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Investments - Gross Unrealized Gains and (Losses) Aggregated by Investment Category (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Investments, Cost	$ 209,453	$ 189,538
Investments, Gross Unrealized Gains	108	35
Investments, Gross Unrealized (Losses)	(127)	(110)
Investments, Estimated Fair Value	209,434	189,463
Time Deposits and Certificates of Deposit [Member]
Schedule of Available-for-sale Securities [Line Items]
Investments, Cost	9,756	23,818
Investments, Gross Unrealized Gains	1
Investments, Estimated Fair Value	9,757	23,818
Bankers Acceptance Drafts [Member]
Schedule of Available-for-sale Securities [Line Items]
Investments, Cost	5,330	1,439
Investments, Estimated Fair Value	5,330	1,439
Asset-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Investments, Cost	37,017	36,847
Investments, Gross Unrealized Gains	15	6
Investments, Gross Unrealized (Losses)	(42)	(44)
Investments, Estimated Fair Value	36,990	36,809
Commercial Paper [Member]
Schedule of Available-for-sale Securities [Line Items]
Investments, Cost	13,810	24,423
Investments, Gross Unrealized (Losses)	(60)	(42)
Investments, Estimated Fair Value	13,750	24,381
Corporate Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Investments, Cost	77,788	46,700
Investments, Gross Unrealized Gains	58	21
Investments, Gross Unrealized (Losses)	(25)	(14)
Investments, Estimated Fair Value	77,821	46,707
Municipal Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Investments, Cost	1,970	591
Investments, Estimated Fair Value	1,970	591
Promissory Note [Member]
Schedule of Available-for-sale Securities [Line Items]
Investments, Cost		675
Investments, Estimated Fair Value		675
U.S. Treasury Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Investments, Cost	28,054	25,414
Investments, Gross Unrealized Gains	24
Investments, Estimated Fair Value	28,078	25,414
U.S. Agency Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Investments, Cost	35,728	29,631
Investments, Gross Unrealized Gains	10	8
Investments, Gross Unrealized (Losses)		(10)
Investments, Estimated Fair Value	35,738	29,629
Group Insurance Contracts [Member] | Long Term Investments [Member]
Schedule of Available-for-sale Securities [Line Items]
Investments, Cost	6,006	5,432
Investments, Gross Unrealized Gains	249	126
Investments, Estimated Fair Value	$ 6,255	$ 5,558

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)
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-Section 50
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-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

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-Topic 320
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-Paragraph 2
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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

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