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v2.4.1.9

Property, Plant and Equipment - Additional Information (Detail) (USD $) In Thousands, unless otherwise specified | 8 Months Ended | 12 Months Ended
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Dec. 31, 2014 | Apr. 30, 2014 | Apr. 30, 2013 | Apr. 30, 2012
Property, Plant and Equipment [Line Items] |  |  |  |
Depreciation expense | $ 21,599 us-gaap_Depreciation | $ 33,094 us-gaap_Depreciation | $ 34,065 us-gaap_Depreciation | $ 37,829 us-gaap_Depreciation
Depletion expense on operating lease contracts | 7,799 cwst_DepletionOfLandfillOperatingLeaseObligations | 9,948 cwst_DepletionOfLandfillOperatingLeaseObligations | 9,372 cwst_DepletionOfLandfillOperatingLeaseObligations | 8,482 cwst_DepletionOfLandfillOperatingLeaseObligations
Landfills [Member] |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |
Amortization expense | 17,912 us-gaap_AdjustmentForAmortization / us-gaap_PropertyPlantAndEquipmentByTypeAxis = us-gaap_LandfillMember | 24,689 us-gaap_AdjustmentForAmortization / us-gaap_PropertyPlantAndEquipmentByTypeAxis = us-gaap_LandfillMember | 21,206 us-gaap_AdjustmentForAmortization / us-gaap_PropertyPlantAndEquipmentByTypeAxis = us-gaap_LandfillMember | 19,957 us-gaap_AdjustmentForAmortization / us-gaap_PropertyPlantAndEquipmentByTypeAxis = us-gaap_LandfillMember
Depletion expense on operating lease contracts | $ 7,799 cwst_DepletionOfLandfillOperatingLeaseObligations / us-gaap_PropertyPlantAndEquipmentByTypeAxis = us-gaap_LandfillMember | $ 9,948 cwst_DepletionOfLandfillOperatingLeaseObligations / us-gaap_PropertyPlantAndEquipmentByTypeAxis = us-gaap_LandfillMember | $ 9,372 cwst_DepletionOfLandfillOperatingLeaseObligations / us-gaap_PropertyPlantAndEquipmentByTypeAxis = us-gaap_LandfillMember | $ 8,482 cwst_DepletionOfLandfillOperatingLeaseObligations / us-gaap_PropertyPlantAndEquipmentByTypeAxis = us-gaap_LandfillMember

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- Definition This element represents the noncash expense charged against earnings to recognize the depletion of landfill operating lease obligation. + References No definition available. + Details
Name: | cwst_DepletionOfLandfillOperatingLeaseObligations
Namespace Prefix: | cwst_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Definition The aggregate amount of recurring noncash expense charged against earnings in the period to allocate the cost of assets over their estimated remaining economic lives. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 28 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3602-108585 + Details
Name: | us-gaap_AdjustmentForAmortization
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Definition The amount of expense recognized in the current period that reflects the allocation of the cost of tangible assets over the assets' useful lives. Includes production and non-production related depreciation. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 28 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3602-108585 + Details
Name: | us-gaap_Depreciation
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Details
Name: | us-gaap_PropertyPlantAndEquipmentLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Details
Name: | us-gaap_PropertyPlantAndEquipmentByTypeAxis=us-gaap_LandfillMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |