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Summary of Significant Accounting Policies - Landfill Development Cost- Additional Information (Detail) (USD $) In Thousands, unless otherwise specified | 8 Months Ended | 12 Months Ended
--- | --- | ---
Dec. 31, 2014 | Apr. 30, 2014 | Apr. 30, 2013 | Apr. 30, 2012
Summary Of Significant Accounting Policies [Line Items] |  |  |  |
Capitalized interest | $ 333 us-gaap_InterestCostsCapitalized | $ 256 us-gaap_InterestCostsCapitalized | $ 368 us-gaap_InterestCostsCapitalized | $ 407 us-gaap_InterestCostsCapitalized
Estimate of inflation rate | 2.30%cwst_InflationRate |  |  |
Accretion expense | 2,275 us-gaap_AssetRetirementObligationAccretionExpense | 3,967 us-gaap_AssetRetirementObligationAccretionExpense | 3,538 us-gaap_AssetRetirementObligationAccretionExpense |
Landfills [Member] |  |  |  |
Summary Of Significant Accounting Policies [Line Items] |  |  |  |
Accretion expense | 2,275 us-gaap_AssetRetirementObligationAccretionExpense / us-gaap_PropertyPlantAndEquipmentByTypeAxis = us-gaap_LandfillMember | 3,967 us-gaap_AssetRetirementObligationAccretionExpense / us-gaap_PropertyPlantAndEquipmentByTypeAxis = us-gaap_LandfillMember | 3,538 us-gaap_AssetRetirementObligationAccretionExpense / us-gaap_PropertyPlantAndEquipmentByTypeAxis = us-gaap_LandfillMember | 3,341 us-gaap_AssetRetirementObligationAccretionExpense / us-gaap_PropertyPlantAndEquipmentByTypeAxis = us-gaap_LandfillMember
Surety bond | 144,633 cwst_SuretyBonds / us-gaap_PropertyPlantAndEquipmentByTypeAxis = us-gaap_LandfillMember | 133,847 cwst_SuretyBonds / us-gaap_PropertyPlantAndEquipmentByTypeAxis = us-gaap_LandfillMember | 128,551 cwst_SuretyBonds / us-gaap_PropertyPlantAndEquipmentByTypeAxis = us-gaap_LandfillMember |
Letters of credit | $ 1,104 us-gaap_LettersOfCreditOutstandingAmount / us-gaap_PropertyPlantAndEquipmentByTypeAxis = us-gaap_LandfillMember | $ 1,104 us-gaap_LettersOfCreditOutstandingAmount / us-gaap_PropertyPlantAndEquipmentByTypeAxis = us-gaap_LandfillMember | $ 1,752 us-gaap_LettersOfCreditOutstandingAmount / us-gaap_PropertyPlantAndEquipmentByTypeAxis = us-gaap_LandfillMember |
Minimum [Member] |  |  |  |
Summary Of Significant Accounting Policies [Line Items] |  |  |  |
Rate of asset retirement obligation | 9.00%us-gaap_FairValueInputsDiscountRate / us-gaap_RangeAxis = us-gaap_MinimumMember |  |  |
Maximum [Member] |  |  |  |
Summary Of Significant Accounting Policies [Line Items] |  |  |  |
Rate of asset retirement obligation | 9.50%us-gaap_FairValueInputsDiscountRate / us-gaap_RangeAxis = us-gaap_MaximumMember |  |  |

X
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- Definition Amount of surety bonds that are available for the future payment of landfill capping, closure and post-closure obligations. + References No definition available. + Details
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- Definition Amount of interest capitalized during the period. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Allowance for Funds Used during Construction -URI http://asc.fasb.org/extlink&oid=6504829 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 835 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=6450988&loc=d3e26243-108391 + Details
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