Subsidiary Guarantors - Schedule of Consolidating Statement of Operations (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended			6 Months Ended
	Oct. 31, 2014	Oct. 31, 2013	Jul. 31, 2013	Oct. 31, 2014	Oct. 31, 2013
Condensed Financial Statements, Captions [Line Items]
Revenues	$ 141,341	$ 132,296		$ 282,728	$ 260,854
Operating expenses:
Cost of operations	98,292	90,545		197,057	178,962
General and administration	16,582	16,429		33,422	31,527
Depreciation and amortization	16,028	15,669		32,396	30,866
Environmental remediation charge	375	150		450	150
Severance and reorganization costs		53			161
Total operating expenses	131,277	122,846		263,325	241,666
Operating income (loss)	10,064	9,450		19,403	19,188
Other expense (income), net:
Interest income	(81)	(101)		(168)	(113)
Interest expense	9,623	9,635		19,173	18,994
(Income) loss from equity method investments		(91)			887
Earnings from consolidated entities
(Gain) loss on derivative instruments	362	629		293	(25)
Other income	(423)	(392)		(614)	(530)
Other expense, net	9,481	9,680		18,684	19,213
Income (loss) from continuing operations before income taxes	583	(230)		719	(25)
Provision (benefit) for income taxes	222	300		512	619
Income (loss) from continuing operations	361	(530)		207	(644)
Discontinued operations:
Income (loss) from discontinued operations, net		(45)			284
Gain (loss) on disposal of discontinued operations, net			(378)		(378)
Net income (loss)	361	(575)		207	(738)
Less: Net income (loss) attributable to noncontrolling interests	102	(236)		238	(207)
Net income (loss) attributable to common stockholders	259	(339)		(31)	(531)
Parent [Member]
Condensed Financial Statements, Captions [Line Items]
Revenues
Operating expenses:
Cost of operations	18			35	1
General and administration	617	595		997	748
Depreciation and amortization	188	225		382	437
Environmental remediation charge
Severance and reorganization costs					4
Total operating expenses	823	820		1,414	1,190
Operating income (loss)	(823)	(820)		(1,414)	(1,190)
Other expense (income), net:
Interest income		(1)			(1)
Interest expense	9,710	9,616		19,300	18,963
(Income) loss from equity method investments		(10,757)			(19,928)
Earnings from consolidated entities	(11,111)			(21,198)
(Gain) loss on derivative instruments	362	629		293	(25)
Other income	(265)	(268)		(290)	(287)
Other expense, net	(1,304)	(781)		(1,895)	(1,278)
Income (loss) from continuing operations before income taxes	481	(39)		481	88
Provision (benefit) for income taxes	222	300		512	619
Income (loss) from continuing operations		(339)			(531)
Discontinued operations:
Income (loss) from discontinued operations, net
Gain (loss) on disposal of discontinued operations, net
Net income (loss)	259	(339)		(31)	(531)
Less: Net income (loss) attributable to noncontrolling interests
Net income (loss) attributable to common stockholders	259	(339)		(31)	(531)
Guarantor [Member]
Condensed Financial Statements, Captions [Line Items]
Revenues	141,040	131,729		282,130	259,423
Operating expenses:
Cost of operations	98,186	89,740		196,903	177,519
General and administration	15,961	15,783		32,431	30,736
Depreciation and amortization	15,840	15,251		32,014	30,055
Environmental remediation charge	375	150		450	150
Severance and reorganization costs		53			157
Total operating expenses	130,362	120,977		261,798	238,617
Operating income (loss)	10,678	10,752		20,332	20,806
Other expense (income), net:
Interest income	(81)	(100)		(168)	(112)
Interest expense	(87)	19		(127)	31
(Income) loss from equity method investments		(15)			(35)
Earnings from consolidated entities
(Gain) loss on derivative instruments
Other income	(158)	(124)		(324)	(242)
Other expense, net	(326)	(220)		(619)	(358)
Income (loss) from continuing operations before income taxes	11,004	10,972		20,951	21,164
Provision (benefit) for income taxes
Income (loss) from continuing operations		10,972			21,164
Discontinued operations:
Income (loss) from discontinued operations, net		(45)			284
Gain (loss) on disposal of discontinued operations, net					(378)
Net income (loss)	11,004	10,927		20,951	21,070
Less: Net income (loss) attributable to noncontrolling interests
Net income (loss) attributable to common stockholders	11,004	10,927		20,951	21,070
Non-Guarantor [Member]
Condensed Financial Statements, Captions [Line Items]
Revenues	301	567		598	1,431
Operating expenses:
Cost of operations	88	805		119	1,442
General and administration	4	51		(6)	43
Depreciation and amortization		193			374
Environmental remediation charge
Severance and reorganization costs
Total operating expenses	92	1,049		113	1,859
Operating income (loss)	209	(482)		485	(428)
Other expense (income), net:
Interest income
Interest expense
(Income) loss from equity method investments		(76)			922
Earnings from consolidated entities
(Gain) loss on derivative instruments
Other income					(1)
Other expense, net		(76)			921
Income (loss) from continuing operations before income taxes	209	(406)		485	(1,349)
Provision (benefit) for income taxes
Income (loss) from continuing operations		(406)			(1,349)
Discontinued operations:
Income (loss) from discontinued operations, net
Gain (loss) on disposal of discontinued operations, net
Net income (loss)	209	(406)		485	(1,349)
Less: Net income (loss) attributable to noncontrolling interests	102	(236)		238	(207)
Net income (loss) attributable to common stockholders	107	(170)		247	(1,142)
Elimination [Member]
Condensed Financial Statements, Captions [Line Items]
Revenues
Operating expenses:
Cost of operations
General and administration
Depreciation and amortization
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		10,757			19,928
Earnings from consolidated entities	11,111			21,198
(Gain) loss on derivative instruments
Other income
Other expense, net	11,111	10,757		21,198	19,928
Income (loss) from continuing operations before income taxes	(11,111)	(10,757)		(21,198)	(19,928)
Provision (benefit) for income taxes
Income (loss) from continuing operations		(10,757)			(19,928)
Discontinued operations:
Income (loss) from discontinued operations, net
Gain (loss) on disposal of discontinued operations, net
Net income (loss)	(11,111)	(10,757)		(21,198)	(19,928)
Less: Net income (loss) attributable to noncontrolling interests
Net income (loss) attributable to common stockholders	$ (11,111)	$ (10,757)		$ (21,198)	$ (19,928)