Income Taxes (Schedule of Deferred Tax Assets and Liabilities) (Details) (USD $) In Thousands, unless otherwise specified	Apr. 30, 2012	Apr. 30, 2011	Apr. 30, 2010
Income Taxes [Abstract]
Deferred tax assets, accrued liabilities	$ 13,143	$ 10,405	$ 9,417
Deferred tax assets, deferred compensation	5,663	5,325	4,941
Deferred tax assets, other	7,644	5,562	3,759
Total gross deferred tax assets	26,450	21,292	18,117
Deferred tax liabilities, excess of tax over book depreciation	(264,757)	(211,415)	(145,433)
Deferred tax liabilities, other	(8,955)	(2,550)	(4,496)
Total gross deferred tax liabilities	(273,712)	(213,965)	(149,929)
Net deferred tax liabilities	$ (247,262)	$ (192,673)	$ (131,812)