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Stockholders' (Deficit) Equity - Summary of Grant Activity for Other Stock Awards (Detail) (USD $) In Thousands, except Per Share data, unless otherwise specified	12 Months Ended
	Apr. 30, 2014	Apr. 30, 2013	Apr. 30, 2012
Schedule Of Stock Option Activity [Line Items]
Stock Options, Granted	542	735	611
Stock Options, Weighted Average Grant Date Fair Value	$ 4.28
Stock Options, Unissued	454	431	62
Restricted Stock Units [Member]
Schedule Of Stock Option Activity [Line Items]
Stock Options, Granted	482	340	305
Stock Options, Weighted Average Grant Date Fair Value	$ 4.09	$ 5.15	$ 6.12
Stock Options, Unissued	454	176	62
Performance Stock Units [Member]
Schedule Of Stock Option Activity [Line Items]
Stock Options, Granted		316	255
Stock Options, Weighted Average Grant Date Fair Value		$ 5.17	$ 6.06
Stock Options, Unissued		255
Restricted Stock Awards [Member]
Schedule Of Stock Option Activity [Line Items]
Stock Options, Granted	60	79	51
Stock Options, Weighted Average Grant Date Fair Value	$ 5.81	$ 4.45	$ 5.83
Stock Options, Unissued

X

- Details

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Namespace Prefix:	cwst_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

X

- Definition

The number of grants made during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan).

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 718

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (c)(2)(iii)(1)

-URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901

+ Details

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Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	duration

X

- Definition

The weighted average fair value at grant date for nonvested equity-based awards issued during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan).

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 718

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (c)(2)(iii)(1)

-URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	num:perShareItemType
Balance Type:	na
Period Type:	duration

X

- Definition

The number of non-vested equity-based payment instruments, excluding stock (or unit) options, that validly exist and are outstanding as of the balance sheet date.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 718

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (c)(2)(i)-(ii)

-URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	instant