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Business Combinations - Schedule of Unaudited Pro forma Combined Information (Detail) (USD $) In Thousands, except Per Share data, unless otherwise specified	8 Months Ended		12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Apr. 30, 2014	Apr. 30, 2013	Apr. 30, 2012
Business Combinations [Abstract]
Revenue	$ 368,641us-gaap_BusinessAcquisitionsProFormaRevenue		$ 502,304us-gaap_BusinessAcquisitionsProFormaRevenue	$ 478,630us-gaap_BusinessAcquisitionsProFormaRevenue
Operating income	22,143cwst_BusinessAcquisitionsProFormaOperatingIncomeLoss		12,770cwst_BusinessAcquisitionsProFormaOperatingIncomeLoss	14,820cwst_BusinessAcquisitionsProFormaOperatingIncomeLoss
Net loss attributable to common stockholders	$ (6,015)us-gaap_BusinessAcquisitionsProFormaNetIncomeLoss		$ (22,775)us-gaap_BusinessAcquisitionsProFormaNetIncomeLoss	$ (53,759)us-gaap_BusinessAcquisitionsProFormaNetIncomeLoss
Basic net loss per common share attributable to common stockholders	$ (0.15)us-gaap_BusinessAcquisitionProFormaEarningsPerShareBasic		$ (0.57)us-gaap_BusinessAcquisitionProFormaEarningsPerShareBasic	$ (1.58)us-gaap_BusinessAcquisitionProFormaEarningsPerShareBasic
Basic and diluted weighted average shares outstanding	40,262us-gaap_WeightedAverageNumberOfShareOutstandingBasicAndDiluted	39,774us-gaap_WeightedAverageNumberOfShareOutstandingBasicAndDiluted	39,820us-gaap_WeightedAverageNumberOfShareOutstandingBasicAndDiluted	34,015us-gaap_WeightedAverageNumberOfShareOutstandingBasicAndDiluted	26,749us-gaap_WeightedAverageNumberOfShareOutstandingBasicAndDiluted

X

- Definition

The pro forma operating income for the period as if the business combination or combinations had been completed at the beginning of the period.

+ References

No definition available.

+ Details

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Namespace Prefix:	cwst_
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- Definition

The pro forma basic net income per share for a period as if the business combination or combinations had been completed at the beginning of a period.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 805

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (h)(2)-(3)

-URI http://asc.fasb.org/extlink&oid=25497992&loc=d3e1392-128463

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X

- Definition

The pro forma net Income or Loss for the period as if the business combination or combinations had been completed at the beginning of a period.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 805

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (h)(2)-(3)

-URI http://asc.fasb.org/extlink&oid=25497992&loc=d3e1392-128463

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- Definition

The pro forma revenue for a period as if the business combination or combinations had been completed at the beginning of the period.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 805

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (h)(2)-(3)

-URI http://asc.fasb.org/extlink&oid=25497992&loc=d3e1392-128463

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- Definition

Average number of shares or units issued and outstanding that are used in calculating basic and diluted earnings per share (EPS).

+ References

No definition available.

+ Details

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Namespace Prefix:	us-gaap_
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