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Summary of Significant Accounting Policies - Self-Insurance Liabilities and Related Costs - Additional Information (Detail) (USD $) In Thousands, unless otherwise specified | 8 Months Ended |  |
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Dec. 31, 2014 | Apr. 30, 2014 | Apr. 30, 2013
Summary Of Significant Accounting Policies [Line Items] |  |  |
Maximum exposure workers' compensation plan per individual event | $ 1,000 cwst_WorkersCompensationBenefits |  |
Self insurance reserves | 10,863 us-gaap_SelfInsuranceReserve | 10,280 us-gaap_SelfInsuranceReserve | 11,362 us-gaap_SelfInsuranceReserve
Automobile Plan [Member] |  |  |
Summary Of Significant Accounting Policies [Line Items] |  |  |
Maximum exposure workers' compensation plan per individual event | $ 1,000 cwst_WorkersCompensationBenefits / us-gaap_MajorPropertyClassAxis = us-gaap_AutomobilesMember |  |

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- Definition Maximum exposure per individual event. + References No definition available. + Details
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- Definition Carrying amount (including both current and noncurrent portions) of accrued known and estimated losses incurred as of the balance sheet date for which no insurance coverage exists, and for which a claim has been made or is probable of being asserted, typically arising from workmen's compensation-type of incidents and personal injury to nonemployees from accidents on the entity's property. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.24) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
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