Summary of Marketable Securities by Asset Type (Detail) - USD ($) $ in Thousands	Aug. 31, 2017	May 31, 2017
Schedule of Available-for-sale Securities [Line Items]
Available-for-Sale Securities, Amortized Cost	$ 144,769	$ 163,113
Available-for-Sale Securities, Gross Unrealized Gains	4,525	4,847
Available-for-Sale Securities, Gross Unrealized Losses	(3,381)	(3,505)
Available-for-Sale Securities, Fair Value (Net Carrying Amount)	145,913	164,455
Equity securities
Schedule of Available-for-sale Securities [Line Items]
Available-for-Sale Securities, Amortized Cost	121,939	140,231
Available-for-Sale Securities, Gross Unrealized Gains	4,288	4,630
Available-for-Sale Securities, Gross Unrealized Losses	(3,205)	(3,322)
Available-for-Sale Securities, Fair Value (Net Carrying Amount)	123,022	141,539
Equity securities \| Stocks \| Domestic
Schedule of Available-for-sale Securities [Line Items]
Available-for-Sale Securities, Amortized Cost	2,110	2,391
Available-for-Sale Securities, Gross Unrealized Gains	76	76
Available-for-Sale Securities, Fair Value (Net Carrying Amount)	2,186	2,467
Equity securities \| Mutual funds \| Domestic
Schedule of Available-for-sale Securities [Line Items]
Available-for-Sale Securities, Amortized Cost	92,075	102,671
Available-for-Sale Securities, Gross Unrealized Gains	1,749	2,084
Available-for-Sale Securities, Gross Unrealized Losses	(2,987)	(3,118)
Available-for-Sale Securities, Fair Value (Net Carrying Amount)	90,837	101,637
Equity securities \| Mutual funds \| Foreign
Schedule of Available-for-sale Securities [Line Items]
Available-for-Sale Securities, Amortized Cost	27,754	35,169
Available-for-Sale Securities, Gross Unrealized Gains	2,463	2,470
Available-for-Sale Securities, Gross Unrealized Losses	(218)	(204)
Available-for-Sale Securities, Fair Value (Net Carrying Amount)	29,999	37,435
Fixed maturity
Schedule of Available-for-sale Securities [Line Items]
Available-for-Sale Securities, Amortized Cost	22,830	22,882
Available-for-Sale Securities, Gross Unrealized Gains	237	217
Available-for-Sale Securities, Gross Unrealized Losses	(176)	(183)
Available-for-Sale Securities, Fair Value (Net Carrying Amount)	22,891	22,916
Fixed maturity \| U.S. Treasury and other government
Schedule of Available-for-sale Securities [Line Items]
Available-for-Sale Securities, Amortized Cost	22,170	22,176
Available-for-Sale Securities, Gross Unrealized Gains	140	120
Available-for-Sale Securities, Gross Unrealized Losses	(170)	(177)
Available-for-Sale Securities, Fair Value (Net Carrying Amount)	22,140	22,119
Corporate bonds
Schedule of Available-for-sale Securities [Line Items]
Available-for-Sale Securities, Amortized Cost	660	706
Available-for-Sale Securities, Gross Unrealized Gains	97	97
Available-for-Sale Securities, Gross Unrealized Losses	(6)	(6)
Available-for-Sale Securities, Fair Value (Net Carrying Amount)	$ 751	$ 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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Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
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Reference 1: http://www.xbrl.org/2003/role/presentationRef
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