Restructuring (Details) (USD $)	3 Months Ended		9 Months Ended		12 Months Ended
	Jan. 31, 2013	Jan. 31, 2012	Jan. 31, 2013	Jan. 31, 2012	Apr. 30, 2012
Restructuring and related charges
Total expected restructuring charge	$ 245,000,000		$ 245,000,000
Beginning Balance			8,778,000	10,198,000	10,198,000
Charge to expense - net	5,400,000	25,600,000	30,134,000	67,300,000	81,141,000
Cash payments			(25,133,000)		(40,336,000)
Noncash utilization			(5,376,000)		(42,225,000)
Ending Balance	8,403,000		8,403,000		8,778,000
Remaining expected restructuring charge	26,057,000		26,057,000
Long-Lived Asset Charges [Member]
Restructuring and related charges
Total expected restructuring charge	105,000,000		105,000,000
Beginning Balance			0	0	0
Charge to expense - net			5,370,000		34,195,000
Cash payments			0		0
Noncash utilization			(5,370,000)		(34,195,000)
Ending Balance	0		0		0
Remaining expected restructuring charge	7,996,000		7,996,000
Employee Separation [Member]
Restructuring and related charges
Total expected restructuring charge	66,000,000		66,000,000
Beginning Balance			8,778,000	10,198,000	10,198,000
Charge to expense - net			3,742,000		20,364,000
Cash payments			(4,111,000)		(13,754,000)
Noncash utilization			(6,000)		(8,030,000)
Ending Balance	8,403,000		8,403,000		8,778,000
Remaining expected restructuring charge	4,745,000		4,745,000
Site Preparation and Equipment Relocation [Member]
Restructuring and related charges
Total expected restructuring charge	35,000,000		35,000,000
Beginning Balance			0	0	0
Charge to expense - net			10,609,000		12,963,000
Cash payments			(10,609,000)		(12,963,000)
Noncash utilization			0		0
Ending Balance	0		0		0
Remaining expected restructuring charge	4,829,000		4,829,000
Production Start-up [Member]
Restructuring and related charges
Total expected restructuring charge	29,000,000		29,000,000
Beginning Balance			0	0	0
Charge to expense - net			8,292,000		10,689,000
Cash payments			(8,292,000)		(10,689,000)
Noncash utilization			0		0
Ending Balance	0		0		0
Remaining expected restructuring charge	4,809,000		4,809,000
Other Costs [Member]
Restructuring and related charges
Total expected restructuring charge	10,000,000		10,000,000
Beginning Balance			0	0	0
Charge to expense - net			2,121,000		2,930,000
Cash payments			(2,121,000)		(2,930,000)
Noncash utilization			0		0
Ending Balance	0		0		0
Remaining expected restructuring charge	$ 3,678,000		$ 3,678,000