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Income Taxes - Additional Information (Detail) (USD $) In Thousands, unless otherwise specified | 8 Months Ended | 12 Months Ended |
--- | --- | --- | ---
Dec. 31, 2014 | Apr. 30, 2014 | Apr. 30, 2013 | Jan. 18, 2011
Income Taxes [Line Items] |  |  |  |
Income tax settlement amount |  |  | $ 800 cwst_IncomeTaxSettlement |
Tax paid for income tax settlement |  |  | 430 cwst_IncomeTaxSettlementTaxPaid |
Interest paid for income tax settlement |  |  | 370 cwst_IncomeTaxSettlementInterestPaid |
Aggregate finance asserting liability total |  |  |  | 3,852 cwst_AssertedLiabilityNet
Taxation payable |  |  |  | 2,220 cwst_AssertedTaxLiability
Penalties and interest payable |  |  |  | 1,632 cwst_AssertedTaxLiabilityPenaltiesAndInterest
Potential cumulative liability |  |  | 8.00%cwst_PercentageOfPotentialCumulativeLiability |
Net operating loss carryforwards | 83,132 us-gaap_DeferredTaxAssetsOperatingLossCarryforwardsDomestic |  |  |
Net operating loss carryforwards expiry period | 2024 through 2034 |  |  |
State net operating loss carryforwards expiry period | 100,363 us-gaap_DeferredTaxAssetsOperatingLossCarryforwardsStateAndLocal |  |  |
Net operating loss carryforwards benefit recorded additional paid-in capital | 383 us-gaap_DeferredTaxAssetsOperatingLossCarryforwardsNotSubjectToExpiration |  |  |
Alternative minimum tax credit carryforwards | 3,457 us-gaap_DeferredTaxAssetsTaxCreditCarryforwardsAlternativeMinimumTax | 3,330 us-gaap_DeferredTaxAssetsTaxCreditCarryforwardsAlternativeMinimumTax | 3,330 us-gaap_DeferredTaxAssetsTaxCreditCarryforwardsAlternativeMinimumTax |
General business tax credit carryforwards | 2,921 us-gaap_DeferredTaxAssetsTaxCreditCarryforwardsGeneralBusiness | 2,666 us-gaap_DeferredTaxAssetsTaxCreditCarryforwardsGeneralBusiness | 2,095 us-gaap_DeferredTaxAssetsTaxCreditCarryforwardsGeneralBusiness |
General business tax credit carryforwards expiration date | 2023 through 2034 |  |  |
Valuation allowance | 14,188 us-gaap_ValuationAllowanceDeferredTaxAssetChangeInAmount | 2,581 us-gaap_ValuationAllowanceDeferredTaxAssetChangeInAmount |  |
Settlements |  |  | (430)us-gaap_UnrecognizedTaxBenefitsDecreasesResultingFromSettlementsWithTaxingAuthorities |
Unrecognized tax benefit net of federal benefit on state issue | 0 us-gaap_UnrecognizedTaxBenefitsThatWouldImpactEffectiveTaxRate | 0 us-gaap_UnrecognizedTaxBenefitsThatWouldImpactEffectiveTaxRate | 0 us-gaap_UnrecognizedTaxBenefitsThatWouldImpactEffectiveTaxRate |
Unrecognized tax benefits | 178 us-gaap_SignificantChangeInUnrecognizedTaxBenefitsIsReasonablyPossibleAmountOfUnrecordedBenefit |  |  |
Uncertain tax positions, accrued interest | 143 us-gaap_UnrecognizedTaxBenefitsInterestOnIncomeTaxesAccrued | 116 us-gaap_UnrecognizedTaxBenefitsInterestOnIncomeTaxesAccrued | 76 us-gaap_UnrecognizedTaxBenefitsInterestOnIncomeTaxesAccrued |
Uncertain tax positions, penalties | 8 us-gaap_UnrecognizedTaxBenefitsIncomeTaxPenaltiesAccrued | 8 us-gaap_UnrecognizedTaxBenefitsIncomeTaxPenaltiesAccrued | 9 us-gaap_UnrecognizedTaxBenefitsIncomeTaxPenaltiesAccrued |
Accrued income tax expense | 26 us-gaap_UnrecognizedTaxBenefitsIncomeTaxPenaltiesAndInterestAccrued | 40 us-gaap_UnrecognizedTaxBenefitsIncomeTaxPenaltiesAndInterestAccrued | 41 us-gaap_UnrecognizedTaxBenefitsIncomeTaxPenaltiesAndInterestAccrued |
Business Acquisition [Member] |  |  |  |
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 |
State [Member] |  |  |  |
Income Taxes [Line Items] |  |  |  |
Net operating loss carryforwards expiry period | 2015 through 2034 |  |  |

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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 increase (decrease) in the valuation allowance for deferred tax assets. + 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 FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 45 -Paragraph 20 -URI http://asc.fasb.org/extlink&oid=37586315&loc=d3e32123-109318 + Details
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- Definition The amount of the unrecognized tax benefit of a position taken for which it is reasonably possible that the total amount thereof will significantly increase or decrease within twelve months of the balance sheet date. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 15 -Subparagraph (d) -URI http://asc.fasb.org/extlink&oid=6907707&loc=d3e32718-109319 + Details
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- Definition Amount of statutory penalties accrued for a tax position claimed or expected to be claimed by the entity, in its tax return. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 15 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=6907707&loc=d3e32718-109319 + Details
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