Summary of Marketable Securities by Asset Type (Detail) - USD ($) $ in Thousands	Feb. 28, 2018	May 31, 2017
Schedule of Available-for-sale Securities [Line Items]
Available-for-Sale Securities, Amortized Cost	$ 176,380	$ 163,113
Available-for-Sale Securities, Gross Unrealized Gains	5,257	4,847
Available-for-Sale Securities, Gross Unrealized Losses	(3,837)	(3,505)
Available-for-Sale Securities, Fair Value (Net Carrying Amount)	177,800	164,455
Equity securities
Schedule of Available-for-sale Securities [Line Items]
Available-for-Sale Securities, Amortized Cost	153,075	140,231
Available-for-Sale Securities, Gross Unrealized Gains	5,179	4,630
Available-for-Sale Securities, Gross Unrealized Losses	(3,237)	(3,322)
Available-for-Sale Securities, Fair Value (Net Carrying Amount)	155,017	141,539
Equity securities \| Stocks \| Domestic
Schedule of Available-for-sale Securities [Line Items]
Available-for-Sale Securities, Amortized Cost	1,235	2,391
Available-for-Sale Securities, Gross Unrealized Gains	76	76
Available-for-Sale Securities, Fair Value (Net Carrying Amount)	1,311	2,467
Equity securities \| Mutual funds \| Domestic
Schedule of Available-for-sale Securities [Line Items]
Available-for-Sale Securities, Amortized Cost	106,443	102,671
Available-for-Sale Securities, Gross Unrealized Gains	1,874	2,084
Available-for-Sale Securities, Gross Unrealized Losses	(2,938)	(3,118)
Available-for-Sale Securities, Fair Value (Net Carrying Amount)	105,379	101,637
Equity securities \| Mutual funds \| Foreign
Schedule of Available-for-sale Securities [Line Items]
Available-for-Sale Securities, Amortized Cost	45,397	35,169
Available-for-Sale Securities, Gross Unrealized Gains	3,229	2,470
Available-for-Sale Securities, Gross Unrealized Losses	(299)	(204)
Available-for-Sale Securities, Fair Value (Net Carrying Amount)	48,327	37,435
Fixed maturity
Schedule of Available-for-sale Securities [Line Items]
Available-for-Sale Securities, Amortized Cost	23,305	22,882
Available-for-Sale Securities, Gross Unrealized Gains	78	217
Available-for-Sale Securities, Gross Unrealized Losses	(600)	(183)
Available-for-Sale Securities, Fair Value (Net Carrying Amount)	22,783	22,916
Fixed maturity \| U.S. Treasury and other government
Schedule of Available-for-sale Securities [Line Items]
Available-for-Sale Securities, Amortized Cost	22,773	22,176
Available-for-Sale Securities, Gross Unrealized Gains	29	120
Available-for-Sale Securities, Gross Unrealized Losses	(593)	(177)
Available-for-Sale Securities, Fair Value (Net Carrying Amount)	22,209	22,119
Corporate bonds
Schedule of Available-for-sale Securities [Line Items]
Available-for-Sale Securities, Amortized Cost	532	706
Available-for-Sale Securities, Gross Unrealized Gains	49	97
Available-for-Sale Securities, Gross Unrealized Losses	(7)	(6)
Available-for-Sale Securities, Fair Value (Net Carrying Amount)	$ 574	$ 797

Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 25
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=75030427&loc=d3e22054-111558

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
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-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27232-111563

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
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-Section 45
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Reference 1: http://www.xbrl.org/2003/role/presentationRef
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