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Fair Value of Financial Instruments - Additional Information (Detail) (USD $)
In Thousands, unless otherwise specified
Mar. 31, 2015
Debt Instrument [Line Items]  
Carrying value of revolver debt $ 83,120us-gaap_LinesOfCreditFairValueDisclosure
Fixed Rate Bonds [Member]  
Debt Instrument [Line Items]  
Fair value of fixed rate debt 460,322us-gaap_DebtInstrumentFairValue
/ us-gaap_LongtermDebtTypeAxis
= cwst_FixedRateBondsMember
Carrying value of fixed rate debt $ 458,400us-gaap_DebtInstrumentCarryingAmount
/ us-gaap_LongtermDebtTypeAxis
= cwst_FixedRateBondsMember