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Summary of Significant Accounting Policies - Asset Impairments - Additional Information (Detail) (USD $) | 8 Months Ended | 12 Months Ended
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Dec. 31, 2014 | Apr. 30, 2014 | Apr. 30, 2013 | Apr. 30, 2012
Summary Of Significant Accounting Policies [Line Items] |  |  |  |
Goodwill impairment incurred | $ 0 us-gaap_GoodwillImpairmentLoss | $ 0 us-gaap_GoodwillImpairmentLoss | $ 0 us-gaap_GoodwillImpairmentLoss | $ 0 us-gaap_GoodwillImpairmentLoss
Impairment of equity method investment |  |  |  | 10,680,000 us-gaap_EquityMethodInvestmentOtherThanTemporaryImpairment
Investment impairment charge | 2,320,000 us-gaap_ImpairmentOfInvestments |  |  | 10,680,000 us-gaap_ImpairmentOfInvestments
Carrying amount of cost method investment | 14,432,000 us-gaap_CostMethodInvestments | 16,752,000 us-gaap_CostMethodInvestments | 16,486,000 us-gaap_CostMethodInvestments |
Recycle Rewards Inc [Member] |  |  |  |
Summary Of Significant Accounting Policies [Line Items] |  |  |  |
Outstanding common stock percentage | 6.7 |  |  |
Investment impairment charge | 2,320,000 us-gaap_ImpairmentOfInvestments / dei_LegalEntityAxis = cwst_RecycleRewardsIncMember |  |  |
Carrying amount of cost method investment | 2,160,000 us-gaap_CostMethodInvestments / dei_LegalEntityAxis = cwst_RecycleRewardsIncMember |  |  |
Eastern Region [Member] |  |  |  |
Summary Of Significant Accounting Policies [Line Items] |  |  |  |
Reporting units exceeded their carrying values | 15.00%us-gaap_ReportingUnitPercentageOfFairValueInExcessOfCarryingAmount / us-gaap_StatementBusinessSegmentsAxis = cwst_EasternRegionMember |  |  |
Western Region [Member] |  |  |  |
Summary Of Significant Accounting Policies [Line Items] |  |  |  |
Reporting units exceeded their carrying values | 28.20%us-gaap_ReportingUnitPercentageOfFairValueInExcessOfCarryingAmount / us-gaap_StatementBusinessSegmentsAxis = cwst_WesternRegionMember |  |  |
Recycling [Member] |  |  |  |
Summary Of Significant Accounting Policies [Line Items] |  |  |  |
Reporting units exceeded their carrying values | 20.80%us-gaap_ReportingUnitPercentageOfFairValueInExcessOfCarryingAmount / us-gaap_StatementBusinessSegmentsAxis = cwst_RecyclingMember |  |  |
Other [Member] | Customer Solutions [Member] |  |  |  |
Summary Of Significant Accounting Policies [Line Items] |  |  |  |
Reporting units exceeded their carrying values | 83.30%us-gaap_ReportingUnitPercentageOfFairValueInExcessOfCarryingAmount / us-gaap_StatementBusinessSegmentsAxis = us-gaap_AllOtherSegmentsMember / us-gaap_SubsegmentsAxis = cwst_CustomerSolutionsMember |  |  |
GreenFiber [Member] |  |  |  |
Summary Of Significant Accounting Policies [Line Items] |  |  |  |
Goodwill impairment incurred |  |  |  | 5,090,000 us-gaap_GoodwillImpairmentLoss / us-gaap_InvestmentTypeAxis = cwst_GreenFiberMember
Impairment of equity method investment |  |  |  | $ 10,680,000 us-gaap_EquityMethodInvestmentOtherThanTemporaryImpairment / us-gaap_InvestmentTypeAxis = cwst_GreenFiberMember

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- Definition Amount, after adjustment, of cost-method investment. Adjustments include, but are not limited to, dividends received in excess of earnings after date of investment that are considered a return of investment and other than temporary impairments. + 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 210.5-02.12) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 325 -SubTopic 20 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6872867&loc=d3e40691-111596 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 325 -SubTopic 20 -Section 35 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=29638308&loc=d3e40346-111594 + Details
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- Definition This item represents an other than temporary decline in value that has been recognized against an investment accounted for under the equity method of accounting. The excess of the carrying amount over the fair value of the investment represents the amount of the write down which is or was reflected in earnings. The written down value is a new cost basis with the adjusted value of the investment becoming its new carrying value subject to the equity accounting method. Evidence of a loss in value might include, but would not necessarily be limited to, absence of an ability to recover the carrying amount of the investment or inability of the investee to sustain an earnings capacity which would justify the carrying amount of the investment. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 323 -SubTopic 10 -Section 35 -Paragraph 32 -URI http://asc.fasb.org/extlink&oid=35687916&loc=d3e32787-111569 + Details
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- Definition Amount of loss from the write-down of an asset representing the future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized. + 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 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 350 -SubTopic 20 -Section 50 -Paragraph 2 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=35741047&loc=d3e13854-109267 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 350 -SubTopic 20 -Section 45 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=6388280&loc=d3e13777-109266 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 350 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (e) -URI http://asc.fasb.org/extlink&oid=35741047&loc=d3e13816-109267 + Details
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