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Summary of Significant Accounting Policies - Landfill Development Cost- Additional Information (Detail) (USD $) In Thousands, unless otherwise specified	8 Months Ended	12 Months Ended
	Dec. 31, 2014	Apr. 30, 2014	Apr. 30, 2013	Apr. 30, 2012
Summary Of Significant Accounting Policies [Line Items]
Capitalized interest	$ 333us-gaap_InterestCostsCapitalized	$ 256us-gaap_InterestCostsCapitalized	$ 368us-gaap_InterestCostsCapitalized	$ 407us-gaap_InterestCostsCapitalized
Estimate of inflation rate	2.30%cwst_InflationRate
Accretion expense	2,275us-gaap_AssetRetirementObligationAccretionExpense	3,967us-gaap_AssetRetirementObligationAccretionExpense	3,538us-gaap_AssetRetirementObligationAccretionExpense
Landfills [Member]
Summary Of Significant Accounting Policies [Line Items]
Accretion expense	2,275us-gaap_AssetRetirementObligationAccretionExpense / us-gaap_PropertyPlantAndEquipmentByTypeAxis = us-gaap_LandfillMember	3,967us-gaap_AssetRetirementObligationAccretionExpense / us-gaap_PropertyPlantAndEquipmentByTypeAxis = us-gaap_LandfillMember	3,538us-gaap_AssetRetirementObligationAccretionExpense / us-gaap_PropertyPlantAndEquipmentByTypeAxis = us-gaap_LandfillMember	3,341us-gaap_AssetRetirementObligationAccretionExpense / us-gaap_PropertyPlantAndEquipmentByTypeAxis = us-gaap_LandfillMember
Surety bond	144,633cwst_SuretyBonds / us-gaap_PropertyPlantAndEquipmentByTypeAxis = us-gaap_LandfillMember	133,847cwst_SuretyBonds / us-gaap_PropertyPlantAndEquipmentByTypeAxis = us-gaap_LandfillMember	128,551cwst_SuretyBonds / us-gaap_PropertyPlantAndEquipmentByTypeAxis = us-gaap_LandfillMember
Letters of credit	$ 1,104us-gaap_LettersOfCreditOutstandingAmount / us-gaap_PropertyPlantAndEquipmentByTypeAxis = us-gaap_LandfillMember	$ 1,104us-gaap_LettersOfCreditOutstandingAmount / us-gaap_PropertyPlantAndEquipmentByTypeAxis = us-gaap_LandfillMember	$ 1,752us-gaap_LettersOfCreditOutstandingAmount / us-gaap_PropertyPlantAndEquipmentByTypeAxis = us-gaap_LandfillMember
Minimum [Member]
Summary Of Significant Accounting Policies [Line Items]
Rate of asset retirement obligation	9.00%us-gaap_FairValueInputsDiscountRate / us-gaap_RangeAxis = us-gaap_MinimumMember
Maximum [Member]
Summary Of Significant Accounting Policies [Line Items]
Rate of asset retirement obligation	9.50%us-gaap_FairValueInputsDiscountRate / us-gaap_RangeAxis = us-gaap_MaximumMember

X

- Definition

Estimated inflation rate.

+ References

No definition available.

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- Definition

Amount of surety bonds that are available for the future payment of landfill capping, closure and post-closure obligations.

+ References

No definition available.

+ Details

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X

- Definition

Amount of accretion expense recognized during the period that is associated with an asset retirement obligation. Accretion expense measures and incorporates changes due to the passage of time into the carrying amount of the liability.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 410

-SubTopic 20

-Section 45

-Paragraph 1

-URI http://asc.fasb.org/extlink&oid=6392676&loc=d3e7480-110848



Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 410

-SubTopic 20

-Section 50

-Paragraph 1

-Subparagraph (c)(3)

-URI http://asc.fasb.org/extlink&oid=6392692&loc=d3e7535-110849

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- Definition

Interest rate used to find the present value of an amount to be paid or received in the future as an input to measure fair value. For example, but not limited to, weighted average cost of capital (WACC), cost of capital, cost of equity and cost of debt.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 820

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (bbb)

-URI http://asc.fasb.org/extlink&oid=36462937&loc=d3e19207-110258

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- Definition

Amount of interest capitalized during the period.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Glossary Allowance for Funds Used during Construction

-URI http://asc.fasb.org/extlink&oid=6504829



Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 835

-SubTopic 20

-Section 50

-Paragraph 1

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-URI http://asc.fasb.org/extlink&oid=6450988&loc=d3e26243-108391

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- Definition

The total amount of the contingent obligation under letters of credit outstanding as of the reporting date.

+ References

No definition available.

+ Details

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