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Investments - Gross Unrealized Gains and (Losses) Aggregated by Investment Category (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Short Term Investments [Member]
Schedule of Available-for-sale Securities [Line Items]
Investments, Cost	$ 219,928	$ 209,453
Investments, Gross Unrealized Gains	84	108
Investments, Gross Unrealized (Losses)	(236)	(127)
Investments, Estimated Fair Value	219,776	209,434
Time Deposits and Certificates of Deposit [Member] | Short Term Investments [Member]
Schedule of Available-for-sale Securities [Line Items]
Investments, Cost	14,552	9,756
Investments, Gross Unrealized Gains		1
Investments, Estimated Fair Value	14,552	9,757
Bankers Acceptance Drafts [Member] | Short Term Investments [Member]
Schedule of Available-for-sale Securities [Line Items]
Investments, Cost	2,107	5,330
Investments, Estimated Fair Value	2,107	5,330
Asset-Backed Securities [Member] | Short Term Investments [Member]
Schedule of Available-for-sale Securities [Line Items]
Investments, Cost	38,961	37,017
Investments, Gross Unrealized Gains	11	15
Investments, Gross Unrealized (Losses)	(25)	(42)
Investments, Estimated Fair Value	38,947	36,990
Commercial Paper [Member] | Short Term Investments [Member]
Schedule of Available-for-sale Securities [Line Items]
Investments, Cost	32,238	13,810
Investments, Gross Unrealized (Losses)	(181)	(60)
Investments, Estimated Fair Value	32,057	13,750
Corporate Obligations [Member] | Short Term Investments [Member]
Schedule of Available-for-sale Securities [Line Items]
Investments, Cost	71,931	77,788
Investments, Gross Unrealized Gains	59	58
Investments, Gross Unrealized (Losses)	(19)	(25)
Investments, Estimated Fair Value	71,971	77,821
Municipal Bonds [Member] | Short Term Investments [Member]
Schedule of Available-for-sale Securities [Line Items]
Investments, Cost	1,290	1,970
Investments, Gross Unrealized (Losses)	(2)
Investments, Estimated Fair Value	1,288	1,970
U.S. Treasury Obligations [Member] | Short Term Investments [Member]
Schedule of Available-for-sale Securities [Line Items]
Investments, Cost	28,200	28,054
Investments, Gross Unrealized Gains	10	24
Investments, Gross Unrealized (Losses)	(2)
Investments, Estimated Fair Value	28,208	28,078
U.S. Agency Obligations [Member] | Short Term Investments [Member]
Schedule of Available-for-sale Securities [Line Items]
Investments, Cost	30,649	35,728
Investments, Gross Unrealized Gains	4	10
Investments, Gross Unrealized (Losses)	(7)
Investments, Estimated Fair Value	30,646	35,738
Group Insurance Contracts [Member] | Long Term Investments [Member]
Schedule of Available-for-sale Securities [Line Items]
Investments, Cost	6,010	6,006
Investments, Gross Unrealized Gains		249
Investments, Gross Unrealized (Losses)	(6)
Investments, Estimated Fair Value	$ 6,004	$ 6,255

X

- Definition

Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

+ References

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