Investments - Gross Unrealized Gains and (Losses) Aggregated by Investment Category (Detail) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Schedule of Available-for-sale Securities [Line Items]
Investments, Cost	$ 44,569	$ 73,994
Investments, Gross Unrealized Gains	4	7
Investments, Gross Unrealized (Losses)	(247)	(175)
Investments, Estimated Fair Value	44,326	73,826
Time Deposits and Certificates of Deposit [Member]
Schedule of Available-for-sale Securities [Line Items]
Investments, Cost	99	102
Investments, Gross Unrealized Gains	2
Investments, Estimated Fair Value	101	102
Bankers' Acceptance Drafts [Member]
Schedule of Available-for-sale Securities [Line Items]
Investments, Cost	1,722	989
Investments, Estimated Fair Value	1,722	989
Asset-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Investments, Cost		9,121
Investments, Gross Unrealized Gains		1
Investments, Gross Unrealized (Losses)		(9)
Investments, Estimated Fair Value		9,113
Commercial Paper [Member]
Schedule of Available-for-sale Securities [Line Items]
Investments, Cost	33,752	19,504
Investments, Gross Unrealized Gains	1
Investments, Gross Unrealized (Losses)	(244)	(145)
Investments, Estimated Fair Value	33,509	19,359
Corporate Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Investments, Cost	8,996	9,367
Investments, Gross Unrealized Gains	1
Investments, Gross Unrealized (Losses)	(3)	(15)
Investments, Estimated Fair Value	8,994	9,352
U.S. Treasury Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Investments, Cost		13,294
Investments, Gross Unrealized Gains		4
Investments, Estimated Fair Value		13,298
U.S. Agency Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Investments, Cost		21,617
Investments, Gross Unrealized Gains		2
Investments, Gross Unrealized (Losses)		(6)
Investments, Estimated Fair Value		21,613
Group Insurance Contracts [Member] \| Long Term Investments [Member]
Schedule of Available-for-sale Securities [Line Items]
Investments, Cost	5,582	5,546
Investments, Gross Unrealized Gains	368	344
Investments, Estimated Fair Value	$ 5,950	$ 5,890

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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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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