Summary of Marketable Securities by Asset Type (Detail) (USD $) In Thousands, unless otherwise specified	Aug. 31, 2012	May 31, 2012
Schedule of Available-for-sale Securities [Line Items]
Available-for-Sale Securities, Amortized Cost	$ 123,680	$ 117,732
Available-for-Sale Securities, Gross Unrealized Gains	4,263	5,475
Available-for-Sale Securities, Gross Unrealized Losses	(986)	(3,078)
Available-for-Sale Securities, Fair Value (Net Carrying Amount)	126,957	120,129
Equity securities
Schedule of Available-for-sale Securities [Line Items]
Available-for-Sale Securities, Amortized Cost	86,716	82,131
Available-for-Sale Securities, Gross Unrealized Gains	3,443	4,490
Available-for-Sale Securities, Gross Unrealized Losses	(968)	(3,059)
Available-for-Sale Securities, Fair Value (Net Carrying Amount)	89,191	83,562
Equity securities | Stocks | Foreign
Schedule of Available-for-sale Securities [Line Items]
Available-for-Sale Securities, Amortized Cost	1,321	1,016
Available-for-Sale Securities, Gross Unrealized Gains	234	79
Available-for-Sale Securities, Gross Unrealized Losses	(1)
Available-for-Sale Securities, Fair Value (Net Carrying Amount)	1,554	1,095
Equity securities | Stocks | Domestic
Schedule of Available-for-sale Securities [Line Items]
Available-for-Sale Securities, Amortized Cost	26,657	24,380
Available-for-Sale Securities, Gross Unrealized Gains	1,877	2,776
Available-for-Sale Securities, Gross Unrealized Losses	(922)	(1,046)
Available-for-Sale Securities, Fair Value (Net Carrying Amount)	27,612	26,110
Equity securities | Mutual funds | Foreign
Schedule of Available-for-sale Securities [Line Items]
Available-for-Sale Securities, Amortized Cost	19,053	17,489
Available-for-Sale Securities, Gross Unrealized Gains	1,167	521
Available-for-Sale Securities, Gross Unrealized Losses	(5)	(936)
Available-for-Sale Securities, Fair Value (Net Carrying Amount)	20,215	17,074
Equity securities | Mutual funds | Domestic
Schedule of Available-for-sale Securities [Line Items]
Available-for-Sale Securities, Amortized Cost	39,685	39,246
Available-for-Sale Securities, Gross Unrealized Gains	165	1,114
Available-for-Sale Securities, Gross Unrealized Losses	(40)	(1,077)
Available-for-Sale Securities, Fair Value (Net Carrying Amount)	39,810	39,283
Fixed maturity
Schedule of Available-for-sale Securities [Line Items]
Available-for-Sale Securities, Amortized Cost	36,964	35,601
Available-for-Sale Securities, Gross Unrealized Gains	820	985
Available-for-Sale Securities, Gross Unrealized Losses	(18)	(19)
Available-for-Sale Securities, Fair Value (Net Carrying Amount)	37,766	36,567
Fixed maturity | U.S. Treasury and other government
Schedule of Available-for-sale Securities [Line Items]
Available-for-Sale Securities, Amortized Cost	20,641	19,347
Available-for-Sale Securities, Gross Unrealized Gains	484	530
Available-for-Sale Securities, Gross Unrealized Losses	(17)	(12)
Available-for-Sale Securities, Fair Value (Net Carrying Amount)	21,108	19,865
Fixed maturity | Kemrock convertible bonds
Schedule of Available-for-sale Securities [Line Items]
Available-for-Sale Securities, Amortized Cost	14,273	13,670
Available-for-Sale Securities, Fair Value (Net Carrying Amount)	14,273	13,670
Fixed maturity | Corporate bonds
Schedule of Available-for-sale Securities [Line Items]
Available-for-Sale Securities, Amortized Cost	1,832	2,305
Available-for-Sale Securities, Gross Unrealized Gains	268	349
Available-for-Sale Securities, Gross Unrealized Losses		(5)
Available-for-Sale Securities, Fair Value (Net Carrying Amount)	2,100	2,649
Fixed maturity | Foreign bonds
Schedule of Available-for-sale Securities [Line Items]
Available-for-Sale Securities, Amortized Cost	38	38
Available-for-Sale Securities, Gross Unrealized Gains	3	1
Available-for-Sale Securities, Fair Value (Net Carrying Amount)	41	39
Fixed maturity | Mortgage-backed securities
Schedule of Available-for-sale Securities [Line Items]
Available-for-Sale Securities, Amortized Cost	180	241
Available-for-Sale Securities, Gross Unrealized Gains	65	105
Available-for-Sale Securities, Gross Unrealized Losses	(1)	(2)
Available-for-Sale Securities, Fair Value (Net Carrying Amount)	$ 244	$ 344