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Income Taxes - Schedule of Difference in Provision (Benefit) for Income Taxes (Detail) (USD $) In Thousands, 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
Income Loss From Operations Before Provision Benefit For Income Taxes [Line Items] |  |  |  |  |
Federal statutory rate | 35.00%us-gaap_EffectiveIncomeTaxRateReconciliationAtFederalStatutoryIncomeTaxRate |  | 35.00%us-gaap_EffectiveIncomeTaxRateReconciliationAtFederalStatutoryIncomeTaxRate | 35.00%us-gaap_EffectiveIncomeTaxRateReconciliationAtFederalStatutoryIncomeTaxRate | 35.00%us-gaap_EffectiveIncomeTaxRateReconciliationAtFederalStatutoryIncomeTaxRate
Tax at statutory rate | $ (1,787)us-gaap_IncomeTaxReconciliationIncomeTaxExpenseBenefitAtFederalStatutoryIncomeTaxRate |  | $ (8,929)us-gaap_IncomeTaxReconciliationIncomeTaxExpenseBenefitAtFederalStatutoryIncomeTaxRate | $ (18,378)us-gaap_IncomeTaxReconciliationIncomeTaxExpenseBenefitAtFederalStatutoryIncomeTaxRate | $ (26,638)us-gaap_IncomeTaxReconciliationIncomeTaxExpenseBenefitAtFederalStatutoryIncomeTaxRate
State income taxes, net of federal benefit | (59)us-gaap_IncomeTaxReconciliationStateAndLocalIncomeTaxes |  | (1,271)us-gaap_IncomeTaxReconciliationStateAndLocalIncomeTaxes | (1,076)us-gaap_IncomeTaxReconciliationStateAndLocalIncomeTaxes | (3,050)us-gaap_IncomeTaxReconciliationStateAndLocalIncomeTaxes
Non-deductible GreenFiber goodwill impairment and equity income in subsidiaries | (73)us-gaap_IncomeTaxReconciliationNondeductibleExpenseOther |  | 1,548 us-gaap_IncomeTaxReconciliationNondeductibleExpenseOther | 180 us-gaap_IncomeTaxReconciliationNondeductibleExpenseOther | 1,182 us-gaap_IncomeTaxReconciliationNondeductibleExpenseOther
Tax over book basis in GreenFiber on sale |  |  | (2,570)us-gaap_IncomeTaxReconciliationDispositionOfBusiness |  |
Non-deductible impairment of investment in GreenFiber |  |  |  |  | 3,738 us-gaap_IncomeTaxReconciliationNondeductibleExpenseImpairmentLosses
Non-deductible expenses | 505 us-gaap_IncomeTaxReconciliationNondeductibleExpense |  | 505 us-gaap_IncomeTaxReconciliationNondeductibleExpense | 494 us-gaap_IncomeTaxReconciliationNondeductibleExpense | 823 us-gaap_IncomeTaxReconciliationNondeductibleExpense
Tax credits | (380)us-gaap_IncomeTaxReconciliationTaxCredits |  | (598)us-gaap_IncomeTaxReconciliationTaxCredits | (660)us-gaap_IncomeTaxReconciliationTaxCredits | (650)us-gaap_IncomeTaxReconciliationTaxCredits
Non-deductible stock option charges |  |  |  |  | 73 us-gaap_IncomeTaxReconciliationNondeductibleExpenseShareBasedCompensationCost
Other, net | (35)us-gaap_IncomeTaxReconciliationOtherReconcilingItems |  | (491)us-gaap_IncomeTaxReconciliationOtherReconcilingItems | (512)us-gaap_IncomeTaxReconciliationOtherReconcilingItems | (1,132)us-gaap_IncomeTaxReconciliationOtherReconcilingItems
Income before provision (benefit) for income tax, net | 703 us-gaap_IncomeTaxExpenseBenefit | 1,162 us-gaap_IncomeTaxExpenseBenefit | 1,799 us-gaap_IncomeTaxExpenseBenefit | (2,526)us-gaap_IncomeTaxExpenseBenefit | 1,593 us-gaap_IncomeTaxExpenseBenefit
Other Valuation Allowance [Member] |  |  |  |  |
Income Loss From Operations Before Provision Benefit For Income Taxes [Line Items] |  |  |  |  |
Increase (decrease) in valuation allowance | 2,532 us-gaap_IncomeTaxReconciliationChangeInDeferredTaxAssetsValuationAllowance / us-gaap_BusinessAcquisitionAxis = cwst_OtherValuationAllowanceMember |  | 13,605 us-gaap_IncomeTaxReconciliationChangeInDeferredTaxAssetsValuationAllowance / us-gaap_BusinessAcquisitionAxis = cwst_OtherValuationAllowanceMember | 22,510 us-gaap_IncomeTaxReconciliationChangeInDeferredTaxAssetsValuationAllowance / us-gaap_BusinessAcquisitionAxis = cwst_OtherValuationAllowanceMember | 27,247 us-gaap_IncomeTaxReconciliationChangeInDeferredTaxAssetsValuationAllowance / us-gaap_BusinessAcquisitionAxis = cwst_OtherValuationAllowanceMember
Business Acquisition [Member] |  |  |  |  |
Income Loss From Operations Before Provision Benefit For Income Taxes [Line Items] |  |  |  |  |
