosyntecConsultantsIncMember
Steadfast [Member]
Contingencies And Commitments [Line Items]
Proceeds from anticipated remediation	177cwst_RemediationSettlement / us-gaap_RelatedPartyTransactionsByRelatedPartyAxis = cwst_SteadfastInsuranceCompanyMember
Minimum [Member]
Contingencies And Commitments [Line Items]
Initial term of employment contract	1 year
Intangible assets, useful life	5 years
Salary and bonuses payment range	1 year
Minimum [Member] | Employment Contracts [Member]
Contingencies And Commitments [Line Items]
Intangible assets, useful life	1 year
Minimum [Member] | New York Attorney General Matter [Member]
Contingencies And Commitments [Line Items]
Minimum acquisition value for post acquisition notice	350cwst_LitigationSettlementAcquisitionNotification / us-gaap_LitigationCaseAxis = cwst_NewYorkAttorneyGeneralMatterMember / us-gaap_RangeAxis = us-gaap_MinimumMember
Maximum [Member]
Contingencies And Commitments [Line Items]
Operating facilities and equipment under operating leases	50us-gaap_PaymentsForRent / us-gaap_RangeAxis = us-gaap_MaximumMember
Initial term of employment contract	3 years
Intangible assets, useful life	10 years
Salary and bonuses payment range	3 years
Maximum [Member] | Employment Contracts [Member]
Contingencies And Commitments [Line Items]
Intangible assets, useful life	2 years
Potsdam Environmental Remediation Liability [Member]
Contingencies And Commitments [Line Items]
Scrap yard and solid waste transfer station	25cwst_ScrapYardAndSolidWasteTransferStation / us-gaap_EnvironmentalRemediationSiteAxis = cwst_PotsdamEnvironmentalLiabilityMember
Estimate of total undiscounted costs associated with implementing the preferred remedies							12,130cwst_UndiscountedCostsPreferredRemedies / us-gaap_EnvironmentalRemediationSiteAxis = cwst_PotsdamEnvironmentalLiabilityMember	10,219cwst_UndiscountedCostsPreferredRemedies / us-gaap_EnvironmentalRemediationSiteAxis = cwst_PotsdamEnvironmentalLiabilityMember
Risk free interest rate	2.00%us-gaap_AccrualForEnvironmentalLossContingenciesDiscountRate / us-gaap_EnvironmentalRemediationSiteAxis = cwst_PotsdamEnvironmentalLiabilityMember
Potsdam Environmental Remediation Liability [Member] | Waste-Stream Inc. [Member]
Contingencies And Commitments [Line Items]
Future remediation costs									3,000cwst_FutureRemediationCostsReimbursed / us-gaap_EnvironmentalRemediationSiteAxis = cwst_PotsdamEnvironmentalLiabilityMember / us-gaap_LossContingenciesByNatureOfContingencyAxis = cwst_WasteStreamIncMember
Potsdam Environmental Remediation Liability [Member] | Motors Liquidation Trust [Member]
Contingencies And Commitments [Line Items]
Future remediation costs										100cwst_FutureRemediationCostsReimbursed / us-gaap_EnvironmentalRemediationSiteAxis = cwst_PotsdamEnvironmentalLiabilityMember / us-gaap_LossContingenciesByNatureOfContingencyAxis = cwst_MotorsLiquidationTrustMember
Southbridge Landfill [Member] | Condition One [Member]
Contingencies And Commitments [Line Items]
Environmental remediation charge	3,337us-gaap_EnvironmentalRemediationExpense / us-gaap_EnvironmentalRemediationSiteAxis = cwst_SouthbridgeLandfillMember / us-gaap_OtherCommitmentsAxis = cwst_ConditionOneMember
Anticipated additional remediation cost	230cwst_Additionalremediationcostanticipated / us-gaap_EnvironmentalRemediationSiteAxis = cwst_SouthbridgeLandfillMember / us-gaap_OtherCommitmentsAxis = cwst_ConditionOneMember
Anticipated remediation costs	$ 3,567cwst_AnticipatedRemediationCost / us-gaap_EnvironmentalRemediationSiteAxis = cwst_SouthbridgeLandfillMember / us-gaap_OtherCommitmentsAxis = cwst_ConditionOneMember

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