Stockholders' Equity - Reclassification out of Accumulated Other Comprehensive Loss (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Oct. 31, 2013	Oct. 31, 2012	Oct. 31, 2013	Oct. 31, 2012
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Loss on derivative instruments	$ (629)	$ (3,896)	$ 25	$ (3,896)
Loss from equity method investments	91	(109)	(887)	(1,875)
Loss from continuing operations before income taxes and discontinued operations	(230)	(20,444)	(25)	(27,957)
Provision for income taxes	300	413	619	1,063
Loss from continuing operations	(530)	(20,857)	(644)	(29,020)
Commodity Hedges [Member] | Reclassification out of Accumulated Other Comprehensive Loss [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Loss from continuing operations before income taxes and discontinued operations	(181)	(3,946)	(345)	(3,990)
Provision for income taxes
Loss from continuing operations	(181)	(3,946)	(345)	(3,990)
Commodity Hedges [Member] | Interest Rate Contracts [Member] | Reclassification out of Accumulated Other Comprehensive Loss [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Loss on derivative instruments		(3,625)		(3,625)
Commodity Hedges [Member] | Commodity Contract [Member] | Reclassification out of Accumulated Other Comprehensive Loss [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Loss from equity method investments	$ (181)	$ (321)	$ (345)	$ (365)