Derivative Financial Instruments (Details) (USD $)	3 Months Ended			9 Months Ended
	Jan. 31, 2012	Oct. 31, 2011	Jan. 31, 2011	Jan. 31, 2012	Jan. 31, 2011	Oct. 18, 2011	Apr. 30, 2011
Cash flow hedging derivative instruments gain loss
Change in accumulated other comprehensive (loss)income	$ 995,000		$ (885,000)	$ (21,131,000)	$ 5,857,000
Gains and losses recognized in cost of products sold on derivatives not designated as qualified hedging instruments
Gains (losses) recognized in cost of products sold (derivatives not designated as hedging instruments)	1,125,000		(1,222,000)	18,584,000	3,895,000
Derivative Financial Instruments (Textual) [Abstract]
Interest rate on notes						3.50%
Derivative Financial Instruments (Textual)
Cash margin accounts related to derivative instruments recognized	10,289,000			10,289,000			12,292,000
Maximum expected recognized period of deferred gain/loss in earnings				P1Y
Gain on early termination agreement		23,900,000
Expected period for Recognition of loss				12 months
6.63% Senior Notes due November 1, 2018 [Member]
Derivative Financial Instruments (Textual) [Abstract]
Interest rate on notes	6.63%			6.63%			6.63%
3.50% Notes due October 15, 2021 [Member]
Derivative Financial Instruments (Textual) [Abstract]
Interest rate on notes	3.50%			3.50%			3.50%
Notes including Unamortized Discount						750,000,000
2012 [Member]
Derivative Financial Instruments (Textual) [Abstract]
Deferred gain recognized over the period				2,500,000
2013 through 2018 [Member]
Derivative Financial Instruments (Textual) [Abstract]
Deferred gain recognized over the period				3,300,000
2019 [Member]
Derivative Financial Instruments (Textual) [Abstract]
Deferred gain recognized over the period				1,600,000
Commodity contracts [Member]
Gains and losses recognized in cost of products sold on derivatives not designated as qualified hedging instruments
Gains (losses) recognized in cost of products sold (derivatives not designated as hedging instruments)	1,008,000		(359,000)	16,812,000	4,488,000
Outstanding derivative contracts
Gross contract notional amount	462,604,000			462,604,000			869,107,000
Derivative Financial Instruments (Textual) [Abstract]
Derivative instrument maturity	1 year			1 year
Tax impact related to deferred losses and gains on cash flow hedges included in accumulated other comprehensive (loss) income	(2,057,000)			(2,057,000)			3,430,000
Deferred pretax loss/gain included in accumulated other comprehensive income (loss)	5,657,000			5,657,000			(9,430,000)
Commodity contracts [Member] | Cash Flow Hedging [Member]
Cash flow hedging derivative instruments gain loss
(Losses) gains recognized in other comprehensive (loss) income (effective portion)	(681,000)		4,788,000	(10,941,000)	17,822,000
(Losses) gains reclassified from accumulated other comprehensive (loss) income to earnings (effective portion)	(1,546,000)		5,673,000	4,146,000	11,965,000
Change in accumulated other comprehensive (loss)income	865,000		(885,000)	(15,087,000)	5,857,000
(Losses) gains recognized in earnings (ineffective portion)	15,000		84,000	(498,000)	458,000
Interest rate contracts [Member]
Outstanding derivative contracts
Gross contract notional amount	0			0			376,000,000
Derivative Financial Instruments (Textual) [Abstract]
Tax impact related to deferred losses and gains on cash flow hedges included in accumulated other comprehensive (loss) income	(2,180,000)			(2,180,000)
Deferred pretax loss/gain included in accumulated other comprehensive income (loss)	6,044,000			6,044,000
Interest rate contracts [Member] | Cash Flow Hedging [Member]
Cash flow hedging derivative instruments gain loss
(Losses) gains recognized in other comprehensive (loss) income (effective portion)	0		0	(6,192,000)	0
(Losses) gains reclassified from accumulated other comprehensive (loss) income to earnings (effective portion)	(130,000)		0	(148,000)	0
Change in accumulated other comprehensive (loss)income	130,000		0	(6,044,000)	0
(Losses) gains recognized in earnings (ineffective portion)	0		0	(19,000)	0
Derivative Financial Instruments (Textual) [Abstract]
Termination loss on interest rate swap		6,200,000
Effective portion of the hedge loss reclassified to interest expense upon termination of the swap	300,000
Foreign currency exchange contracts [Member]
Gains and losses recognized in cost of products sold on derivatives not designated as qualified hedging instruments
Gains (losses) recognized in cost of products sold (derivatives not designated as hedging instruments)	117,000		(863,000)	1,772,000	(593,000)
