Subsidiary Guarantors - Schedule of Condensed Consolidating Statement of Cash Flows (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Jul. 31, 2013	Jul. 31, 2012
Condensed Financial Statements, Captions [Line Items]
Net cash provided by (used in) operating activities	$ 19,526	$ 7,722
Cash Flows from Investing Activities:
Acquisitions, net of cash acquired	(29)	(3,150)
Additions to property, plant and equipment - acquisitions	(1,072)	(288)
Additions to property, plant and equipment - growth	(1,785)	(2,002)
Additions to property, plant and equipment - maintenance	(11,622)	(14,179)
Payments on landfill operating lease contracts	(1,982)	(1,814)
Payment for capital related to divestiture		(618)
Investments in unconsolidated entities	(2,148)	(1,000)
Proceeds from sale of property and equipment	284	265
Net cash provided by (used in) investing activities	(18,354)	(22,786)
Cash Flows from Financing Activities:
Proceeds from long-term borrowings	29,890	62,310
Principal payments on long-term debt	(29,310)	(48,689)
Payments of financing costs	(359)	(96)
Excess tax benefit on the vesting of share based awards	(63)	205
Contributions from noncontrolling interest holders		721
Net cash provided by (used in) financing activities	158	14,451
Net cash provided by (used in) discontinued operations	(166)	(416)
Net increase (decrease) in cash and cash equivalents	1,164	(1,029)
Cash and cash equivalents, beginning of period	1,755	4,534
Cash and cash equivalents, end of period	2,919	3,505
Parent [Member]
Condensed Financial Statements, Captions [Line Items]
Net cash provided by (used in) operating activities	(6,951)	(5,192)
Cash Flows from Investing Activities:
Additions to property, plant and equipment - maintenance	(237)	(105)
Investments in unconsolidated entities	(2,148)	(1,000)
Net cash provided by (used in) investing activities	(2,385)	(1,105)
Cash Flows from Financing Activities:
Proceeds from long-term borrowings	29,890	62,310
Principal payments on long-term debt	(28,585)	(47,944)
Payments of financing costs	(359)	(96)
Excess tax benefit on the vesting of share based awards	(63)	205
Intercompany borrowings	9,513	(9,554)
Net cash provided by (used in) financing activities	10,396	4,921
Net increase (decrease) in cash and cash equivalents	1,060	(1,376)
Cash and cash equivalents, beginning of period	1,260	3,799
Cash and cash equivalents, end of period	2,320	2,423
Guarantors [Member]
Condensed Financial Statements, Captions [Line Items]
Net cash provided by (used in) operating activities	26,358	13,231
Cash Flows from Investing Activities:
Acquisitions, net of cash acquired	(29)	(3,150)
Additions to property, plant and equipment - acquisitions	(1,072)	(288)
Additions to property, plant and equipment - growth	(1,724)	(1,232)
Additions to property, plant and equipment - maintenance	(11,385)	(14,074)
Payments on landfill operating lease contracts	(1,982)	(1,814)
Payment for capital related to divestiture		(618)
Investments in unconsolidated entities		(750)
Proceeds from sale of property and equipment	284	265
Net cash provided by (used in) investing activities	(15,908)	(21,661)
Cash Flows from Financing Activities:
Principal payments on long-term debt	(725)	(745)
Intercompany borrowings	(9,513)	9,554
Net cash provided by (used in) financing activities	(10,238)	8,809
Net cash provided by (used in) discontinued operations	(166)	(416)
Net increase (decrease) in cash and cash equivalents	46	(37)
Cash and cash equivalents, beginning of period	253	368
Cash and cash equivalents, end of period	299	331
Non-Guarantors [Member]
Condensed Financial Statements, Captions [Line Items]
Net cash provided by (used in) operating activities	119	(317)
Cash Flows from Investing Activities:
Additions to property, plant and equipment - growth	(61)	(770)
Investments in unconsolidated entities		750
Net cash provided by (used in) investing activities	(61)	(20)
Cash Flows from Financing Activities:
Contributions from noncontrolling interest holders		721
Net cash provided by (used in) financing activities		721
Net increase (decrease) in cash and cash equivalents	58	384
Cash and cash equivalents, beginning of period	242	367
Cash and cash equivalents, end of period	300	751
Elimination [Member]
Condensed Financial Statements, Captions [Line Items]
Net cash provided by (used in) 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
Payments on landfill operating lease contracts
Payment for capital related to divestiture
Investments in unconsolidated entities
Proceeds from sale of property and equipment
Net cash provided by (used in) investing activities
Cash Flows from Financing Activities:
Proceeds from long-term borrowings
Principal payments on long-term debt
Payments of financing costs
Excess tax benefit on the vesting of share based awards
Contributions from noncontrolling interest holders
Intercompany borrowings
Net cash provided by (used in) financing activities
Net cash provided by (used in) discontinued operations
Net increase (decrease) in cash and cash equivalents
Cash and cash equivalents, beginning of period
Cash and cash equivalents, end of period