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Restricted Cash / Restricted Assets - Additional Information (Detail) (USD $) In Thousands, unless otherwise specified | Dec. 31, 2014 | Apr. 30, 2014 | Apr. 30, 2013
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Restricted Cash and Investments [Line Items] |  |  |
Restricted cash / restricted assets | $ 6,632 us-gaap_RestrictedCashAndInvestmentsNoncurrent | $ 681 us-gaap_RestrictedCashAndInvestmentsNoncurrent | $ 545 us-gaap_RestrictedCashAndInvestmentsNoncurrent
Capital Projects [Member] |  |  |
Restricted Cash and Investments [Line Items] |  |  |
Restricted cash / restricted assets | 5,819 us-gaap_RestrictedCashAndInvestmentsNoncurrent / us-gaap_PropertyPlantAndEquipmentByTypeAxis = cwst_CapitalProjectsMember |  |
New York [Member] | Capital Projects [Member] |  |  |
Restricted Cash and Investments [Line Items] |  |  |
Restricted cash / restricted assets | $ 5,819 us-gaap_RestrictedCashAndInvestmentsNoncurrent / us-gaap_PropertyPlantAndEquipmentByTypeAxis = cwst_CapitalProjectsMember / us-gaap_StatementGeographicalAxis = stpr_NY |  |

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- Details
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Namespace Prefix: | cwst_
Data Type: | xbrli:stringItemType
Balance Type: | na
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- Definition The noncurrent cash, cash equivalents and investments that is restricted as to withdrawal or usage. Restrictions may include legally restricted deposits held as compensating balances against borrowing arrangements, contracts entered into with others, or entity statements of intention with regard to particular deposits classified as long-term; that is not expected to be released from such existing restrictions within one year of the balance sheet date or operating cycle, whichever is longer. Excludes compensating balance arrangements that are not agreements which legally restrict the use of cash amounts shown on the balance sheet. Includes noncurrent cash equivalents and investments that are similarly restricted as to withdrawal, usage or disposal. + References No definition available. + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
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- Details
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Balance Type: |
Period Type: |

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- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |