Summary of Marketable Securities by Asset Type (Detail) - USD ($) $ in Thousands	Nov. 30, 2017	May 31, 2017
Schedule of Available-for-sale Securities [Line Items]
Available-for-Sale Securities, Amortized Cost	$ 168,890	$ 163,113
Available-for-Sale Securities, Gross Unrealized Gains	7,471	4,847
Available-for-Sale Securities, Gross Unrealized Losses	(2,602)	(3,505)
Available-for-Sale Securities, Fair Value (Net Carrying Amount)	173,759	164,455
Equity securities
Schedule of Available-for-sale Securities [Line Items]
Available-for-Sale Securities, Amortized Cost	144,672	140,231
Available-for-Sale Securities, Gross Unrealized Gains	7,320	4,630
Available-for-Sale Securities, Gross Unrealized Losses	(2,253)	(3,322)
Available-for-Sale Securities, Fair Value (Net Carrying Amount)	149,739	141,539
Equity securities \| Stocks \| Domestic
Schedule of Available-for-sale Securities [Line Items]
Available-for-Sale Securities, Amortized Cost	1,800	2,391
Available-for-Sale Securities, Gross Unrealized Gains	77	76
Available-for-Sale Securities, Fair Value (Net Carrying Amount)	1,877	2,467
Equity securities \| Mutual funds \| Domestic
Schedule of Available-for-sale Securities [Line Items]
Available-for-Sale Securities, Amortized Cost	100,971	102,671
Available-for-Sale Securities, Gross Unrealized Gains	4,282	2,084
Available-for-Sale Securities, Gross Unrealized Losses	(2,042)	(3,118)
Available-for-Sale Securities, Fair Value (Net Carrying Amount)	103,211	101,637
Equity securities \| Mutual funds \| Foreign
Schedule of Available-for-sale Securities [Line Items]
Available-for-Sale Securities, Amortized Cost	41,901	35,169
Available-for-Sale Securities, Gross Unrealized Gains	2,961	2,470
Available-for-Sale Securities, Gross Unrealized Losses	(211)	(204)
Available-for-Sale Securities, Fair Value (Net Carrying Amount)	44,651	37,435
Fixed maturity
Schedule of Available-for-sale Securities [Line Items]
Available-for-Sale Securities, Amortized Cost	24,218	22,882
Available-for-Sale Securities, Gross Unrealized Gains	151	217
Available-for-Sale Securities, Gross Unrealized Losses	(349)	(183)
Available-for-Sale Securities, Fair Value (Net Carrying Amount)	24,020	22,916
Fixed maturity \| U.S. Treasury and other government
Schedule of Available-for-sale Securities [Line Items]
Available-for-Sale Securities, Amortized Cost	23,567	22,176
Available-for-Sale Securities, Gross Unrealized Gains	66	120
Available-for-Sale Securities, Gross Unrealized Losses	(343)	(177)
Available-for-Sale Securities, Fair Value (Net Carrying Amount)	23,290	22,119
Corporate bonds
Schedule of Available-for-sale Securities [Line Items]
Available-for-Sale Securities, Amortized Cost	651	706
Available-for-Sale Securities, Gross Unrealized Gains	85	97
Available-for-Sale Securities, Gross Unrealized Losses	(6)	(6)
Available-for-Sale Securities, Fair Value (Net Carrying Amount)	$ 730	$ 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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