Subsidiary Guarantors - Schedule of Condensed Consolidating Statement of Cash Flows (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Jul. 31, 2014	Jul. 31, 2013
Condensed Financial Statements, Captions [Line Items]
Net cash provided by (used in) operating activities	$ 13,581	$ 19,526
Cash Flows from Investing Activities:
Acquisitions, net of cash acquired		(29)
Additions to property, plant and equipment	(16,881)	(13,407)
Payments on landfill operating lease contracts	(962)	(1,982)
Payments related to investments		(2,148)
Proceeds from sale of property and equipment	163	284
Net cash provided by (used in) investing activities	(17,725)	(18,354)
Cash Flows from Financing Activities:
Proceeds from long-term borrowings	38,300	29,890
Principal payments on long-term debt	(34,314)	(29,310)
Payments of financing costs	(1)	(359)
Proceeds from the exercise of share based awards	143
Excess tax benefit on the vesting of share based awards	59	(63)
Intercompany borrowings
Net cash provided by (used in) financing activities	4,187	158
Net cash provided by (used in) discontinued operations	88	(166)
Net increase (decrease) in cash and cash equivalents	131	1,164
Cash and cash equivalents, beginning of period	2,464	1,755
Cash and cash equivalents, end of period	2,595	2,919
Parent [Member]
Condensed Financial Statements, Captions [Line Items]
Net cash provided by (used in) operating activities	(9,238)	(6,951)
Cash Flows from Investing Activities:
Acquisitions, net of cash acquired
Additions to property, plant and equipment	(227)	(237)
Payments on landfill operating lease contracts
Payments related to investments		(2,148)
Proceeds from sale of property and equipment
Net cash provided by (used in) investing activities	(227)	(2,385)
Cash Flows from Financing Activities:
Proceeds from long-term borrowings		29,890
Principal payments on long-term debt	(34,181)	(28,585)
Payments of financing costs	(1)	(359)
Proceeds from the exercise of share based awards
Excess tax benefit on the vesting of share based awards	59	(63)
Intercompany borrowings	43,429	9,513
Net cash provided by (used in) financing activities	9,306	10,396
Net cash provided by (used in) discontinued operations
Net increase (decrease) in cash and cash equivalents	(159)	1,060
Cash and cash equivalents, beginning of period	2,151	1,260
Cash and cash equivalents, end of period	1,992	2,320
Guarantor [Member]
Condensed Financial Statements, Captions [Line Items]
Net cash provided by (used in) operating activities	22,829	26,358
Cash Flows from Investing Activities:
Acquisitions, net of cash acquired		(29)
Additions to property, plant and equipment	(16,654)	(13,109)
Payments on landfill operating lease contracts	(962)	(1,982)
Payments related to investments	(310)
Proceeds from sale of property and equipment	163	284
Net cash provided by (used in) investing activities	(17,808)	(15,908)
Cash Flows from Financing Activities:
Proceeds from long-term borrowings	38,300
Principal payments on long-term debt	(133)	(725)
Payments of financing costs
Proceeds from the exercise of share based awards	143
Excess tax benefit on the vesting of share based awards
Intercompany borrowings	(43,429)	(9,513)
Net cash provided by (used in) financing activities	(5,119)	(10,238)
Net cash provided by (used in) discontinued operations	88	(166)
Net increase (decrease) in cash and cash equivalents	(10)	46
Cash and cash equivalents, beginning of period	271	253
Cash and cash equivalents, end of period	261	299
Non-Guarantor [Member]
Condensed Financial Statements, Captions [Line Items]
Net cash provided by (used in) operating activities	(10)	119
Cash Flows from Investing Activities:
Acquisitions, net of cash acquired
Additions to property, plant and equipment		(61)
Payments on landfill operating lease contracts
Payments related to investments	310
Proceeds from sale of property and equipment
Net cash provided by (used in) investing activities	310	(61)
Cash Flows from Financing Activities:
Proceeds from long-term borrowings
Principal payments on long-term debt
Payments of financing costs
Proceeds from the exercise of share based awards
Excess tax benefit on the vesting of share based awards
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	300	58
Cash and cash equivalents, beginning of period	42	242
Cash and cash equivalents, end of period	342	300
Acquisition [Member]
Cash Flows from Investing Activities:
Additions to property, plant and equipment	(45)	(1,072)
Acquisition [Member] | Parent [Member]
Cash Flows from Investing Activities:
Additions to property, plant and equipment
Acquisition [Member] | Guarantor [Member]
Cash Flows from Investing Activities:
Additions to property, plant and equipment	(45)	(1,072)
Acquisition [Member] | Non-Guarantor [Member]
Cash Flows from Investing Activities:
Additions to property, plant and equipment
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
Payments on landfill operating lease contracts
Payments related to investments
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
Proceeds from the exercise of share based awards
Excess tax benefit on the vesting of share based awards
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
Elimination [Member] | Acquisition [Member]
Cash Flows from Investing Activities:
Additions to property, plant and equipment