Derivative Financial Instruments (Details 1) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Oct. 31, 2013	Oct. 31, 2012	Oct. 31, 2013	Oct. 31, 2012
Cash flow hedging derivative instruments gain loss
Change in accumulated other comprehensive loss	$ (0.7)	$ (0.6)	$ 2.1	$ 3.7
Commodity Contracts [Member]
Cash flow hedging derivative instruments gain loss
Losses recognized in other comprehensive (loss) income (effective portion)	(4.6)	(13.1)	(10.6)	(15.3)
Losses reclassified from accumulated other comprehensive loss to earnings (effective portion)	(3.9)	(12.5)	(12.7)	(19.0)
Gains (losses) recognized in earnings (ineffective portion)	0.1	(0.1)	0.1	(0.2)
Interest rate contract [Member]
Cash flow hedging derivative instruments gain loss
Losses reclassified from accumulated other comprehensive loss to earnings (effective portion)	$ (0.2)	$ (0.2)	$ (0.3)	$ (0.3)