Restructuring (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		12 Months Ended
	Jul. 31, 2014	Jul. 31, 2013	Apr. 30, 2014
Restructuring and related charges
Total expected restructuring charge	$ 265.0
Beginning Balance	1.7	7.7	7.7
Charge to expense	2.8	5.2	20.8
Cash payments	(3.6)		(23.9)
Noncash utilization	(0.1)		(2.9)
Ending Balance	0.8		1.7
Remaining expected restructuring charge	13.8
Long-Lived Asset Charges [Member]
Restructuring and related charges
Total expected restructuring charge	102.8
Beginning Balance	0	0	0
Charge to expense	0.1		2.7
Cash payments	0		0
Noncash utilization	(0.1)		(2.7)
Ending Balance	0		0
Remaining expected restructuring charge	0.1
Employee Separation [Member]
Restructuring and related charges
Total expected restructuring charge	63.8
Beginning Balance	1.7	7.7	7.7
Charge to expense	0.2		2.6
Cash payments	(1.1)		(8.4)
Noncash utilization	0		(0.2)
Ending Balance	0.8		1.7
Remaining expected restructuring charge	0.1
Site Preparation and Equipment Relocation [Member]
Restructuring and related charges
Total expected restructuring charge	45.4
Beginning Balance	0	0	0
Charge to expense	0.9		7.2
Cash payments	(0.9)		(7.2)
Noncash utilization	0		0
Ending Balance	0		0
Remaining expected restructuring charge	4.3
Production Start-up [Member]
Restructuring and related charges
Total expected restructuring charge	42.8
Beginning Balance	0	0	0
Charge to expense	1.3		7.2
Cash payments	(1.3)		(7.2)
Noncash utilization	0		0
Ending Balance	0		0
Remaining expected restructuring charge	7.7
Other Costs [Member]
Restructuring and related charges
Total expected restructuring charge	10.2
Beginning Balance	0	0	0
Charge to expense	0.3		1.1
Cash payments	(0.3)		(1.1)
Noncash utilization	0		0
Ending Balance	0		0
Remaining expected restructuring charge	$ 1.6