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Business Combinations - Additional Information (Detail) | 6 Months Ended
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Oct. 31, 2014 Business | Oct. 31, 2013 Business
Solid Waste Operations [Member] |  |
Business Acquisition [Line Items] |  |
Number of businesses acquired |  | 3
Solid Waste Operations [Member] | Western Region [Member] |  |
Business Acquisition [Line Items] |  |
Number of businesses acquired | 1 | 2
Solid Waste Operations [Member] | Eastern Region [Member] |  |
Business Acquisition [Line Items] |  |
Number of businesses acquired | 1 | 1
Industrial Services [Member] | Other [Member] |  |
Business Acquisition [Line Items] |  |
Number of businesses acquired |  | 1
Minimum [Member] |  |
Business Acquisition [Line Items] |  |
Acquired intangible assets, amortization period | 5 years |
Maximum [Member] |  |
Business Acquisition [Line Items] |  |
Acquired intangible assets, amortization period | 10 years |

X
- Details
Name: | us-gaap_BusinessAcquisitionLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition Useful life of finite-lived intangible assets, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. + References No definition available. + Details
Name: | us-gaap_FiniteLivedIntangibleAssetUsefulLife
Namespace Prefix: | us-gaap_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

X
- Definition The number of businesses acquired by the entity during the period. + References No definition available. + Details
Name: | us-gaap_NumberOfBusinessesAcquired
Namespace Prefix: | us-gaap_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | duration