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Income Taxes (Schedule of Deferred Tax Assets and Liabilities) (Details) (USD $)
In Thousands, unless otherwise specified
Apr. 30, 2013
Apr. 30, 2012
Apr. 30, 2011
Income Taxes [Abstract]      
Deferred tax assets, accrued liabilities $ 9,916 $ 13,143 $ 10,405
Deferred tax assets, deferred compensation 6,050 5,663 5,325
Deferred tax assets, unrecognized tax benefits 3,128 2,638 2,135
Deferred tax assets, state net operation loss 3,902 4,395 2,771
Deferred tax assets, other 1,016 611 656
Total gross deferred tax assets 24,012 26,450 21,292
Deferred tax liabilities, excess of tax over book depreciation (295,951) (264,757) (211,415)
Deferred tax liabilites, goodwill (11,919) (8,955) (2,550)
Deferred tax liabilities, other 66    
Total gross deferred tax liabilities (307,804) (273,712) (213,965)
Net deferred tax liabilities $ (283,792) $ (247,262) $ (192,673)