Fair Value Of Financial Instruments And Long-Term Debt (Narrative) (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Apr. 30, 2012	Apr. 30, 2011	Apr. 30, 2010
Fair Value Of Financial Instruments And Long-Term Debt [Line Items]
Fair value of long-term debt excluding capital lease obligations	$ 691,000	$ 647,000
Number of Promissory notes outstanding	2
Outstanding amount of each Promissory note	50,000
Line of credit, maximum borrowing capacity	100,000
Line of credit, amount owed	0	600
Weighted average interest rate	0.85%
Interest expense net of interest income	208	360	300
Interest expense capitalized	$ 674	$ 406	$ 431
Note 1 [Member]
Fair Value Of Financial Instruments And Long-Term Debt [Line Items]
Interest over Index	0.75%
Interest rate	0.85%
Note 2 [Member]
Fair Value Of Financial Instruments And Long-Term Debt [Line Items]
Interest over Index	1.00%
Interest rate	1.10%