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Property, Plant and Equipment - Additional Information (Detail) (USD $) In Thousands, unless otherwise specified | 12 Months Ended
--- | ---
Apr. 30, 2014 | Apr. 30, 2013 | Apr. 30, 2012
Property, Plant and Equipment [Line Items] |  |  |
Depreciation expense | $ 33,094 | $ 34,065 | $ 37,829
Depletion expense on operating lease contracts | 9,948 | 9,372 | 8,482
Landfills [Member] |  |  |
Property, Plant and Equipment [Line Items] |  |  |
Amortization expense | 24,689 | 21,206 | 19,957
Depletion expense on operating lease contracts | $ 9,948 | $ 9,372 | $ 8,482

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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