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Income Taxes - Schedule of Deferred Tax Assets and Liabilities (Detail) (USD $)
In Thousands, unless otherwise specified
Apr. 30, 2013
Apr. 30, 2012
Deferred tax assets:    
Net operating loss carryforwards $ 34,217 $ 9,724
Accrued expenses and reserves 29,884 28,383
Book over tax depreciation of property and equipment 19,881 24,899
Alternative minimum tax credit carryforwards 3,330 3,330
General business tax credit carryforwards 2,095 1,438
Unrealized loss on commodity hedges 1,852 688
Stock awards 1,177 1,140
Deferred revenue   218
Other 964 370
Total deferred tax assets 93,400 70,190
Less: valuation allowance (70,352) (50,700)
Total deferred tax assets after valuation allowance 23,048 19,490
Deferred tax liabilities:    
Amortization of intangibles (25,973) (21,114)
Other (143)  
Total deferred tax liabilities (26,116) (21,114)
Net deferred tax (liability) asset $ (3,068) $ (1,624)