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Summary of Significant Accounting Policies - Derivatives and Hedging - Additional Information (Detail) (USD $) In Thousands, unless otherwise specified | 8 Months Ended | 12 Months Ended | 1 Months Ended |
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Dec. 31, 2014 Agreement | Dec. 31, 2013 | Apr. 30, 2014 | Apr. 30, 2013 | Feb. 28, 2015 | Jan. 15, 2013
Summary Of Significant Accounting Policies [Line Items] |  |  |  |  |  |
Number of interest rate derivative agreements | 2 us-gaap_DerivativeNumberOfInstrumentsHeld |  |  |  |  |
Derivative liability notional amount | $ 150,000 us-gaap_DerivativeLiabilityNotionalAmount |  |  | $ 75,000 us-gaap_DerivativeLiabilityNotionalAmount |  |
LIBOR base rate | 1.40%us-gaap_DerivativeFixedInterestRate |  |  |  |  |
Derivative Instrument maturity term | Mar. 15, 2016 |  |  |  |  |
Loss on derivative instruments | (225)us-gaap_DerivativeInstrumentsNotDesignatedAsHedgingInstrumentsGainLossNet | 69 us-gaap_DerivativeInstrumentsNotDesignatedAsHedgingInstrumentsGainLossNet | (280)us-gaap_DerivativeInstrumentsNotDesignatedAsHedgingInstrumentsGainLossNet | (4,512)us-gaap_DerivativeInstrumentsNotDesignatedAsHedgingInstrumentsGainLossNet |  |
Interest Rate Swap [Member] |  |  |  |  |  |
Summary Of Significant Accounting Policies [Line Items] |  |  |  |  |  |
Obligation recorded in other accrued liabilities and other long-term liabilities | 1,668 us-gaap_DerivativeInstrumentsNotDesignatedAsHedgingInstrumentsLiabilityAtFairValue / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_InterestRateSwapMember |  |  |  |  |
Reclassification Out of Accumulated Other Comprehensive Income (Loss) [Member] | Interest Rate Swap [Member] |  |  |  |  |  |
Summary Of Significant Accounting Policies [Line Items] |  |  |  |  |  |
Loss on derivative instruments |  |  |  | 3,626 us-gaap_DerivativeInstrumentsNotDesignatedAsHedgingInstrumentsGainLossNet / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_InterestRateSwapMember / us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis = us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeMember |  |
Subsequent Event [Member] |  |  |  |  |  |
Summary Of Significant Accounting Policies [Line Items] |  |  |  |  |  |
Settlement of forward starting interest rate derivative agreements |  |  |  |  | $ 830 cwst_DerivativeSettlementOfForwardStartingInterestRateSwapAgreement / us-gaap_SubsequentEventTypeAxis = us-gaap_SubsequentEventMember |
Second Lien Notes [Member] |  |  |  |  |  |
Summary Of Significant Accounting Policies [Line Items] |  |  |  |  |  |
Derivative instrument interest rate |  |  |  |  |  | 11.00%us-gaap_DebtInstrumentInterestRateStatedPercentage / us-gaap_FinancialInstrumentAxis = cwst_SecondLienNotesMember

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- Definition Cash settlement of interest rate swap. + References No definition available. + Details
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- Definition Contractual interest rate for funds borrowed, under the debt agreement. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 22 -Article 5 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.22(a)(1)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
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- Definition Fixed interest rate related to the interest rate derivative. + References No definition available. + Details
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- Definition Amount of realized and unrealized gain (loss) of derivative instruments not designated or qualifying as hedging instruments. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 4C -Subparagraph (e) -URI http://asc.fasb.org/extlink&oid=35706170&loc=SL5624171-113959 + Details
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- Definition Fair value as of the balance sheet date of derivative instrument not designated as hedging instrument and classified as a liability. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 4B -Subparagraph (c)(2) -URI http://asc.fasb.org/extlink&oid=35706170&loc=SL5624163-113959 + Details
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- Definition Nominal or face amount used to calculate payments on the derivative liability. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 1B -URI http://asc.fasb.org/extlink&oid=35706170&loc=SL5580258-113959 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 1A -Subparagraph (d) -URI http://asc.fasb.org/extlink&oid=35706170&loc=SL5579245-113959 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Notional Amount -URI http://asc.fasb.org/extlink&oid=6519104 + Details
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