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Summary of Significant Accounting Policies - Investments in Unconsolidated Entities - Additional Information (Detail) (USD $) In Thousands, unless otherwise specified	8 Months Ended	12 Months Ended			0 Months Ended		8 Months Ended
	Dec. 31, 2013	Apr. 30, 2014	Apr. 30, 2013	Apr. 30, 2012	Dec. 31, 2013	Dec. 05, 2013	Dec. 31, 2014
Summary Of Significant Accounting Policies [Line Items]
Net cash proceeds		$ 3,442us-gaap_ProceedsFromSaleOfEquityMethodInvestments
Gain on sale of equity method investment	815us-gaap_EquityMethodInvestmentRealizedGainLossOnDisposal	593us-gaap_EquityMethodInvestmentRealizedGainLossOnDisposal
Gain through loss from equity method investments	(1,027)us-gaap_IncomeLossFromEquityMethodInvestments	(936)us-gaap_IncomeLossFromEquityMethodInvestments	(4,441)us-gaap_IncomeLossFromEquityMethodInvestments	(9,994)us-gaap_IncomeLossFromEquityMethodInvestments
Tompkins County Recycling LLC [Member]
Summary Of Significant Accounting Policies [Line Items]
Equity method investment membership interest	50.00%us-gaap_EquityMethodInvestmentOwnershipPercentage / us-gaap_InvestmentTypeAxis = cwst_TompkinsCountyRecyclingLlcMember				50.00%us-gaap_EquityMethodInvestmentOwnershipPercentage / us-gaap_InvestmentTypeAxis = cwst_TompkinsCountyRecyclingLlcMember
Equity contribution					425us-gaap_PaymentsToAcquireEquityMethodInvestments / us-gaap_InvestmentTypeAxis = cwst_TompkinsCountyRecyclingLlcMember
Price to purchase acquired assets prior to the acquisition date		300us-gaap_BusinessCombinationStepAcquisitionEquityInterestInAcquireeFairValue1 / us-gaap_InvestmentTypeAxis = cwst_TompkinsCountyRecyclingLlcMember
Gain through loss from equity method investments		106us-gaap_IncomeLossFromEquityMethodInvestments / us-gaap_InvestmentTypeAxis = cwst_TompkinsCountyRecyclingLlcMember
GreenFiber [Member]
Summary Of Significant Accounting Policies [Line Items]
Sale of equity method investment						18,000cwst_EquityMethodInvestmentAgreedToConsideration / us-gaap_InvestmentTypeAxis = cwst_GreenFiberMember
Gross cash proceeds						19,194cwst_EquityMethodInvestmentFinalConsideration / us-gaap_InvestmentTypeAxis = cwst_GreenFiberMember
Adjustment of working capital						1,194cwst_EquityMethodInvestmentWorkingCapitalAdjustmentConsideration / us-gaap_InvestmentTypeAxis = cwst_GreenFiberMember
Net cash proceeds							3,442us-gaap_ProceedsFromSaleOfEquityMethodInvestments / us-gaap_InvestmentTypeAxis = cwst_GreenFiberMember
Unrealized losses relating to derivative instruments							593us-gaap_OtherComprehensiveIncomeUnrealizedGainLossOnDerivativesArisingDuringPeriodNetOfTax / us-gaap_InvestmentTypeAxis = cwst_GreenFiberMember
Gain on sale of equity method investment							$ 593us-gaap_EquityMethodInvestmentRealizedGainLossOnDisposal / us-gaap_InvestmentTypeAxis = cwst_GreenFiberMember
Equity method investment membership interest						50.00%us-gaap_EquityMethodInvestmentOwnershipPercentage / us-gaap_InvestmentTypeAxis = cwst_GreenFiberMember

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Fair value at acquisition-date of the equity interest in the acquiree held by the acquirer, immediately before the acquisition date for businesses combined in stages.

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The percentage of ownership of common stock or equity participation in the investee accounted for under the equity method of accounting.

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Amount after tax of increase (decrease) in accumulated gain (loss) from derivative instruments designated and qualifying as the effective portion of cash flow hedges and an entity's share of an equity investee's increase (decrease) in deferred hedging gain (loss).

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The cash outflow associated with the purchase of or advances to an equity method investments, which are investments in joint ventures and entities in which the entity has an equity ownership interest normally of 20 to 50 percent and exercises significant influence.

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