Investments - Gross Unrealized Gains and (Losses) Aggregated by Investment Category (Detail) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]
Investments, Cost	$ 204,082	$ 209,453
Investments, Gross Unrealized Gains	92	108
Investments, Gross Unrealized (Losses)	(488)	(127)
Investments, Estimated Fair Value	203,686	209,434
Time Deposits and Certificates of Deposit [Member]
Schedule of Available-for-sale Securities [Line Items]
Investments, Cost	18,452	9,756
Investments, Gross Unrealized Gains	3	1
Investments, Estimated Fair Value	18,455	9,757
Bankers Acceptance Drafts [Member]
Schedule of Available-for-sale Securities [Line Items]
Investments, Cost	2,522	5,330
Investments, Estimated Fair Value	2,522	5,330
Asset-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Investments, Cost	34,760	37,017
Investments, Gross Unrealized Gains	9	15
Investments, Gross Unrealized (Losses)	(42)	(42)
Investments, Estimated Fair Value	34,727	36,990
Commercial Paper [Member]
Schedule of Available-for-sale Securities [Line Items]
Investments, Cost	26,337	13,810
Investments, Gross Unrealized (Losses)	(412)	(60)
Investments, Estimated Fair Value	25,925	13,750
Corporate Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Investments, Cost	66,632	77,788
Investments, Gross Unrealized Gains	59	58
Investments, Gross Unrealized (Losses)	(28)	(25)
Investments, Estimated Fair Value	66,663	77,821
Municipal Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Investments, Cost	2,620	1,970
Investments, Gross Unrealized (Losses)	(5)
Investments, Estimated Fair Value	2,615	1,970
U.S. Treasury Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Investments, Cost	31,418	28,054
Investments, Gross Unrealized Gains	16	24
Investments, Estimated Fair Value	31,434	28,078
U.S. Agency Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Investments, Cost	21,341	35,728
Investments, Gross Unrealized Gains	5	10
Investments, Gross Unrealized (Losses)	(1)
Investments, Estimated Fair Value	21,345	35,738
Group Insurance Contracts [Member] \| Long Term Investments [Member]
Schedule of Available-for-sale Securities [Line Items]
Investments, Cost	6,051	6,006
Investments, Gross Unrealized Gains	25	249
Investments, Estimated Fair Value	$ 6,076	$ 6,255

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

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- Definition

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://fasb.org/us-gaap/role/ref/legacyRef
-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

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This item represents the cost of debt and equity securities, which are categorized neither as held-to-maturity nor trading, net of adjustments including accretion, amortization, collection of cash, previous other-than-temporary impairments recognized in earnings (less any cumulative-effect adjustments recognized, as defined), and fair value hedge accounting adjustments, if any.

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

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