Summary of Significant Accounting Policies - Self-Insurance Liabilities and Related Costs - Additional Information (Detail) (USD $) In Thousands, unless otherwise specified	8 Months Ended
Dec. 31, 2014	Apr. 30, 2014	Apr. 30, 2013
Summary Of Significant Accounting Policies [Line Items]
Maximum exposure workers' compensation plan per individual event	$ 1,000cwst_WorkersCompensationBenefits
Self insurance reserves	10,863us-gaap_SelfInsuranceReserve	10,280us-gaap_SelfInsuranceReserve	11,362us-gaap_SelfInsuranceReserve
Automobile Plan [Member]
Summary Of Significant Accounting Policies [Line Items]
Maximum exposure workers' compensation plan per individual event	$ 1,000

Carrying amount (including both current and noncurrent portions) of accrued known and estimated losses incurred as of the balance sheet date for which no insurance coverage exists, and for which a claim has been made or is probable of being asserted, typically arising from workmen's compensation-type of incidents and personal injury to nonemployees from accidents on the entity's property.

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.5-02.24)

-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Name:	us-gaap_SelfInsuranceReserve
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant