Other Financial Instruments and Fair Value Measurements (Details 1) (Fair value measurements recurring [Member], USD $) In Thousands, unless otherwise specified	Jan. 31, 2012	Apr. 30, 2011
Financial assets (liabilities) measured at fair value on a recurring basis
Marketable securities	$ 0	$ 18,600
Total Financial Assets Measured at Fair Value	39,403	69,175
Equity mutual funds [Member]
Financial assets (liabilities) measured at fair value on a recurring basis
Other investments	13,152	14,011
Municipal obligations [Member]
Financial assets (liabilities) measured at fair value on a recurring basis
Other investments	20,575	20,042
Other investments [Member]
Financial assets (liabilities) measured at fair value on a recurring basis
Other investments	7,946	7,507
Commodity contracts [Member]
Financial assets (liabilities) measured at fair value on a recurring basis
Derivative	(2,102)	7,863
Foreign currency exchange contracts [Member]
Financial assets (liabilities) measured at fair value on a recurring basis
Derivative	(168)	(2,887)
Interest rate contracts [Member]
Financial assets (liabilities) measured at fair value on a recurring basis
Derivative	0	4,039
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]
Financial assets (liabilities) measured at fair value on a recurring basis
Marketable securities	0
Total Financial Assets Measured at Fair Value	12,938
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Equity mutual funds [Member]
Financial assets (liabilities) measured at fair value on a recurring basis
Other investments	13,152
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Municipal obligations [Member]
Financial assets (liabilities) measured at fair value on a recurring basis
Other investments	0
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Other investments [Member]
Financial assets (liabilities) measured at fair value on a recurring basis
Other investments	903
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Commodity contracts [Member]
Financial assets (liabilities) measured at fair value on a recurring basis
Derivative	(1,173)
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Foreign currency exchange contracts [Member]
Financial assets (liabilities) measured at fair value on a recurring basis
Derivative	56
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Interest rate contracts [Member]
Financial assets (liabilities) measured at fair value on a recurring basis
Derivative	0
Significant Observable Inputs (Level 2) [Member]
Financial assets (liabilities) measured at fair value on a recurring basis
Marketable securities	0
Total Financial Assets Measured at Fair Value	26,465
Significant Observable Inputs (Level 2) [Member] | Equity mutual funds [Member]
Financial assets (liabilities) measured at fair value on a recurring basis
Other investments	0
Significant Observable Inputs (Level 2) [Member] | Municipal obligations [Member]
Financial assets (liabilities) measured at fair value on a recurring basis
Other investments	20,575
Significant Observable Inputs (Level 2) [Member] | Other investments [Member]
Financial assets (liabilities) measured at fair value on a recurring basis
Other investments	7,043
Significant Observable Inputs (Level 2) [Member] | Commodity contracts [Member]
Financial assets (liabilities) measured at fair value on a recurring basis
Derivative	(929)
Significant Observable Inputs (Level 2) [Member] | Foreign currency exchange contracts [Member]
Financial assets (liabilities) measured at fair value on a recurring basis
Derivative	(224)
Significant Observable Inputs (Level 2) [Member] | Interest rate contracts [Member]
Financial assets (liabilities) measured at fair value on a recurring basis
Derivative	0
Significant Unobservable Inputs (Level 3) [Member]
Financial assets (liabilities) measured at fair value on a recurring basis
Marketable securities	0
Total Financial Assets Measured at Fair Value	0
Significant Unobservable Inputs (Level 3) [Member] | Equity mutual funds [Member]
Financial assets (liabilities) measured at fair value on a recurring basis
Other investments	0
Significant Unobservable Inputs (Level 3) [Member] | Municipal obligations [Member]
Financial assets (liabilities) measured at fair value on a recurring basis
Other investments	0
Significant Unobservable Inputs (Level 3) [Member] | Other investments [Member]
Financial assets (liabilities) measured at fair value on a recurring basis
Other investments	0
Significant Unobservable Inputs (Level 3) [Member] | Commodity contracts [Member]
Financial assets (liabilities) measured at fair value on a recurring basis
Derivative	0
Significant Unobservable Inputs (Level 3) [Member] | Foreign currency exchange contracts [Member]
Financial assets (liabilities) measured at fair value on a recurring basis
Derivative	0
Significant Unobservable Inputs (Level 3) [Member] | Interest rate contracts [Member]
Financial assets (liabilities) measured at fair value on a recurring basis
Derivative	$ 0