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Goodwill and Intangible Assets - Schedule of Activity and Balances Related to Goodwill (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
Goodwill And Intangible Assets [Line Items] |  |  |  |
Goodwill, beginning balance | $ 119,139 us-gaap_Goodwill | $ 115,928 us-gaap_Goodwill | $ 101,706 us-gaap_Goodwill |
Acquisitions | 31 us-gaap_GoodwillAcquiredDuringPeriod | 3,183 us-gaap_GoodwillAcquiredDuringPeriod | 14,222 us-gaap_GoodwillAcquiredDuringPeriod |
Other |  | 28 us-gaap_GoodwillOtherChanges |  |
Goodwill, ending balance | 119,170 us-gaap_Goodwill | 119,139 us-gaap_Goodwill | 115,928 us-gaap_Goodwill |
Recycling [Member] |  |  |  |
Goodwill And Intangible Assets [Line Items] |  |  |  |
Goodwill, beginning balance | 12,315 us-gaap_Goodwill / us-gaap_StatementBusinessSegmentsAxis = cwst_RecyclingMember | 12,190 us-gaap_Goodwill / us-gaap_StatementBusinessSegmentsAxis = cwst_RecyclingMember |  | 12,190 us-gaap_Goodwill / us-gaap_StatementBusinessSegmentsAxis = cwst_RecyclingMember
Acquisitions |  | 125 us-gaap_GoodwillAcquiredDuringPeriod / us-gaap_StatementBusinessSegmentsAxis = cwst_RecyclingMember |  |
Goodwill, ending balance | 12,315 us-gaap_Goodwill / us-gaap_StatementBusinessSegmentsAxis = cwst_RecyclingMember | 12,315 us-gaap_Goodwill / us-gaap_StatementBusinessSegmentsAxis = cwst_RecyclingMember |  | 12,190 us-gaap_Goodwill / us-gaap_StatementBusinessSegmentsAxis = cwst_RecyclingMember
Other [Member] |  |  |  |
Goodwill And Intangible Assets [Line Items] |  |  |  |
Goodwill, beginning balance | 1,729 us-gaap_Goodwill / us-gaap_StatementBusinessSegmentsAxis = us-gaap_AllOtherSegmentsMember |  |  |
Acquisitions |  | 1,729 us-gaap_GoodwillAcquiredDuringPeriod / us-gaap_StatementBusinessSegmentsAxis = us-gaap_AllOtherSegmentsMember |  |
Goodwill, ending balance | 1,729 us-gaap_Goodwill / us-gaap_StatementBusinessSegmentsAxis = us-gaap_AllOtherSegmentsMember | 1,729 us-gaap_Goodwill / us-gaap_StatementBusinessSegmentsAxis = us-gaap_AllOtherSegmentsMember |  |
Eastern Region [Member] |  |  |  |
Goodwill And Intangible Assets [Line Items] |  |  |  |
Goodwill, beginning balance | 17,429 us-gaap_Goodwill / us-gaap_StatementGeographicalAxis = cwst_EasternRegionMember | 16,858 us-gaap_Goodwill / us-gaap_StatementGeographicalAxis = cwst_EasternRegionMember | 58 us-gaap_Goodwill / us-gaap_StatementGeographicalAxis = cwst_EasternRegionMember |
Acquisitions |  | 539 us-gaap_GoodwillAcquiredDuringPeriod / us-gaap_StatementGeographicalAxis = cwst_EasternRegionMember | 12,939 us-gaap_GoodwillAcquiredDuringPeriod / us-gaap_StatementGeographicalAxis = cwst_EasternRegionMember |
Other |  | 32 us-gaap_GoodwillOtherChanges / us-gaap_StatementGeographicalAxis = cwst_EasternRegionMember | 3,861 us-gaap_GoodwillOtherChanges / us-gaap_StatementGeographicalAxis = cwst_EasternRegionMember |
Goodwill, ending balance | 17,429 us-gaap_Goodwill / us-gaap_StatementGeographicalAxis = cwst_EasternRegionMember | 17,429 us-gaap_Goodwill / us-gaap_StatementGeographicalAxis = cwst_EasternRegionMember | 16,858 us-gaap_Goodwill / us-gaap_StatementGeographicalAxis = cwst_EasternRegionMember |
Western Region [Member] |  |  |  |
Goodwill And Intangible Assets [Line Items] |  |  |  |
Goodwill, beginning balance | 87,666 us-gaap_Goodwill / us-gaap_StatementGeographicalAxis = cwst_WesternRegionMember | 86,880 us-gaap_Goodwill / us-gaap_StatementGeographicalAxis = cwst_WesternRegionMember | 89,458 us-gaap_Goodwill / us-gaap_StatementGeographicalAxis = cwst_WesternRegionMember |
Acquisitions | 31 us-gaap_GoodwillAcquiredDuringPeriod / us-gaap_StatementGeographicalAxis = cwst_WesternRegionMember | 790 us-gaap_GoodwillAcquiredDuringPeriod / us-gaap_StatementGeographicalAxis = cwst_WesternRegionMember | 1,283 us-gaap_GoodwillAcquiredDuringPeriod / us-gaap_StatementGeographicalAxis = cwst_WesternRegionMember |
Other |  | (4)us-gaap_GoodwillOtherChanges / us-gaap_StatementGeographicalAxis = cwst_WesternRegionMember | (3,861)us-gaap_GoodwillOtherChanges / us-gaap_StatementGeographicalAxis = cwst_WesternRegionMember |
Goodwill, ending balance | $ 87,697 us-gaap_Goodwill / us-gaap_StatementGeographicalAxis = cwst_WesternRegionMember | $ 87,666 us-gaap_Goodwill / us-gaap_StatementGeographicalAxis = cwst_WesternRegionMember | $ 86,880 us-gaap_Goodwill / us-gaap_StatementGeographicalAxis = cwst_WesternRegionMember |

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