Assets and Liabilities Measured at Fair Value on Recurring Basis and Categorized using Fair Value Hierarchy (Detail) (Fair Value, Measurements, Recurring, USD $) In Thousands, unless otherwise specified	Aug. 31, 2012	May 31, 2012
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets (liabilities) at fair value	$ 123,301	$ 125,645
U.S. Treasury and other government
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value on recurring basis	21,108	19,865
Foreign bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value on recurring basis	41	39
Mortgage-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value on recurring basis	244	344
Corporate bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value on recurring basis	2,100	2,649
Stocks | Foreign
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value on recurring basis	1,554	1,095
Stocks | Domestic
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value on recurring basis	27,612	26,110
Mutual funds | Foreign
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value on recurring basis	20,215	17,074
Mutual funds | Domestic
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value on recurring basis	39,810	39,283
Cross-currency swap
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Foreign currency contracts, fair value of liability	(4,503)	(2,159)
Conversion option, Kemrock 5.5% bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value on recurring basis	847	9,031
Investment in Kemrock convertible debt
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value on recurring basis	14,273	13,670
Foreign currency forward contract
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Foreign currency contracts, fair value of liability		(1,356)
Quoted Prices in Active Markets for Identical Assets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets (liabilities) at fair value	29,166	27,205
Quoted Prices in Active Markets for Identical Assets (Level 1) | Stocks | Foreign
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value on recurring basis	1,554	1,095
Quoted Prices in Active Markets for Identical Assets (Level 1) | Stocks | Domestic
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value on recurring basis	27,612	26,110
Significant Other Observable Inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets (liabilities) at fair value	93,288	89,409
Significant Other Observable Inputs (Level 2) | U.S. Treasury and other government
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value on recurring basis	21,108	19,865
Significant Other Observable Inputs (Level 2) | Foreign bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value on recurring basis	41	39
Significant Other Observable Inputs (Level 2) | Mortgage-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value on recurring basis	244	344
Significant Other Observable Inputs (Level 2) | Corporate bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value on recurring basis	2,100	2,649
Significant Other Observable Inputs (Level 2) | Mutual funds | Foreign
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value on recurring basis	20,215	17,074
Significant Other Observable Inputs (Level 2) | Mutual funds | Domestic
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value on recurring basis	39,810	39,283
Significant Other Observable Inputs (Level 2) | Cross-currency swap
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Foreign currency contracts, fair value of liability	(4,503)	(2,159)
Significant Other Observable Inputs (Level 2) | Investment in Kemrock convertible debt
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value on recurring basis	14,273	13,670
Significant Other Observable Inputs (Level 2) | Foreign currency forward contract
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Foreign currency contracts, fair value of liability		(1,356)
Significant Unobservable Inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets (liabilities) at fair value	847	9,031
Significant Unobservable Inputs (Level 3) | Conversion option, Kemrock 5.5% bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value on recurring basis	$ 847	$ 9,031