Subsidiary Guarantors (Schedule of Condensed Consolidating Statements of Cash Flows) (Detail) (USD $) In Thousands, unless otherwise specified	9 Months Ended
	Jan. 31, 2013	Jan. 31, 2012
Condensed Financial Statements, Captions [Line Items]
Net cash provided by operating activities	$ 30,526	$ 49,742
Cash Flows from Investing Activities:
Acquisitions, net of cash acquired	(25,106)	(2,102)
Additions to property, plant and equipment - acquisitions	(528)	(168)
Additions to property, plant and equipment - growth	(10,415)	(9,833)
Additions to property, plant and equipment- maintenance	(33,526)	(39,279)
Payment for capital related to divestiture	(618)
Payments on landfill operating lease contracts	(5,726)	(6,052)
Proceeds from sale of property and equipment	795	1,337
Investments in unconsolidated entities	(1,000)	(4,146)
Net cash provided by (used in) investing activities	(76,124)	(60,243)
Cash Flows from Financing Activities:
Proceeds from long-term borrowings	334,497	127,900
Principal payments on long-term debt	(320,483)	(119,433)
Payment of tender premium on second lien notes	(10,743)
Net proceeds from the issuance of class A common stock	42,184
Contributions from noncontrolling interest holder	1,195	174
Other	(4,474)	623
Net cash provided by (used in) financing activities	42,176	9,090
Cash provided by discontinued operations		725
Net increase (decrease) in cash and cash equivalents	(3,422)	(686)
Cash and cash equivalents, beginning of period	4,534	1,817
Cash and cash equivalents, end of period	1,112	1,131
Parent [Member]
Condensed Financial Statements, Captions [Line Items]
Net cash provided by operating activities	(3,983)	5,176
Cash Flows from Investing Activities:
Acquisitions, net of cash acquired
Additions to property, plant and equipment - acquisitions
Additions to property, plant and equipment - growth
Additions to property, plant and equipment- maintenance	(233)	(977)
Payment for capital related to divestiture
Payments on landfill operating lease contracts
Proceeds from sale of property and equipment
Investments in unconsolidated entities	(4,736)	(1,820)
Net cash provided by (used in) investing activities	(4,969)	(2,797)
Cash Flows from Financing Activities:
Proceeds from long-term borrowings	334,497	127,900
Principal payments on long-term debt	(319,365)	(118,356)
Payment of tender premium on second lien notes	(10,743)
Net proceeds from the issuance of class A common stock	42,184
Contributions from noncontrolling interest holder
Other	(4,474)	449
Intercompany borrowings	(36,126)	(12,961)
Net cash provided by (used in) financing activities	5,973	(2,968)
Net increase (decrease) in cash and cash equivalents	(2,979)	(589)
Cash and cash equivalents, beginning of period	3,799	1,531
Cash and cash equivalents, end of period	820	942
Guarantors [Member]
Condensed Financial Statements, Captions [Line Items]
Net cash provided by operating activities	35,357	44,524
Cash Flows from Investing Activities:
Acquisitions, net of cash acquired	(25,106)	(2,102)
Additions to property, plant and equipment - acquisitions	(528)	(168)
Additions to property, plant and equipment - growth	(5,530)	(8,209)
Additions to property, plant and equipment- maintenance	(33,293)	(38,302)
Payment for capital related to divestiture	(618)
Payments on landfill operating lease contracts	(5,726)	(6,052)
Proceeds from sale of property and equipment	795	1,337
Investments in unconsolidated entities	(500)	(3,785)
Net cash provided by (used in) investing activities	(70,506)	(57,281)
Cash Flows from Financing Activities:
Proceeds from long-term borrowings
Principal payments on long-term debt	(1,118)	(1,077)
Payment of tender premium on second lien notes
Net proceeds from the issuance of class A common stock
Contributions from noncontrolling interest holder
Other
Intercompany borrowings	36,126	12,961
Net cash provided by (used in) financing activities	35,008	11,884
Cash provided by discontinued operations		725
Net increase (decrease) in cash and cash equivalents	(141)	(148)
Cash and cash equivalents, beginning of period	368	286
Cash and cash equivalents, end of period	227	138
Non-Guarantors [Member]
Condensed Financial Statements, Captions [Line Items]
Net cash provided by operating activities	(848)	42
Cash Flows from Investing Activities:
Acquisitions, net of cash acquired
Additions to property, plant and equipment - acquisitions
Additions to property, plant and equipment - growth	(4,885)	(1,624)
Additions to property, plant and equipment- maintenance
Payment for capital related to divestiture
Payments on landfill operating lease contracts
Proceeds from sale of property and equipment
Investments in unconsolidated entities	4,236	1,459
Net cash provided by (used in) investing activities	(649)	(165)
Cash Flows from Financing Activities:
Proceeds from long-term borrowings
Principal payments on long-term debt
Payment of tender premium on second lien notes
Net proceeds from the issuance of class A common stock
Contributions from noncontrolling interest holder	1,195	174
Other
Intercompany borrowings
Net cash provided by (used in) financing activities	1,195	174
Net increase (decrease) in cash and cash equivalents	(302)	51
Cash and cash equivalents, beginning of period	367
Cash and cash equivalents, end of period	65	51
Elimination [Member]
Condensed Financial Statements, Captions [Line Items]
Net cash provided by operating activities
Cash Flows from Investing Activities:
Acquisitions, net of cash acquired
Additions to property, plant and equipment - acquisitions
Additions to property, plant and equipment - growth
Additions to property, plant and equipment- maintenance
Payment for capital related to divestiture
Payments on landfill operating lease contracts
Proceeds from sale of property and equipment
Investments in unconsolidated entities
Net cash provided by (used in) investing activities
Cash Flows from Financing Activities:
Proceeds from long-term borrowings
Principal payments on long-term debt
Payment of tender premium on second lien notes
Net proceeds from the issuance of class A common stock
Contributions from noncontrolling interest holder
Other
Intercompany borrowings
Net cash provided by (used in) financing activities
Net increase (decrease) in cash and cash equivalents
Cash and cash equivalents, beginning of period
Cash and cash equivalents, end of period