Subsidiary Guarantors - Schedule of Consolidating Statement of Operations (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Jul. 31, 2014	Jul. 31, 2013
Condensed Financial Statements, Captions [Line Items]
Revenues	$ 141,387	$ 128,558
Operating expenses:
Cost of operations	98,766	88,419
General and administration	16,840	15,078
Depreciation and amortization	16,368	15,197
Expense from divestiture, acquisition and financing costs		20
Environmental remediation charge	75
Severance and reorganization costs		107
Total operating expenses	132,049	118,821
Operating income (loss)	9,338	9,737
Other expense (income), net:
Interest income	(88)	(12)
Interest expense	9,551	9,359
(Income) loss from equity method investments		977
(Gain) loss on derivative instruments	(69)	(654)
Other income	(192)	(138)
Other expense, net	9,202	9,532
Income (loss) from continuing operations before income taxes	136	205
Provision (benefit) for income taxes	290	319
Income (loss) from continuing operations	(154)	(114)
Discontinued operations:
Income (loss) from discontinued operations, net		329
Gain (loss) on disposal of discontinued operations, net		(378)
Net loss	(154)	(163)
Less: Net income (loss) attributable to noncontrolling interests	136	28
Net income (loss) attributable to common stockholders	(290)	(191)
Parent [Member]
Condensed Financial Statements, Captions [Line Items]
Revenues
Operating expenses:
Cost of operations	17
General and administration	380	157
Depreciation and amortization	195	214
Expense from divestiture, acquisition and financing costs
Environmental remediation charge
Severance and reorganization costs
Total operating expenses	592	371
Operating income (loss)	(592)	(371)
Other expense (income), net:
Interest income		(1)
Interest expense	9,589	9,347
(Income) loss from equity method investments	(10,089)	(9,172)
(Gain) loss on derivative instruments	(69)	(654)
Other income	(23)	(19)
Other expense, net	(592)	(499)
Income (loss) from continuing operations before income taxes		128
Provision (benefit) for income taxes	290	319
Income (loss) from continuing operations		(191)
Discontinued operations:
Income (loss) from discontinued operations, net
Gain (loss) on disposal of discontinued operations, net
Net loss	(290)	(191)
Less: Net income (loss) attributable to noncontrolling interests
Net income (loss) attributable to common stockholders	(290)	(191)
Guarantor [Member]
Condensed Financial Statements, Captions [Line Items]
Revenues	141,090	127,693
Operating expenses:
Cost of operations	98,718	87,782
General and administration	16,470	14,927
Depreciation and amortization	16,173	14,802
Expense from divestiture, acquisition and financing costs		20
Environmental remediation charge	75
Severance and reorganization costs		107
Total operating expenses	131,436	117,638
Operating income (loss)	9,654	10,055
Other expense (income), net:
Interest income	(88)	(11)
Interest expense	(38)	12
(Income) loss from equity method investments		(20)
(Gain) loss on derivative instruments
Other income	(169)	(118)
Other expense, net	(295)	(137)
Income (loss) from continuing operations before income taxes	9,949	10,192
Provision (benefit) for income taxes
Income (loss) from continuing operations		10,192
Discontinued operations:
Income (loss) from discontinued operations, net		329
Gain (loss) on disposal of discontinued operations, net		(378)
Net loss	9,949	10,143
Less: Net income (loss) attributable to noncontrolling interests
Net income (loss) attributable to common stockholders	9,949	10,143
Non-Guarantor [Member]
Condensed Financial Statements, Captions [Line Items]
Revenues	297	865
Operating expenses:
Cost of operations	31	637
General and administration	(10)	(6)
Depreciation and amortization		181
Expense from divestiture, acquisition and financing costs
Environmental remediation charge
Severance and reorganization costs
Total operating expenses	21	812
Operating income (loss)	276	53
Other expense (income), net:
Interest income
Interest expense
(Income) loss from equity method investments		997
(Gain) loss on derivative instruments
Other income		(1)
Other expense, net		996
Income (loss) from continuing operations before income taxes	276	(943)
Provision (benefit) for income taxes
Income (loss) from continuing operations		(943)
Discontinued operations:
Income (loss) from discontinued operations, net
Gain (loss) on disposal of discontinued operations, net
Net loss	276	(943)
Less: Net income (loss) attributable to noncontrolling interests	136	28
Net income (loss) attributable to common stockholders	140	(971)
Elimination [Member]
Condensed Financial Statements, Captions [Line Items]
Revenues
Operating expenses:
Cost of operations
General and administration
Depreciation and amortization
Expense from divestiture, acquisition and financing costs
Environmental remediation charge
Severance and reorganization costs
Total operating expenses
Operating income (loss)
Other expense (income), net:
Interest income
Interest expense
(Income) loss from equity method investments	10,089	9,172
(Gain) loss on derivative instruments
Other income
Other expense, net	10,089	9,172
Income (loss) from continuing operations before income taxes	(10,089)	(9,172)
Provision (benefit) for income taxes
Income (loss) from continuing operations		(9,172)
Discontinued operations:
Income (loss) from discontinued operations, net
Gain (loss) on disposal of discontinued operations, net
Net loss	(10,089)	(9,172)
Less: Net income (loss) attributable to noncontrolling interests
Net income (loss) attributable to common stockholders	$ (10,089)	$ (9,172)