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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
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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 |  | $ 3,442 us-gaap_ProceedsFromSaleOfEquityMethodInvestments |  |  |  |  |
Gain on sale of equity method investment | 815 us-gaap_EquityMethodInvestmentRealizedGainLossOnDisposal | 593 us-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 |  |  |  |  | 425 us-gaap_PaymentsToAcquireEquityMethodInvestments / us-gaap_InvestmentTypeAxis = cwst_TompkinsCountyRecyclingLlcMember |  |
Price to purchase acquired assets prior to the acquisition date |  | 300 us-gaap_BusinessCombinationStepAcquisitionEquityInterestInAcquireeFairValue1 / us-gaap_InvestmentTypeAxis = cwst_TompkinsCountyRecyclingLlcMember |  |  |  |  |
Gain through loss from equity method investments |  | 106 us-gaap_IncomeLossFromEquityMethodInvestments / us-gaap_InvestmentTypeAxis = cwst_TompkinsCountyRecyclingLlcMember |  |  |  |  |
GreenFiber [Member] |  |  |  |  |  |  |
Summary Of Significant Accounting Policies [Line Items] |  |  |  |  |  |  |
Sale of equity method investment |  |  |  |  |  | 18,000 cwst_EquityMethodInvestmentAgreedToConsideration / us-gaap_InvestmentTypeAxis = cwst_GreenFiberMember |
Gross cash proceeds |  |  |  |  |  | 19,194 cwst_EquityMethodInvestmentFinalConsideration / us-gaap_InvestmentTypeAxis = cwst_GreenFiberMember |
Adjustment of working capital |  |  |  |  |  | 1,194 cwst_EquityMethodInvestmentWorkingCapitalAdjustmentConsideration / us-gaap_InvestmentTypeAxis = cwst_GreenFiberMember |
Net cash proceeds |  |  |  |  |  |  | 3,442 us-gaap_ProceedsFromSaleOfEquityMethodInvestments / us-gaap_InvestmentTypeAxis = cwst_GreenFiberMember
Unrealized losses relating to derivative instruments |  |  |  |  |  |  | 593 us-gaap_OtherComprehensiveIncomeUnrealizedGainLossOnDerivativesArisingDuringPeriodNetOfTax / us-gaap_InvestmentTypeAxis = cwst_GreenFiberMember
Gain on sale of equity method investment |  |  |  |  |  |  | $ 593 us-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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- Definition 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. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 805 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (g)(1) -URI http://asc.fasb.org/extlink&oid=25497992&loc=d3e1392-128463 + Details
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- Definition 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. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 13 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3213-108585 + Details
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