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Investments - Gross Unrealized Gains and (Losses) Aggregated by Investment Category (Detail) - USD ($) $ in Thousands | Dec. 31, 2017 | Dec. 31, 2016
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Schedule of Available-for-sale Securities [Line Items] |  |
Investments, Cost | $ 209,453 | $ 189,538
Investments, Gross Unrealized Gains | 108 | 35
Investments, Gross Unrealized (Losses) | (127) | (110)
Investments, Estimated Fair Value | 209,434 | 189,463
Time Deposits and Certificates of Deposit [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Investments, Cost | 9,756 | 23,818
Investments, Gross Unrealized Gains | 1 |
Investments, Estimated Fair Value | 9,757 | 23,818
Bankers Acceptance Drafts [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Investments, Cost | 5,330 | 1,439
Investments, Estimated Fair Value | 5,330 | 1,439
Asset-Backed Securities [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Investments, Cost | 37,017 | 36,847
Investments, Gross Unrealized Gains | 15 | 6
Investments, Gross Unrealized (Losses) | (42) | (44)
Investments, Estimated Fair Value | 36,990 | 36,809
Commercial Paper [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Investments, Cost | 13,810 | 24,423
Investments, Gross Unrealized (Losses) | (60) | (42)
Investments, Estimated Fair Value | 13,750 | 24,381
Corporate Obligations [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Investments, Cost | 77,788 | 46,700
Investments, Gross Unrealized Gains | 58 | 21
Investments, Gross Unrealized (Losses) | (25) | (14)
Investments, Estimated Fair Value | 77,821 | 46,707
Municipal Bonds [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Investments, Cost | 1,970 | 591
Investments, Estimated Fair Value | 1,970 | 591
Promissory Note [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Investments, Cost |  | 675
Investments, Estimated Fair Value |  | 675
U.S. Treasury Obligations [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Investments, Cost | 28,054 | 25,414
Investments, Gross Unrealized Gains | 24 |
Investments, Estimated Fair Value | 28,078 | 25,414
U.S. Agency Obligations [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Investments, Cost | 35,728 | 29,631
Investments, Gross Unrealized Gains | 10 | 8
Investments, Gross Unrealized (Losses) |  | (10)
Investments, Estimated Fair Value | 35,738 | 29,629
Group Insurance Contracts [Member] | Long Term Investments [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Investments, Cost | 6,006 | 5,432
Investments, Gross Unrealized Gains | 249 | 126
Investments, Estimated Fair Value | $ 6,255 | $ 5,558

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- 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 -SubTopic 10 -Section 50 -Paragraph 5 -Subparagraph (aa) -URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27232-111563 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=82896770&loc=d3e26610-111562 + Details
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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://www.xbrl.org/2003/role/presentationRef -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 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 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563 + Details
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