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Investments - Gross Unrealized Gains and (Losses) Aggregated by Investment Category (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]
Investments, Cost	$ 200,695	$ 209,453
Investments, Gross Unrealized Gains	82	108
Investments, Gross Unrealized (Losses)	(163)	(127)
Investments, Estimated Fair Value	200,614	209,434
Time Deposits and Certificates of Deposit [Member]
Schedule of Available-for-sale Securities [Line Items]
Investments, Cost	10,458	9,756
Investments, Gross Unrealized Gains	1	1
Investments, Estimated Fair Value	10,459	9,757
Bankers Acceptance Drafts [Member]
Schedule of Available-for-sale Securities [Line Items]
Investments, Cost	2,179	5,330
Investments, Estimated Fair Value	2,179	5,330
Asset-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Investments, Cost	38,056	37,017
Investments, Gross Unrealized Gains	13	15
Investments, Gross Unrealized (Losses)	(57)	(42)
Investments, Estimated Fair Value	38,012	36,990
Commercial Paper [Member]
Schedule of Available-for-sale Securities [Line Items]
Investments, Cost	16,065	13,810
Investments, Gross Unrealized (Losses)	(55)	(60)
Investments, Estimated Fair Value	16,010	13,750
Corporate Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Investments, Cost	78,778	77,788
Investments, Gross Unrealized Gains	38	58
Investments, Gross Unrealized (Losses)	(49)	(25)
Investments, Estimated Fair Value	78,767	77,821
Municipal Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Investments, Cost	2,620	1,970
Investments, Gross Unrealized (Losses)	(2)
Investments, Estimated Fair Value	2,618	1,970
U.S. Treasury Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Investments, Cost	30,196	28,054
Investments, Gross Unrealized Gains	24	24
Investments, Estimated Fair Value	30,220	28,078
U.S. Agency Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Investments, Cost	22,343	35,728
Investments, Gross Unrealized Gains	6	10
Investments, Estimated Fair Value	22,349	35,738
Group Insurance Contracts [Member] | Long Term Investments [Member]
Schedule of Available-for-sale Securities [Line Items]
Investments, Cost	6,382	6,006
Investments, Gross Unrealized Gains	59	249
Investments, Estimated Fair Value	$ 6,441	$ 6,255

X

- Definition

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

+ References

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
-Paragraph 5
-Subparagraph (aa)
-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
-SubTopic 10
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=82896770&loc=d3e26610-111562

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Amount before tax of unrealized gain in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563

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Amount before tax of unrealized loss in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

+ References

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

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
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