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Investments - Gross Unrealized Gains and (Losses) Aggregated by Investment Category (Detail) - USD ($) $ in Thousands | Mar. 31, 2018 | Dec. 31, 2017
--- | --- | ---
Schedule of Available-for-sale Securities [Line Items] |  |
Investments, Cost | $ 200,695 | $ 209,453
Investments, Gross Unrealized Gains | 82 | 108
Investments, Gross Unrealized (Losses) | (163) | (127)
Investments, Estimated Fair Value | 200,614 | 209,434
Time Deposits and Certificates of Deposit [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Investments, Cost | 10,458 | 9,756
Investments, Gross Unrealized Gains | 1 | 1
Investments, Estimated Fair Value | 10,459 | 9,757
Bankers Acceptance Drafts [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Investments, Cost | 2,179 | 5,330
Investments, Estimated Fair Value | 2,179 | 5,330
Asset-Backed Securities [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Investments, Cost | 38,056 | 37,017
Investments, Gross Unrealized Gains | 13 | 15
Investments, Gross Unrealized (Losses) | (57) | (42)
Investments, Estimated Fair Value | 38,012 | 36,990
Commercial Paper [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Investments, Cost | 16,065 | 13,810
Investments, Gross Unrealized (Losses) | (55) | (60)
Investments, Estimated Fair Value | 16,010 | 13,750
Corporate Obligations [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Investments, Cost | 78,778 | 77,788
Investments, Gross Unrealized Gains | 38 | 58
Investments, Gross Unrealized (Losses) | (49) | (25)
Investments, Estimated Fair Value | 78,767 | 77,821
Municipal Bonds [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Investments, Cost | 2,620 | 1,970
Investments, Gross Unrealized (Losses) | (2) |
Investments, Estimated Fair Value | 2,618 | 1,970
U.S. Treasury Obligations [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Investments, Cost | 30,196 | 28,054
Investments, Gross Unrealized Gains | 24 | 24
Investments, Estimated Fair Value | 30,220 | 28,078
U.S. Agency Obligations [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Investments, Cost | 22,343 | 35,728
Investments, Gross Unrealized Gains | 6 | 10
Investments, Estimated Fair Value | 22,349 | 35,738
Group Insurance Contracts [Member] | Long Term Investments [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Investments, Cost | 6,382 | 6,006
Investments, Gross Unrealized Gains | 59 | 249
Investments, Estimated Fair Value | $ 6,441 | $ 6,255

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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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