Subsidiary Guarantors - Schedule of Condensed Consolidating Statement of Cash Flows (Detail) (USD $) In Thousands, unless otherwise specified	6 Months Ended
	Oct. 31, 2013	Oct. 31, 2012
Condensed Financial Statements, Captions [Line Items]
Net cash provided by (used in) operating activities	$ 28,119	$ 22,768
Cash Flows from Investing Activities:
Acquisitions, net of cash acquired	(2,822)	(4,635)
Additions to property, plant and equipment - acquisition	(1,365)	(417)
Additions to property, plant and equipment - growth	(3,249)	(8,198)
Additions to property, plant and equipment - maintenance	(22,810)	(24,776)
Payments on landfill operating lease contracts	(3,471)	(3,298)
Payment for capital related to divestiture		(618)
Investments in unconsolidated entities	(2,148)	(1,000)
Proceeds from sale of property and equipment	929	543
Net cash used in investing activities	(34,936)	(42,399)
Cash Flows from Financing Activities:
Proceeds from long-term borrowings	83,190	236,177
Principal payments on long-term debt	(72,586)	(227,028)
Change in restricted cash		(23,579)
Payment of tender premium and costs on second lien notes		(6,745)
Payments of financing costs	(388)	(4,329)
Net proceeds from the sale of Class A common stock		42,149
Excess tax benefit on the vesting of share based awards		188
Contributions from noncontrolling interest holders		1,195
Net cash provided by financing activities	10,216	18,028
Net cash provided by (used in) discontinued operations	(201)	(1,030)
Net increase (decrease) in cash and cash equivalents	3,198	(2,633)
Cash and cash equivalents, beginning of period	1,755	4,534
Cash and cash equivalents, end of period	4,953	1,901
Parent [Member]
Condensed Financial Statements, Captions [Line Items]
Net cash provided by (used in) operating activities	(25,720)	(27,935)
Cash Flows from Investing Activities:
Additions to property, plant and equipment - maintenance	(1,059)	(190)
Investments in unconsolidated entities	(2,148)	(1,000)
Net cash used in investing activities	(3,207)	(1,190)
Cash Flows from Financing Activities:
Proceeds from long-term borrowings	83,190	236,177
Principal payments on long-term debt	(71,735)	(226,098)
Change in restricted cash		(23,579)
Payment of tender premium and costs on second lien notes		(6,745)
Payments of financing costs	(388)	(4,329)
Net proceeds from the sale of Class A common stock		42,149
Excess tax benefit on the vesting of share based awards		188
Intercompany borrowings	21,145	9,049
Net cash provided by financing activities	32,212	26,812
Net increase (decrease) in cash and cash equivalents	3,285	(2,313)
Cash and cash equivalents, beginning of period	1,260	3,799
Cash and cash equivalents, end of period	4,545	1,486
Guarantors [Member]
Condensed Financial Statements, Captions [Line Items]
Net cash provided by (used in) operating activities	53,481	49,057
Cash Flows from Investing Activities:
Acquisitions, net of cash acquired	(2,822)	(4,635)
Additions to property, plant and equipment - acquisition	(1,365)	(417)
Additions to property, plant and equipment - growth	(2,824)	(3,520)
Additions to property, plant and equipment - maintenance	(21,751)	(24,586)
Payments on landfill operating lease contracts	(3,471)	(3,298)
Payment for capital related to divestiture		(618)
Investments in unconsolidated entities		(1,566)
Proceeds from sale of property and equipment	929	543
Net cash used in investing activities	(31,304)	(38,097)
Cash Flows from Financing Activities:
Principal payments on long-term debt	(851)	(930)
Intercompany borrowings	(21,145)	(9,049)
Net cash provided by financing activities	(21,996)	(9,979)
Net cash provided by (used in) discontinued operations	(201)	(1,030)
Net increase (decrease) in cash and cash equivalents	(20)	(49)
Cash and cash equivalents, beginning of period	253	368
Cash and cash equivalents, end of period	233	319
Non-Guarantors [Member]
Condensed Financial Statements, Captions [Line Items]
Net cash provided by (used in) operating activities	358	1,646
Cash Flows from Investing Activities:
Additions to property, plant and equipment - growth	(425)	(4,678)
Investments in unconsolidated entities		1,566
Net cash used in investing activities	(425)	(3,112)
Cash Flows from Financing Activities:
Contributions from noncontrolling interest holders		1,195
Net cash provided by financing activities		1,195
Net increase (decrease) in cash and cash equivalents	(67)	(271)
Cash and cash equivalents, beginning of period	242	367
Cash and cash equivalents, end of period	$ 175	$ 96