Increase (decrease) in valuation allowance |  |  | $ 5,084 us-gaap_IncomeTaxReconciliationChangeInDeferredTaxAssetsValuationAllowance / us-gaap_BusinessAcquisitionAxis = cwst_BusinessAcquisitionMember | $ (5,084)us-gaap_IncomeTaxReconciliationChangeInDeferredTaxAssetsValuationAllowance / us-gaap_BusinessAcquisitionAxis = cwst_BusinessAcquisitionMember |

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- Definition Percentage of domestic federal statutory tax rate applicable to pretax income (loss). + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 12 -URI http://asc.fasb.org/extlink&oid=6907707&loc=d3e32687-109319 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SAB TOPIC 6.I) -URI http://asc.fasb.org/extlink&oid=34349781&loc=d3e330036-122817 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 13 -URI http://asc.fasb.org/extlink&oid=6907707&loc=d3e32698-109319 + Details
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- Definition Amount of current income tax expense (benefit) and deferred income tax expense (benefit) pertaining to continuing operations. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 235 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.4-08.(h)) -URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e23780-122690 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 08 -Paragraph h -Article 4 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Income Tax Expense (or Benefit) -URI http://asc.fasb.org/extlink&oid=6515339 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 9 -Subparagraph (a),(b) -URI http://asc.fasb.org/extlink&oid=6907707&loc=d3e32639-109319 + Details
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- Definition Amount of the difference between reported income tax expense (benefit) and expected income tax expense (benefit) computed by applying the domestic federal statutory income tax rates to pretax income (loss) from continuing operations attributable to the disposition of a business not qualifying as a discontinued operation. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 12 -URI http://asc.fasb.org/extlink&oid=6907707&loc=d3e32687-109319 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 13 -URI http://asc.fasb.org/extlink&oid=6907707&loc=d3e32698-109319 + Details
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- Definition The amount of income tax expense or benefit for the period computed by applying the domestic federal statutory tax rates to pretax income from continuing operations. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 12 -URI http://asc.fasb.org/extlink&oid=6907707&loc=d3e32687-109319 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 13 -URI http://asc.fasb.org/extlink&oid=6907707&loc=d3e32698-109319 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 08 -Paragraph h -Subparagraph 2 -Article 4 + Details
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- Definition Amount of the difference between reported income tax expense (benefit) and expected income tax expense (benefit) computed by applying the domestic federal statutory income tax rates to pretax income (loss) from continuing operations attributable to other nondeductible expenses. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 12 -URI http://asc.fasb.org/extlink&oid=6907707&loc=d3e32687-109319 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 13 -URI http://asc.fasb.org/extlink&oid=6907707&loc=d3e32698-109319 + Details
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- Definition Amount of the difference between reported income tax expense (benefit) and expected income tax expense (benefit) computed by applying the domestic federal statutory income tax rates to pretax income (loss) from continuing operations attributable to tax credits. Including, but not limited to, research credit, foreign tax credit, investment tax credit, and other tax credits. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 12 -URI http://asc.fasb.org/extlink&oid=6907707&loc=d3e32687-109319 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 13 -URI http://asc.fasb.org/extlink&oid=6907707&loc=d3e32698-109319 + Details
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