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Investments - Gross Unrealized Gains and (Losses) Aggregated by Investment Category (Detail) - USD ($) $ in Thousands | Sep. 30, 2018 | Dec. 31, 2017
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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

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- 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 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 + Details
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- Definition Amount, before tax, of unrealized loss 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 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 (c) -URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563 + Details
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- Definition Amortized cost of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale). + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563 + Details
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