Derivative Financial Instruments (Details Textual) (USD $)	9 Months Ended
	Jan. 31, 2013	Apr. 30, 2012
Derivative Financial Instruments (Additional Textual) [Abstract]
Cash margin accounts related to derivative instruments recognized	$ 5,900,000	$ 32,500,000
Commodity Contracts [Member]
Derivative Financial Instruments (Textual) [Abstract]
Derivative instrument maturity	1 year
Deferred pre-tax loss included in accumulated other comprehensive loss	(16,600,000)	(24,300,000)
Tax impact related to deferred losses and gains on cash flow hedges included in accumulated other comprehensive loss	6,000,000	8,800,000
Maximum expected recognized period of deferred loss in earnings	1 year
Interest Rate Contract [Member]
Derivative Financial Instruments (Textual) [Abstract]
Deferred pre-tax loss included in accumulated other comprehensive loss	(5,500,000)	(5,900,000)
Tax impact related to deferred losses and gains on cash flow hedges included in accumulated other comprehensive loss	2,000,000	2,100,000
Effective portion of the hedge loss reclassified to interest expense upon termination of the swap	$ 500,000
Expected period for recognition of the hedge loss reclassified to interest expense under termination of the swap	12 months
Foreign currency exchange contracts [Member]
Derivative Financial Instruments (Textual) [Abstract]
Derivative instrument maturity	1 year