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Stockholders' (Deficit) Equity - Additional Information (Detail) (USD $) | 8 Months Ended | 12 Months Ended | 0 Months Ended |
--- | --- | --- | --- | ---
Dec. 31, 2014 | Apr. 30, 2014 | Apr. 30, 2013 | Apr. 30, 2012 | Feb. 09, 2014 | Apr. 30, 2007
Limited Partners' Capital Account [Line Items] |  |  |  |  |  |
Shares issued | 0 us-gaap_PreferredStockSharesIssued | 0 us-gaap_PreferredStockSharesIssued | 0 us-gaap_PreferredStockSharesIssued |  |  |
Weighted average fair value stock options granted | $ 4.86 us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageExercisePrice | $ 4.22 us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageExercisePrice | $ 3.03 us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageExercisePrice | $ 4.14 us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageExercisePrice |  |
Expected dividend yield | 0.00%us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedDividendRate | 0.00%us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedDividendRate | 0.00%us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedDividendRate | 0.00%us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedDividendRate |  |
Total fair value of other stock awards vested | $ 1,866,000 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodTotalFairValue | $ 1,458,000 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodTotalFairValue | $ 2,475,000 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodTotalFairValue | $ 2,056,000 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodTotalFairValue |  |
Tax benefits from additional paid-in-capital adjustments | 84,000 us-gaap_AdjustmentsToAdditionalPaidInCapitalTaxEffectFromShareBasedCompensation | 0 us-gaap_AdjustmentsToAdditionalPaidInCapitalTaxEffectFromShareBasedCompensation | 96,000 us-gaap_AdjustmentsToAdditionalPaidInCapitalTaxEffectFromShareBasedCompensation | 254,000 us-gaap_AdjustmentsToAdditionalPaidInCapitalTaxEffectFromShareBasedCompensation |  |
Stock Options [Member] |  |  |  |  |  |
Limited Partners' Capital Account [Line Items] |  |  |  |  |  |
Stock-based compensation expense | 386,000 us-gaap_AllocatedShareBasedCompensationExpense / us-gaap_AwardTypeAxis = us-gaap_StockOptionMember | 464,000 us-gaap_AllocatedShareBasedCompensationExpense / us-gaap_AwardTypeAxis = us-gaap_StockOptionMember | 528,000 us-gaap_AllocatedShareBasedCompensationExpense / us-gaap_AwardTypeAxis = us-gaap_StockOptionMember | 258,000 us-gaap_AllocatedShareBasedCompensationExpense / us-gaap_AwardTypeAxis = us-gaap_StockOptionMember |  |
Aggregate intrinsic value of stock options exercised | 31,000 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisesInPeriodTotalIntrinsicValue / us-gaap_AwardTypeAxis = us-gaap_StockOptionMember | 23,000 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisesInPeriodTotalIntrinsicValue / us-gaap_AwardTypeAxis = us-gaap_StockOptionMember | 0 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisesInPeriodTotalIntrinsicValue / us-gaap_AwardTypeAxis = us-gaap_StockOptionMember | 0 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisesInPeriodTotalIntrinsicValue / us-gaap_AwardTypeAxis = us-gaap_StockOptionMember |  |
Unrecognized stock-based compensation expense | 1,093,000 us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedShareBasedAwardsOtherThanOptions / us-gaap_AwardTypeAxis = us-gaap_StockOptionMember |  |  |  |  |
Unrecognized stock-based compensation expense, weighted average period | 1 year 7 months 6 days |  |  |  |  |
Restricted Stock, Restricted Stock Units and Performance Stock Units [Member] |  |  |  |  |  |
Limited Partners' Capital Account [Line Items] |  |  |  |  |  |
Stock-based compensation expense | 1,202,000 us-gaap_AllocatedShareBasedCompensationExpense / us-gaap_AwardTypeAxis = cwst_StockOptionsPerformanceStockUnitsRestrictedStockUnitsAndRestrictedStockMember | 1,861,000 us-gaap_AllocatedShareBasedCompensationExpense / us-gaap_AwardTypeAxis = cwst_StockOptionsPerformanceStockUnitsRestrictedStockUnitsAndRestrictedStockMember | 1,609,000 us-gaap_AllocatedShareBasedCompensationExpense / us-gaap_AwardTypeAxis = cwst_StockOptionsPerformanceStockUnitsRestrictedStockUnitsAndRestrictedStockMember | 1,485,000 us-gaap_AllocatedShareBasedCompensationExpense / us-gaap_AwardTypeAxis = cwst_StockOptionsPerformanceStockUnitsRestrictedStockUnitsAndRestrictedStockMember |  |
