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Stockholders' Equity - Additional Information (Detail) (USD $) In Thousands, except Share data, unless otherwise specified	12 Months Ended															12 Months Ended				3 Months Ended	12 Months Ended
	Apr. 30, 2013	Apr. 30, 2012	Apr. 30, 2011	Apr. 30, 2013 Stock Options [Member]	Apr. 30, 2012 Stock Options [Member]	Apr. 30, 2011 Stock Options [Member]	Apr. 30, 2013 Employee Stock Purchase Plan [Member]	Apr. 30, 2012 Employee Stock Purchase Plan [Member]	Apr. 30, 2011 Employee Stock Purchase Plan [Member]	Apr. 30, 2013 Restricted Stock Restricted Stock Units and Performance Stock Units [Member]	Apr. 30, 2012 Restricted Stock Restricted Stock Units and Performance Stock Units [Member]	Apr. 30, 2011 Restricted Stock Restricted Stock Units and Performance Stock Units [Member]	Apr. 30, 2013 Restricted Stock And Restricted Stock Unit [Member] Y	Apr. 30, 2013 Performance Stock Units [Member] Y	Apr. 30, 2013 Performance Stock Units [Member]	Apr. 30, 2013 1997 Non-Employee Director Stock Option Plan [Member]	Apr. 30, 2013 Casella-Altela Regional Environmental Services LLC [Member] Altela, Inc. [Member]	Apr. 30, 2013 Maximum [Member] Altela, Inc. [Member]	Apr. 30, 2013 Minimum [Member] Altela, Inc. [Member]	Oct. 31, 2012 Class A Common Stock [Member]	Apr. 30, 2013 Class A Common Stock [Member]	Apr. 30, 2011 Class A Common Stock [Member]	Apr. 30, 2013 Class A Common Stock [Member] 1997 Stock Option Plan [Member]	Apr. 30, 2012 Class A Common Stock [Member] 1997 Stock Option Plan [Member]	Apr. 30, 2013 Class A Common Stock [Member] 1997 Non-Employee Director Stock Option Plan [Member]	Apr. 30, 2012 Class A Common Stock [Member] 1997 Non-Employee Director Stock Option Plan [Member]	Apr. 30, 2013 Class A Common Stock [Member] 2006 Stock Incentive Plan [Member]	Apr. 30, 2012 Class A Common Stock [Member] 2006 Stock Incentive Plan [Member]	Apr. 30, 2013 Class A Common Stock [Member] Maximum [Member] 1997 Non-Employee Director Stock Option Plan [Member]	Apr. 30, 2013 Preferred Stock [Member]	Apr. 30, 2012 Preferred Stock [Member]	Apr. 30, 2013 Preferred Stock [Member] Maximum [Member]
Limited Partners' Capital Account [Line Items]
Issuances of Class A common stock																				11,500,000	11,500,000
Share price																				$ 4.00
Net proceeds from the registered public offering		$ 337	$ 476																	$ 42,184
Preferred stock authorized to issue																																944,000
Shares issued																														0	0
Options to purchase	1,442	1,661																					599	1,081	75	95	768	485
Weighted average exercise																							$ 12.23	$ 11.82	$ 12.45	$ 11.04	$ 5.16	$ 7.64
Common stock, issued shares																											2,475		200,000
Non-Employee Director Plan termination																July 31, 2007
Number of shares available for future grant																											1,602
Stock-based compensation expense	2,236	1,855	1,592	528	258	387	99	113	122	1,609	1,485	1,083
Income taxes associated with stock-based compensation expense	0	0	97
Issuance of share																						259,000
Recognition of total compensation expense			1,438
Unrecognized stock-based compensation expense													2,750	3,805	0
Unrecognized stock-based compensation expense, weighted average period													1.64	1.64	1.64
Tax benefits from additional paid in capital adjustments	$ 96	$ 254	$ 129
Membership interest																	49.00%	66.10%	51.00%

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