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Business Combinations - Schedule of Purchase Price Paid for Acquisitions (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
Business Acquisition [Line Items]
Cash used in acquisitions, net of cash acquired	$ 360us-gaap_PaymentsToAcquireBusinessesNetOfCashAcquired	$ 8,305us-gaap_PaymentsToAcquireBusinessesNetOfCashAcquired	$ 25,225us-gaap_PaymentsToAcquireBusinessesNetOfCashAcquired	$ 2,102us-gaap_PaymentsToAcquireBusinessesNetOfCashAcquired
Excess purchase price to be allocated to goodwill	119,170us-gaap_Goodwill	119,139us-gaap_Goodwill	115,928us-gaap_Goodwill	101,706us-gaap_Goodwill
BBI Acquisition [Member]
Business Acquisition [Line Items]
Cash used in acquisitions, net of cash acquired	314us-gaap_PaymentsToAcquireBusinessesNetOfCashAcquired / us-gaap_InvestmentTypeAxis = cwst_BbiWasteServicesMember	7,860us-gaap_PaymentsToAcquireBusinessesNetOfCashAcquired / us-gaap_InvestmentTypeAxis = cwst_BbiWasteServicesMember	25,225us-gaap_PaymentsToAcquireBusinessesNetOfCashAcquired / us-gaap_InvestmentTypeAxis = cwst_BbiWasteServicesMember
Common stock issued			2,650us-gaap_BusinessAcquisitionEquityInterestIssuedOrIssuableValueAssigned / us-gaap_InvestmentTypeAxis = cwst_BbiWasteServicesMember
Other non-cash considerations		555us-gaap_BusinessCombinationConsiderationTransferredOther1 / us-gaap_InvestmentTypeAxis = cwst_BbiWasteServicesMember
Contingent consideration and holdbacks	67us-gaap_BusinessCombinationContingentConsiderationLiability / us-gaap_InvestmentTypeAxis = cwst_BbiWasteServicesMember	1,653us-gaap_BusinessCombinationContingentConsiderationLiability / us-gaap_InvestmentTypeAxis = cwst_BbiWasteServicesMember	33us-gaap_BusinessCombinationContingentConsiderationLiability / us-gaap_InvestmentTypeAxis = cwst_BbiWasteServicesMember
Total	381us-gaap_BusinessCombinationConsiderationTransferred1 / us-gaap_InvestmentTypeAxis = cwst_BbiWasteServicesMember	10,068us-gaap_BusinessCombinationConsiderationTransferred1 / us-gaap_InvestmentTypeAxis = cwst_BbiWasteServicesMember	27,908us-gaap_BusinessCombinationConsiderationTransferred1 / us-gaap_InvestmentTypeAxis = cwst_BbiWasteServicesMember
Current assets		814us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentAssets / us-gaap_InvestmentTypeAxis = cwst_BbiWasteServicesMember	1,422us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentAssets / us-gaap_InvestmentTypeAxis = cwst_BbiWasteServicesMember
Equipment	99us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedPropertyPlantAndEquipment / us-gaap_InvestmentTypeAxis = cwst_BbiWasteServicesMember	2,010us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedPropertyPlantAndEquipment / us-gaap_InvestmentTypeAxis = cwst_BbiWasteServicesMember	9,423us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedPropertyPlantAndEquipment / us-gaap_InvestmentTypeAxis = cwst_BbiWasteServicesMember
Other liabilities, net		(241)us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedLiabilities / us-gaap_InvestmentTypeAxis = cwst_BbiWasteServicesMember	(7,009)us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedLiabilities / us-gaap_InvestmentTypeAxis = cwst_BbiWasteServicesMember
Intangible assets	251us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedIntangibleAssetsOtherThanGoodwill / us-gaap_InvestmentTypeAxis = cwst_BbiWasteServicesMember	4,302us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedIntangibleAssetsOtherThanGoodwill / us-gaap_InvestmentTypeAxis = cwst_BbiWasteServicesMember	9,850us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedIntangibleAssetsOtherThanGoodwill / us-gaap_InvestmentTypeAxis = cwst_BbiWasteServicesMember
Fair value of assets acquired and liabilities assumed	350us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedNet / us-gaap_InvestmentTypeAxis = cwst_BbiWasteServicesMember	6,885us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedNet / us-gaap_InvestmentTypeAxis = cwst_BbiWasteServicesMember	13,686us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedNet / us-gaap_InvestmentTypeAxis = cwst_BbiWasteServicesMember
Excess purchase price to be allocated to goodwill	$ 31us-gaap_Goodwill / us-gaap_InvestmentTypeAxis = cwst_BbiWasteServicesMember	$ 3,183us-gaap_Goodwill / us-gaap_InvestmentTypeAxis = cwst_BbiWasteServicesMember	$ 14,222us-gaap_Goodwill / us-gaap_InvestmentTypeAxis = cwst_BbiWasteServicesMember

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Value of equity interests (such as common shares, preferred shares, or partnership interest) issued or issuable to acquire the entity.

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Amount of consideration transferred, consisting of acquisition-date fair value of assets transferred by the acquirer, liabilities incurred by the acquirer, and equity interest issued by the acquirer.

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Amount of tangible or intangible assets, including a business or subsidiary of the acquirer transferred by the entity to the former owners of the acquiree. Excludes cash.

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Amount of assets that are expected to be realized or consumed within one year or the normal operating cycle, if longer, acquired at the acquisition date.

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Amount recognized as of the acquisition date for the identifiable assets acquired in excess of (less than) the aggregate liabilities assumed.

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The amount of property, plant, and equipment recognized as of the acquisition date.

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Amount after accumulated impairment loss of an asset representing future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized.

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