Outstanding derivative contracts
Gross contract notional amount	74,255,000			74,255,000			73,158,000
Derivative Financial Instruments (Textual) [Abstract]
Derivative instrument maturity	1 year			1 year
Other Current Assets [Member]
Fair value of derivative instruments [Line Items]
Derivatives Instruments, Assets	2,629,000			2,629,000			19,035,000
Other Current Assets [Member] | Designated as hedging instruments [Member]
Fair value of derivative instruments [Line Items]
Derivatives Instruments, Assets	350,000			350,000			8,831,000
Other Current Assets [Member] | Not designated as hedging instruments [Member]
Fair value of derivative instruments [Line Items]
Derivatives Instruments, Assets	2,279,000			2,279,000			10,204,000
Other Current Assets [Member] | Commodity contracts [Member] | Designated as hedging instruments [Member]
Fair value of derivative instruments [Line Items]
Derivatives Instruments, Assets	350,000			350,000			3,408,000
Other Current Assets [Member] | Commodity contracts [Member] | Not designated as hedging instruments [Member]
Fair value of derivative instruments [Line Items]
Derivatives Instruments, Assets	2,223,000			2,223,000			9,887,000
Other Current Assets [Member] | Interest rate contracts [Member] | Designated as hedging instruments [Member]
Fair value of derivative instruments [Line Items]
Derivatives Instruments, Assets	0			0			5,423,000
Other Current Assets [Member] | Foreign currency exchange contracts [Member] | Not designated as hedging instruments [Member]
Fair value of derivative instruments [Line Items]
Derivatives Instruments, Assets	56,000			56,000			317,000
Other Current Liabilities [Member]
Fair value of derivative instruments [Line Items]
Derivatives Instruments, Liabilities	4,899,000			4,899,000			8,636,000
Other Current Liabilities [Member] | Designated as hedging instruments [Member]
Fair value of derivative instruments [Line Items]
Derivatives Instruments, Liabilities	1,754,000			1,754,000			0
Other Current Liabilities [Member] | Not designated as hedging instruments [Member]
Fair value of derivative instruments [Line Items]
Derivatives Instruments, Liabilities	3,145,000			3,145,000			8,636,000
Other Current Liabilities [Member] | Commodity contracts [Member] | Designated as hedging instruments [Member]
Fair value of derivative instruments [Line Items]
Derivatives Instruments, Liabilities	1,754,000			1,754,000			0
Other Current Liabilities [Member] | Commodity contracts [Member] | Not designated as hedging instruments [Member]
Fair value of derivative instruments [Line Items]
Derivatives Instruments, Liabilities	2,921,000			2,921,000			5,432,000
Other Current Liabilities [Member] | Interest rate contracts [Member] | Designated as hedging instruments [Member]
Fair value of derivative instruments [Line Items]
Derivatives Instruments, Liabilities	0			0			0
Other Current Liabilities [Member] | Foreign currency exchange contracts [Member] | Not designated as hedging instruments [Member]
Fair value of derivative instruments [Line Items]
Derivatives Instruments, Liabilities	224,000			224,000			3,204,000
Other Noncurrent Liabilities [Member]
Fair value of derivative instruments [Line Items]
Derivatives Instruments, Liabilities							1,384,000
Other Noncurrent Liabilities [Member] | Designated as hedging instruments [Member]
Fair value of derivative instruments [Line Items]
Derivatives Instruments, Liabilities							1,384,000
Other Noncurrent Liabilities [Member] | Not designated as hedging instruments [Member]
Fair value of derivative instruments [Line Items]
Derivatives Instruments, Liabilities							0
Other Noncurrent Liabilities [Member] | Commodity contracts [Member] | Designated as hedging instruments [Member]
Fair value of derivative instruments [Line Items]
Derivatives Instruments, Liabilities							0
Other Noncurrent Liabilities [Member] | Commodity contracts [Member] | Not designated as hedging instruments [Member]
Fair value of derivative instruments [Line Items]
Derivatives Instruments, Liabilities							0
Other Noncurrent Liabilities [Member] | Interest rate contracts [Member] | Designated as hedging instruments [Member]
Fair value of derivative instruments [Line Items]
Derivatives Instruments, Liabilities							1,384,000
Other Noncurrent Liabilities [Member] | Foreign currency exchange contracts [Member] | Not designated as hedging instruments [Member]
Fair value of derivative instruments [Line Items]
Derivatives Instruments, Liabilities							$ 0