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Property, Plant and Equipment - 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
Property, Plant and Equipment [Line Items]
Depreciation expense	$ 21,599us-gaap_Depreciation	$ 33,094us-gaap_Depreciation	$ 34,065us-gaap_Depreciation	$ 37,829us-gaap_Depreciation
Depletion expense on operating lease contracts	7,799cwst_DepletionOfLandfillOperatingLeaseObligations	9,948cwst_DepletionOfLandfillOperatingLeaseObligations	9,372cwst_DepletionOfLandfillOperatingLeaseObligations	8,482cwst_DepletionOfLandfillOperatingLeaseObligations
Landfills [Member]
Property, Plant and Equipment [Line Items]
Amortization expense	17,912us-gaap_AdjustmentForAmortization / us-gaap_PropertyPlantAndEquipmentByTypeAxis = us-gaap_LandfillMember	24,689us-gaap_AdjustmentForAmortization / us-gaap_PropertyPlantAndEquipmentByTypeAxis = us-gaap_LandfillMember	21,206us-gaap_AdjustmentForAmortization / us-gaap_PropertyPlantAndEquipmentByTypeAxis = us-gaap_LandfillMember	19,957us-gaap_AdjustmentForAmortization / us-gaap_PropertyPlantAndEquipmentByTypeAxis = us-gaap_LandfillMember
Depletion expense on operating lease contracts	$ 7,799cwst_DepletionOfLandfillOperatingLeaseObligations / us-gaap_PropertyPlantAndEquipmentByTypeAxis = us-gaap_LandfillMember	$ 9,948cwst_DepletionOfLandfillOperatingLeaseObligations / us-gaap_PropertyPlantAndEquipmentByTypeAxis = us-gaap_LandfillMember	$ 9,372cwst_DepletionOfLandfillOperatingLeaseObligations / us-gaap_PropertyPlantAndEquipmentByTypeAxis = us-gaap_LandfillMember	$ 8,482cwst_DepletionOfLandfillOperatingLeaseObligations / us-gaap_PropertyPlantAndEquipmentByTypeAxis = us-gaap_LandfillMember

X

- Definition

This element represents the noncash expense charged against earnings to recognize the depletion of landfill operating lease obligation.

+ References

No definition available.

+ Details

Name:	cwst_DepletionOfLandfillOperatingLeaseObligations
Namespace Prefix:	cwst_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

X

- Definition

The aggregate amount of recurring noncash expense charged against earnings in the period to allocate the cost of assets over their estimated remaining economic lives.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 230

-SubTopic 10

-Section 45

-Paragraph 28

-Subparagraph (b)

-URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3602-108585

+ Details

Name:	us-gaap_AdjustmentForAmortization
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

X

- Definition

The amount of expense recognized in the current period that reflects the allocation of the cost of tangible assets over the assets' useful lives. Includes production and non-production related depreciation.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 230

-SubTopic 10

-Section 45

-Paragraph 28

-Subparagraph (b)

-URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3602-108585

+ Details

Name:	us-gaap_Depreciation
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

X

- Details

Name:	us-gaap_PropertyPlantAndEquipmentLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

X

- Details

Name:	us-gaap_PropertyPlantAndEquipmentByTypeAxis=us-gaap_LandfillMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: