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Investments - Gross Unrealized Gains and (Losses) Aggregated by Investment Category (Detail) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Investments, Cost	$ 228,592	$ 189,538
Investments, Gross Unrealized Gains	137	35
Investments, Gross Unrealized (Losses)	(98)	(110)
Investments, Estimated Fair Value	228,631	189,463
Time Deposits and Certificates of Deposit [Member]
Schedule of Available-for-sale Securities [Line Items]
Investments, Cost	21,501	23,818
Investments, Gross Unrealized Gains	2
Investments, Estimated Fair Value	21,503	23,818
Bankers Acceptance Drafts [Member]
Schedule of Available-for-sale Securities [Line Items]
Investments, Cost	3,452	1,439
Investments, Estimated Fair Value	3,452	1,439
Asset-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Investments, Cost	39,709	36,847
Investments, Gross Unrealized Gains	17	6
Investments, Gross Unrealized (Losses)	(24)	(44)
Investments, Estimated Fair Value	39,702	36,809
Commercial Paper [Member]
Schedule of Available-for-sale Securities [Line Items]
Investments, Cost	21,278	24,423
Investments, Gross Unrealized (Losses)	(63)	(42)
Investments, Estimated Fair Value	21,215	24,381
Corporate Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Investments, Cost	80,020	46,700
Investments, Gross Unrealized Gains	87	21
Investments, Gross Unrealized (Losses)	(8)	(14)
Investments, Estimated Fair Value	80,099	46,707
Municipal Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Investments, Cost	45	591
Investments, Estimated Fair Value	45	591
Promissory Note [Member]
Schedule of Available-for-sale Securities [Line Items]
Investments, Cost	1,075	675
Investments, Estimated Fair Value	1,075	675
U.S. Treasury Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Investments, Cost	25,756	25,414
Investments, Gross Unrealized Gains	21
Investments, Estimated Fair Value	25,777	25,414
U.S. Agency Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Investments, Cost	35,756	29,631
Investments, Gross Unrealized Gains	10	8
Investments, Gross Unrealized (Losses)	(3)	(10)
Investments, Estimated Fair Value	35,763	29,629
Group Insurance Contracts [Member] | Long Term Investments [Member]
Schedule of Available-for-sale Securities [Line Items]
Investments, Cost	6,193	6,276
Investments, Gross Unrealized (Losses)		(718)
Investments, Estimated Fair Value	$ 6,193	$ 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)
-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
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-Section 50
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Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
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-Topic 320
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-Section 45
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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

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-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

Reference 1: http://www.xbrl.org/2003/role/presentationRef
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+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
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-Name Accounting Standards Codification
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