Income taxes associated with stock-based compensation expense | 0 us-gaap_EmployeeServiceShareBasedCompensationTaxBenefitRealizedFromExerciseOfStockOptions / us-gaap_AwardTypeAxis = cwst_StockOptionsPerformanceStockUnitsRestrictedStockUnitsAndRestrictedStockMember | 0 us-gaap_EmployeeServiceShareBasedCompensationTaxBenefitRealizedFromExerciseOfStockOptions / us-gaap_AwardTypeAxis = cwst_StockOptionsPerformanceStockUnitsRestrictedStockUnitsAndRestrictedStockMember | 0 us-gaap_EmployeeServiceShareBasedCompensationTaxBenefitRealizedFromExerciseOfStockOptions / us-gaap_AwardTypeAxis = cwst_StockOptionsPerformanceStockUnitsRestrictedStockUnitsAndRestrictedStockMember | 0 us-gaap_EmployeeServiceShareBasedCompensationTaxBenefitRealizedFromExerciseOfStockOptions / us-gaap_AwardTypeAxis = cwst_StockOptionsPerformanceStockUnitsRestrictedStockUnitsAndRestrictedStockMember |  |
Restricted Stock and Restricted Stock Unit [Member] |  |  |  |  |  |
Limited Partners' Capital Account [Line Items] |  |  |  |  |  |
Unrecognized stock-based compensation expense | 2,438,000 us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedShareBasedAwardsOtherThanOptions / us-gaap_AwardTypeAxis = cwst_RestrictedStockAndRestrictedStockUnitMember |  |  |  |  |
Unrecognized stock-based compensation expense, weighted average period | 1 year 8 months 12 days |  |  |  |  |
Performance Stock Units [Member] |  |  |  |  |  |
Limited Partners' Capital Account [Line Items] |  |  |  |  |  |
Unrecognized stock-based compensation expense | 2,505,000 us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedShareBasedAwardsOtherThanOptions / us-gaap_AwardTypeAxis = cwst_PerformanceShareUnitsMember |  |  |  |  |
Unrecognized stock-based compensation expense, weighted average period | 3 months 18 days |  |  |  |  |
Amended and Restated 1997 Employee Stock Purchase Plan [Member] |  |  |  |  |  |
Limited Partners' Capital Account [Line Items] |  |  |  |  |  |
Stock-based compensation expense | 52,000 us-gaap_AllocatedShareBasedCompensationExpense / us-gaap_AwardTypeAxis = cwst_AmendedAndRestatedNineteenNinetySevenEmployeeStockPurchasePlanMember | 79,000 us-gaap_AllocatedShareBasedCompensationExpense / us-gaap_AwardTypeAxis = cwst_AmendedAndRestatedNineteenNinetySevenEmployeeStockPurchasePlanMember | 99,000 us-gaap_AllocatedShareBasedCompensationExpense / us-gaap_AwardTypeAxis = cwst_AmendedAndRestatedNineteenNinetySevenEmployeeStockPurchasePlanMember | 113,000 us-gaap_AllocatedShareBasedCompensationExpense / us-gaap_AwardTypeAxis = cwst_AmendedAndRestatedNineteenNinetySevenEmployeeStockPurchasePlanMember |  |
Non-Employee Director [Member] | Restricted Stock Awards [Member] |  |  |  |  |  |
Limited Partners' Capital Account [Line Items] |  |  |  |  |  |
Options vesting period | 3 years |  |  |  |  |
Casella-Altela Regional Environmental Services, LLC [Member] |  |  |  |  |  |
Limited Partners' Capital Account [Line Items] |  |  |  |  |  |
Membership interest | 51.00%us-gaap_MinorityInterestOwnershipPercentageByParent / us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis = cwst_CasellaAltelaRegionalEnvironmentalServicesLlcMember |  |  |  |  |
Casella-Altela Regional Environmental Services, LLC [Member] | Subsequent Event [Member] |  |  |  |  |  |
Limited Partners' Capital Account [Line Items] |  |  |  |  |  |
Undiscounted purchase consideration |  |  |  |  | 3,500,000 us-gaap_ProceedsFromDivestitureOfInterestInConsolidatedSubsidiaries / us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis = cwst_CasellaAltelaRegionalEnvironmentalServicesLlcMember / us-gaap_SubsequentEventTypeAxis = us-gaap_SubsequentEventMember |
Gain on divestiture |  |  |  |  | 2,850,000 us-gaap_GainLossOnDispositionOfAssets / us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis = cwst_CasellaAltelaRegionalEnvironmentalServicesLlcMember / us-gaap_SubsequentEventTypeAxis = us-gaap_SubsequentEventMember |
Casella-Altela Regional Environmental Services, LLC [Member] | Altela, Inc. [Member] | Subsequent Event [Member] |  |  |  |  |  |
