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Business Combinations - Schedule of Unaudited Pro forma Combined Information (Detail) (USD $) In Thousands, except Per Share data, unless otherwise specified | 8 Months Ended | 12 Months Ended
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Dec. 31, 2014 | Dec. 31, 2013 | Apr. 30, 2014 | Apr. 30, 2013 | Apr. 30, 2012
Business Combinations [Abstract] |  |  |  |  |
Revenue | $ 368,641 us-gaap_BusinessAcquisitionsProFormaRevenue |  | $ 502,304 us-gaap_BusinessAcquisitionsProFormaRevenue | $ 478,630 us-gaap_BusinessAcquisitionsProFormaRevenue |
Operating income | 22,143 cwst_BusinessAcquisitionsProFormaOperatingIncomeLoss |  | 12,770 cwst_BusinessAcquisitionsProFormaOperatingIncomeLoss | 14,820 cwst_BusinessAcquisitionsProFormaOperatingIncomeLoss |
Net loss attributable to common stockholders | $ (6,015)us-gaap_BusinessAcquisitionsProFormaNetIncomeLoss |  | $ (22,775)us-gaap_BusinessAcquisitionsProFormaNetIncomeLoss | $ (53,759)us-gaap_BusinessAcquisitionsProFormaNetIncomeLoss |
Basic net loss per common share attributable to common stockholders | $ (0.15)us-gaap_BusinessAcquisitionProFormaEarningsPerShareBasic |  | $ (0.57)us-gaap_BusinessAcquisitionProFormaEarningsPerShareBasic | $ (1.58)us-gaap_BusinessAcquisitionProFormaEarningsPerShareBasic |
Basic and diluted weighted average shares outstanding | 40,262 us-gaap_WeightedAverageNumberOfShareOutstandingBasicAndDiluted | 39,774 us-gaap_WeightedAverageNumberOfShareOutstandingBasicAndDiluted | 39,820 us-gaap_WeightedAverageNumberOfShareOutstandingBasicAndDiluted | 34,015 us-gaap_WeightedAverageNumberOfShareOutstandingBasicAndDiluted | 26,749 us-gaap_WeightedAverageNumberOfShareOutstandingBasicAndDiluted

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- Definition The pro forma basic net income per share for a period as if the business combination or combinations had been completed at the beginning of a period. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 805 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (h)(2)-(3) -URI http://asc.fasb.org/extlink&oid=25497992&loc=d3e1392-128463 + Details
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- Definition The pro forma net Income or Loss for the period as if the business combination or combinations had been completed at the beginning of a period. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 805 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (h)(2)-(3) -URI http://asc.fasb.org/extlink&oid=25497992&loc=d3e1392-128463 + Details
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- Definition The pro forma revenue for a period as if the business combination or combinations had been completed at the beginning of the period. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 805 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (h)(2)-(3) -URI http://asc.fasb.org/extlink&oid=25497992&loc=d3e1392-128463 + Details
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- Definition Average number of shares or units issued and outstanding that are used in calculating basic and diluted earnings per share (EPS). + References No definition available. + Details
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