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Income Taxes - Schedule of Difference in Provision (Benefit) for Income Taxes (Detail) (USD $) In Thousands, unless otherwise specified	8 Months Ended		12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Apr. 30, 2014	Apr. 30, 2013	Apr. 30, 2012
Income Loss From Operations Before Provision Benefit For Income Taxes [Line Items]
Federal statutory rate	35.00%us-gaap_EffectiveIncomeTaxRateReconciliationAtFederalStatutoryIncomeTaxRate		35.00%us-gaap_EffectiveIncomeTaxRateReconciliationAtFederalStatutoryIncomeTaxRate	35.00%us-gaap_EffectiveIncomeTaxRateReconciliationAtFederalStatutoryIncomeTaxRate	35.00%us-gaap_EffectiveIncomeTaxRateReconciliationAtFederalStatutoryIncomeTaxRate
Tax at statutory rate	$ (1,787)us-gaap_IncomeTaxReconciliationIncomeTaxExpenseBenefitAtFederalStatutoryIncomeTaxRate		$ (8,929)us-gaap_IncomeTaxReconciliationIncomeTaxExpenseBenefitAtFederalStatutoryIncomeTaxRate	$ (18,378)us-gaap_IncomeTaxReconciliationIncomeTaxExpenseBenefitAtFederalStatutoryIncomeTaxRate	$ (26,638)us-gaap_IncomeTaxReconciliationIncomeTaxExpenseBenefitAtFederalStatutoryIncomeTaxRate
State income taxes, net of federal benefit	(59)us-gaap_IncomeTaxReconciliationStateAndLocalIncomeTaxes		(1,271)us-gaap_IncomeTaxReconciliationStateAndLocalIncomeTaxes	(1,076)us-gaap_IncomeTaxReconciliationStateAndLocalIncomeTaxes	(3,050)us-gaap_IncomeTaxReconciliationStateAndLocalIncomeTaxes
Non-deductible GreenFiber goodwill impairment and equity income in subsidiaries	(73)us-gaap_IncomeTaxReconciliationNondeductibleExpenseOther		1,548us-gaap_IncomeTaxReconciliationNondeductibleExpenseOther	180us-gaap_IncomeTaxReconciliationNondeductibleExpenseOther	1,182us-gaap_IncomeTaxReconciliationNondeductibleExpenseOther
Tax over book basis in GreenFiber on sale			(2,570)us-gaap_IncomeTaxReconciliationDispositionOfBusiness
Non-deductible impairment of investment in GreenFiber					3,738us-gaap_IncomeTaxReconciliationNondeductibleExpenseImpairmentLosses
Non-deductible expenses	505us-gaap_IncomeTaxReconciliationNondeductibleExpense		505us-gaap_IncomeTaxReconciliationNondeductibleExpense	494us-gaap_IncomeTaxReconciliationNondeductibleExpense	823us-gaap_IncomeTaxReconciliationNondeductibleExpense
Tax credits	(380)us-gaap_IncomeTaxReconciliationTaxCredits		(598)us-gaap_IncomeTaxReconciliationTaxCredits	(660)us-gaap_IncomeTaxReconciliationTaxCredits	(650)us-gaap_IncomeTaxReconciliationTaxCredits
Non-deductible stock option charges					73us-gaap_IncomeTaxReconciliationNondeductibleExpenseShareBasedCompensationCost
Other, net	(35)us-gaap_IncomeTaxReconciliationOtherReconcilingItems		(491)us-gaap_IncomeTaxReconciliationOtherReconcilingItems	(512)us-gaap_IncomeTaxReconciliationOtherReconcilingItems	(1,132)us-gaap_IncomeTaxReconciliationOtherReconcilingItems
Income before provision (benefit) for income tax, net	703us-gaap_IncomeTaxExpenseBenefit	1,162us-gaap_IncomeTaxExpenseBenefit	1,799us-gaap_IncomeTaxExpenseBenefit	(2,526)us-gaap_IncomeTaxExpenseBenefit	1,593us-gaap_IncomeTaxExpenseBenefit
Other Valuation Allowance [Member]
Income Loss From Operations Before Provision Benefit For Income Taxes [Line Items]
Increase (decrease) in valuation allowance	2,532us-gaap_IncomeTaxReconciliationChangeInDeferredTaxAssetsValuationAllowance / us-gaap_BusinessAcquisitionAxis = cwst_OtherValuationAllowanceMember		13,605us-gaap_IncomeTaxReconciliationChangeInDeferredTaxAssetsValuationAllowance / us-gaap_BusinessAcquisitionAxis = cwst_OtherValuationAllowanceMember	22,510us-gaap_IncomeTaxReconciliationChangeInDeferredTaxAssetsValuationAllowance / us-gaap_BusinessAcquisitionAxis = cwst_OtherValuationAllowanceMember	27,247us-gaap_IncomeTaxReconciliationChangeInDeferredTaxAssetsValuationAllowance / us-gaap_BusinessAcquisitionAxis = cwst_OtherValuationAllowanceMember
Business Acquisition [Member]
Income Loss From Operations Before Provision Benefit For 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

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Percentage of domestic federal statutory tax rate applicable to pretax income (loss).

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Amount of current income tax expense (benefit) and deferred income tax expense (benefit) pertaining to continuing operations.

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Amount of the difference between reported income tax expense (benefit) and expected income tax expense (benefit) computed by applying the domestic federal statutory income tax rates to pretax income (loss) from continuing operations attributable to increase (decrease) in the valuation allowance for deferred tax assets.

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Amount of the difference between reported income tax expense (benefit) and expected income tax expense (benefit) computed by applying the domestic federal statutory income tax rates to pretax income (loss) from continuing operations attributable to the disposition of a business not qualifying as a discontinued operation.

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The amount of income tax expense or benefit for the period computed by applying the domestic federal statutory tax rates to pretax income from continuing operations.

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Amount of the difference between reported income tax expense (benefit) and expected income tax expense (benefit) computed by applying the domestic federal statutory income tax rates to pretax income (loss) from continuing operations attributable to nondeductible expenses.

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Amount of the difference between reported income tax expense (benefit) and expected income tax expense (benefit) computed by applying the domestic federal statutory income tax rates to pretax income (loss) from continuing operations attributable to nondeductible impairment loss.

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Amount of the difference between reported income tax expense (benefit) and expected income tax expense (benefit) computed by applying the domestic federal statutory income tax rates to pretax income (loss) from continuing operations attributable to other nondeductible expenses.

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Amount of the difference between reported income tax expense (benefit) and expected income tax expense (benefit) computed by applying the domestic federal statutory income tax rates to pretax income (loss) from continuing operations attributable to tax credits. Including, but not limited to, research credit, foreign tax credit, investment tax credit, and other tax credits.

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