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Stockholders' (Deficit) Equity - Summary of Grant Activity for Other Stock Awards (Detail) (USD $) In Thousands, except Per Share data, unless otherwise specified | 8 Months Ended | 12 Months Ended
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Dec. 31, 2014 | Apr. 30, 2014 | Apr. 30, 2013 | Apr. 30, 2012
Schedule Of Stock Option Activity [Line Items] |  |  |  |
Stock Awards, Granted | 370 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriod | 542 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriod | 735 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriod | 611 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriod
Stock Awards, Weighted Average Grant Date Fair Value | $ 4.91 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue |  |  |
Restricted Stock Units [Member] |  |  |  |
Schedule Of Stock Option Activity [Line Items] |  |  |  |
Stock Awards, Granted | 277 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriod / us-gaap_AwardTypeAxis = us-gaap_RestrictedStockUnitsRSUMember | 482 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriod / us-gaap_AwardTypeAxis = us-gaap_RestrictedStockUnitsRSUMember | 340 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriod / us-gaap_AwardTypeAxis = us-gaap_RestrictedStockUnitsRSUMember | 305 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriod / us-gaap_AwardTypeAxis = us-gaap_RestrictedStockUnitsRSUMember
Stock Awards, Weighted Average Grant Date Fair Value | $ 5.28 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue / us-gaap_AwardTypeAxis = us-gaap_RestrictedStockUnitsRSUMember | 4.09 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue / us-gaap_AwardTypeAxis = us-gaap_RestrictedStockUnitsRSUMember | 5.15 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue / us-gaap_AwardTypeAxis = us-gaap_RestrictedStockUnitsRSUMember | 6.12 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue / us-gaap_AwardTypeAxis = us-gaap_RestrictedStockUnitsRSUMember
Performance Stock Units [Member] |  |  |  |
Schedule Of Stock Option Activity [Line Items] |  |  |  |
Stock Awards, Weighted Average Grant Date Fair Value |  |  | 5.17 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue / us-gaap_AwardTypeAxis = cwst_PerformanceStockUnitMember | 6.06 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue / us-gaap_AwardTypeAxis = cwst_PerformanceStockUnitMember
Stock Awards, Granted |  |  | 316 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod / us-gaap_AwardTypeAxis = cwst_PerformanceStockUnitMember | 255 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod / us-gaap_AwardTypeAxis = cwst_PerformanceStockUnitMember
Restricted Stock Awards [Member] |  |  |  |
Schedule Of Stock Option Activity [Line Items] |  |  |  |
Stock Awards, Granted | 93 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriod / us-gaap_AwardTypeAxis = cwst_RestrictedStockAwardsMember | 60 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriod / us-gaap_AwardTypeAxis = cwst_RestrictedStockAwardsMember | 79 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriod / us-gaap_AwardTypeAxis = cwst_RestrictedStockAwardsMember | 51 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriod / us-gaap_AwardTypeAxis = cwst_RestrictedStockAwardsMember
Stock Awards, Weighted Average Grant Date Fair Value | $ 3.78 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue / us-gaap_AwardTypeAxis = cwst_RestrictedStockAwardsMember | 5.81 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue / us-gaap_AwardTypeAxis = cwst_RestrictedStockAwardsMember | 4.45 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue / us-gaap_AwardTypeAxis = cwst_RestrictedStockAwardsMember | 5.83 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue / us-gaap_AwardTypeAxis = cwst_RestrictedStockAwardsMember
2013 Grants [Member] |  |  |  |
Schedule Of Stock Option Activity [Line Items] |  |  |  |
Stock Awards, Unvested | 360 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber / us-gaap_PlanNameAxis = cwst_TwoThousandThirteenGrantsMember |  |  |
2013 Grants [Member] | Restricted Stock Units [Member] |  |  |  |
Schedule Of Stock Option Activity [Line Items] |  |  |  |
Stock Awards, Unvested | 84 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber / us-gaap_AwardTypeAxis = us-gaap_RestrictedStockUnitsRSUMember / us-gaap_PlanNameAxis = cwst_TwoThousandThirteenGrantsMember |  |  |
2013 Grants [Member] | Performance Stock Units [Member] |  |  |  |
Schedule Of Stock Option Activity [Line Items] |  |  |  |
Stock Awards, Unvested | 250 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber / us-gaap_AwardTypeAxis = cwst_PerformanceStockUnitMember / us-gaap_PlanNameAxis = cwst_TwoThousandThirteenGrantsMember |  |  |
2013 Grants [Member] | Restricted Stock Awards [Member] |  |  |  |
Schedule Of Stock Option Activity [Line Items] |  |  |  |
Stock Awards, Unvested | 26 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber / us-gaap_AwardTypeAxis = cwst_RestrictedStockAwardsMember / us-gaap_PlanNameAxis = cwst_TwoThousandThirteenGrantsMember |  |  |
2014 Grants [Member] |  |  |  |
Schedule Of Stock Option Activity [Line Items] |  |  |  |
Stock Awards, Unvested | 324 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber / us-gaap_PlanNameAxis = cwst_TwoThousandFourteenGrantsMember |  |  |
2014 Grants [Member] | Restricted Stock Units [Member] |  |  |  |
Schedule Of Stock Option Activity [Line Items] |  |  |  |
Stock Awards, Unvested | 284 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber / us-gaap_AwardTypeAxis = us-gaap_RestrictedStockUnitsRSUMember / us-gaap_PlanNameAxis = cwst_TwoThousandFourteenGrantsMember |  |  |
2014 Grants [Member] | Restricted Stock Awards [Member] |  |  |  |
Schedule Of Stock Option Activity [Line Items] |  |  |  |
Stock Awards, Unvested | 40 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber / us-gaap_AwardTypeAxis = cwst_RestrictedStockAwardsMember / us-gaap_PlanNameAxis = cwst_TwoThousandFourteenGrantsMember |  |  |
Transition Period 2014 Grands [Member] |  |  |  |
Schedule Of Stock Option Activity [Line Items] |  |  |  |
Stock Awards, Unvested | 364 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber / us-gaap_PlanNameAxis = cwst_TransitionPeriodTwoThousandFourteenGrantsMember |  |  |
Transition Period 2014 Grands [Member] | Restricted Stock Units [Member] |  |  |  |
Schedule Of Stock Option Activity [Line Items] |  |  |  |
Stock Awards, Unvested | 271 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber / us-gaap_AwardTypeAxis = us-gaap_RestrictedStockUnitsRSUMember / us-gaap_PlanNameAxis = cwst_TransitionPeriodTwoThousandFourteenGrantsMember |  |  |
Transition Period 2014 Grands [Member] | Restricted Stock Awards [Member] |  |  |  |
Schedule Of Stock Option Activity [Line Items] |  |  |  |
Stock Awards, Unvested | 93 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber / us-gaap_AwardTypeAxis = cwst_RestrictedStockAwardsMember / us-gaap_PlanNameAxis = cwst_TransitionPeriodTwoThousandFourteenGrantsMember |  |  |

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- Definition The number of grants made during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan). + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(2)(iii)(1) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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- Definition The weighted average fair value at grant date for nonvested equity-based awards issued during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan). + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(2)(iii)(1) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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- Definition The number of non-vested equity-based payment instruments, excluding stock (or unit) options, that validly exist and are outstanding as of the balance sheet date. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(2)(i)-(ii) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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- Definition Net number of share options (or share units) granted during the period. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(1)(iv)(1) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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