Stockholders' Equity - Reclassification out of Accumulated Other Comprehensive Income (Loss) (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended	9 Months Ended
Jan. 31, 2014	Jan. 31, 2013	Jan. 31, 2014	Jan. 31, 2013
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Loss on derivative instruments	$ (233)	$ 24	$ (208)	$ (3,871)
Loss from equity method investments	(112)	(1,436)	(998)	(3,311)
Loss from continuing operations before income taxes and discontinued operations	(10,377)	(16,109)	(10,402)	(44,066)
Provision for income taxes	656	(4,963)	1,275	(3,899)
Loss from continuing operations	(11,033)	(11,146)	(11,677)	(40,167)
Commodity Hedges [Member] \| Reclassification out of Accumulated Other Comprehensive Income (Loss) [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Loss from continuing operations before income taxes and discontinued operations	(60)	(149)	(405)	(4,139)
Provision for income taxes	(250)		(250)
Loss from continuing operations	(310)	(149)	(655)	(4,139)
Commodity Hedges [Member] \| Interest Rate Contracts [Member] \| Reclassification out of Accumulated Other Comprehensive Income (Loss) [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Loss on derivative instruments				(3,625)
Commodity Hedges [Member] \| Commodity Contract [Member] \| Reclassification out of Accumulated Other Comprehensive Income (Loss) [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Loss from equity method investments	$ (60)	$ (149)	$ (405)	$ (514)

Amount of realized and unrealized gain (loss) of derivative instruments not designated or qualifying as hedging instruments.

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 815

-SubTopic 10

-Section 50

-Paragraph 4C

-Subparagraph (e)

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This element represents the income or loss from continuing operations attributable to the economic entity which may also be defined as revenue less expenses from ongoing operations, after income or loss from equity method investments, but before income taxes, extraordinary items, and noncontrolling interest.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 03

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This element represents the income or loss from continuing operations attributable to the economic entity which may also be defined as revenue less expenses and taxes from ongoing operations before extraordinary items, and noncontrolling interest.

+ References

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This item represents the entity's proportionate share for the period of the net income (loss) of its investee (such as unconsolidated subsidiaries and joint ventures) to which the equity method of accounting is applied. This item includes income or expense related to stock-based compensation based on the investor's grant of stock to employees of an equity method investee.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

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Amount of current income tax expense (benefit) and deferred income tax expense (benefit) pertaining to continuing operations.

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-Name Accounting Standards Codification

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-Glossary Income Tax Expense (or Benefit)

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