Restructuring (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended		12 Months Ended
	Jan. 31, 2014	Jan. 31, 2013	Jan. 31, 2014	Jan. 31, 2013	Apr. 30, 2013
Restructuring and related charges
Total expected restructuring charge			$ 265.0
Beginning Balance			7.7	8.8	8.8
Charge to expense	5.1	5.4	17.1	30.1	38.8
Cash payments			(17.9)		(31.7)
Noncash utilization			(2.9)		(8.2)
Ending Balance	4.0		4.0		7.7
Remaining expected restructuring charge			20.3
Long-Lived Asset Charges [Member]
Restructuring and related charges
Total expected restructuring charge			102.8
Beginning Balance			0	0	0
Charge to expense			2.7		8.2
Cash payments			0		0
Noncash utilization			(2.7)		(8.2)
Ending Balance	0		0		0
Remaining expected restructuring charge			0.2
Employee Separation [Member]
Restructuring and related charges
Total expected restructuring charge			64.2
Beginning Balance			7.7	8.8	8.8
Charge to expense			2.6		3.4
Cash payments			(6.1)		(4.5)
Noncash utilization			(0.2)		0
Ending Balance	4.0		4.0		7.7
Remaining expected restructuring charge			0.7
Site Preparation and Equipment Relocation [Member]
Restructuring and related charges
Total expected restructuring charge			45.6
Beginning Balance			0	0	0
Charge to expense			5.5		13.4
Cash payments			(5.5)		(13.4)
Noncash utilization			0		0
Ending Balance	0		0		0
Remaining expected restructuring charge			7.1
Production Start-up [Member]
Restructuring and related charges
Total expected restructuring charge			42.2
Beginning Balance			0	0	0
Charge to expense			5.5		10.8
Cash payments			(5.5)		(10.8)
Noncash utilization			0		0
Ending Balance	0		0		0
Remaining expected restructuring charge			10.1
Other Costs [Member]
Restructuring and related charges
Total expected restructuring charge			10.2
Beginning Balance			0	0	0
Charge to expense			0.8		3.0
Cash payments			(0.8)		(3.0)
Noncash utilization			0		0
Ending Balance	0		0		0
Remaining expected restructuring charge			$ 2.2