Derivative Financial Instruments (Details Textual) (USD $) In Thousands, unless otherwise specified	3 Months Ended	12 Months Ended
	Jul. 31, 2012	Apr. 30, 2012
Derivative Financial Instruments (Additional Textual) [Abstract]
Cash margin accounts related to derivative instruments recognized	$ 5,390	$ 32,529
Maximum expected recognized period of deferred loss in earnings	1 year
Expected period for Recognition of loss		12 months
Commodity contracts [Member]
Derivative Financial Instruments (Textual) [Abstract]
Derivative instrument maturity	1 year
Deferred pretax (loss) gain included in accumulated other comprehensive income (loss)	(19,959)	(24,287)
Tax impact related to deferred (losses) and gains on cash flow hedges included in accumulated other comprehensive (loss) income	7,249	8,820
Interest rate contracts [Member]
Derivative Financial Instruments (Textual) [Abstract]
Deferred pretax (loss) gain included in accumulated other comprehensive income (loss)	(5,782)	(5,914)
Tax impact related to deferred (losses) and gains on cash flow hedges included in accumulated other comprehensive (loss) income	2,085	2,133
Foreign currency exchange contracts [Member]
Derivative Financial Instruments (Textual) [Abstract]
Derivative instrument maturity	1 year
Cash Flow Hedging [Member] | Interest rate contracts [Member]
Derivative Financial Instruments (Textual) [Abstract]
Effective portion of the hedge loss reclassified to interest expense upon termination of the swap	$ 500