Limited Partners' Capital Account [Line Items] |  |  |  |  |  |
Membership interest |  |  |  |  | 49.00%us-gaap_MinorityInterestOwnershipPercentageByNoncontrollingOwners / dei_LegalEntityAxis = cwst_AltelaIncMember / us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis = cwst_CasellaAltelaRegionalEnvironmentalServicesLlcMember / us-gaap_SubsequentEventTypeAxis = us-gaap_SubsequentEventMember |
Maximum [Member] |  |  |  |  |  |
Limited Partners' Capital Account [Line Items] |  |  |  |  |  |
Preferred stock authorized to issue | 944,000 us-gaap_PreferredStockSharesAuthorized / us-gaap_RangeAxis = us-gaap_MaximumMember |  |  |  |  |
Maximum [Member] | 2006 Stock Incentive Plan [Member] |  |  |  |  |  |
Limited Partners' Capital Account [Line Items] |  |  |  |  |  |
Options vesting period | 4 years |  |  |  |  |
Options granted period | 10 years |  |  |  |  |
Maximum [Member] | Casella-Altela Regional Environmental Services, LLC [Member] | Subsequent Event [Member] |  |  |  |  |  |
Limited Partners' Capital Account [Line Items] |  |  |  |  |  |
Gain on sale of assets |  |  |  |  | 1,050,000 us-gaap_GainLossOnDispositionOfAssets1 / us-gaap_RangeAxis = us-gaap_MaximumMember / us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis = cwst_CasellaAltelaRegionalEnvironmentalServicesLlcMember / us-gaap_SubsequentEventTypeAxis = us-gaap_SubsequentEventMember |
Minimum [Member] | 2006 Stock Incentive Plan [Member] |  |  |  |  |  |
Limited Partners' Capital Account [Line Items] |  |  |  |  |  |
Options vesting period | 1 year |  |  |  |  |
Class A Common Stock [Member] |  |  |  |  |  |
Limited Partners' Capital Account [Line Items] |  |  |  |  |  |
Issuances of Class A common stock |  |  | 11,500,000 us-gaap_StockIssuedDuringPeriodSharesNewIssues / us-gaap_StatementClassOfStockAxis = us-gaap_CommonClassAMember |  |  |
Share price |  |  | $ 4.00 us-gaap_SharePrice / us-gaap_StatementClassOfStockAxis = us-gaap_CommonClassAMember |  |  |
Net proceeds from the registered public offering |  |  | $ 42,184,000 us-gaap_ProceedsFromIssuanceOrSaleOfEquity / us-gaap_StatementClassOfStockAxis = us-gaap_CommonClassAMember |  |  |
Common stock, issued shares | 1,200,000 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAuthorized / us-gaap_StatementClassOfStockAxis = us-gaap_CommonClassAMember |  |  |  |  |
Number of shares available for future grant | 376,000 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant / us-gaap_StatementClassOfStockAxis = us-gaap_CommonClassAMember |  |  |  |  |
Class A Common Stock [Member] | 1997 Non-Employee Director Stock Option Plan [Member] |  |  |  |  |  |
Limited Partners' Capital Account [Line Items] |  |  |  |  |  |
Common stock, issued shares | 200,000 cwst_MaximumPotentialSharesIssuedUnderPlan / us-gaap_PlanNameAxis = cwst_NonEmployeeDirectorsStockOptionPlanMember / us-gaap_StatementClassOfStockAxis = us-gaap_CommonClassAMember |  |  |  |  |
Class A Common Stock [Member] | 2006 Stock Incentive Plan [Member] |  |  |  |  |  |
Limited Partners' Capital Account [Line Items] |  |  |  |  |  |
Common stock, issued shares |  |  |  |  |  | 2,475,000 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAuthorized / us-gaap_PlanNameAxis = cwst_TwoThousandSixStockIncentivePlanMember / us-gaap_StatementClassOfStockAxis = us-gaap_CommonClassAMember
Number of shares available for future grant | 1,010,000 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant / us-gaap_PlanNameAxis = cwst_TwoThousandSixStockIncentivePlanMember / us-gaap_StatementClassOfStockAxis = us-gaap_CommonClassAMember |  |  |  |  |

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- Definition The difference between the maximum number of shares (or other type of equity) authorized for issuance under the plan (including the effects of amendments and adjustments), and the sum of: 1) the number of shares (or other type of equity) already issued upon exercise of options or other equity-based awards under the plan; and 2) shares (or other type of equity) reserved for issuance on granting of outstanding awards, net of cancellations and forfeitures, if applicable. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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