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Investments - Gross Unrealized Gains and (Losses) Aggregated by Investment Category (Detail) - USD ($) $ in Thousands | Jun. 30, 2018 | Dec. 31, 2017
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Schedule of Available-for-sale Securities [Line Items] |  |
Investments, Cost | $ 204,082 | $ 209,453
Investments, Gross Unrealized Gains | 92 | 108
Investments, Gross Unrealized (Losses) | (488) | (127)
Investments, Estimated Fair Value | 203,686 | 209,434
Time Deposits and Certificates of Deposit [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Investments, Cost | 18,452 | 9,756
Investments, Gross Unrealized Gains | 3 | 1
Investments, Estimated Fair Value | 18,455 | 9,757
Bankers Acceptance Drafts [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Investments, Cost | 2,522 | 5,330
Investments, Estimated Fair Value | 2,522 | 5,330
Asset-Backed Securities [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Investments, Cost | 34,760 | 37,017
Investments, Gross Unrealized Gains | 9 | 15
Investments, Gross Unrealized (Losses) | (42) | (42)
Investments, Estimated Fair Value | 34,727 | 36,990
Commercial Paper [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Investments, Cost | 26,337 | 13,810
Investments, Gross Unrealized (Losses) | (412) | (60)
Investments, Estimated Fair Value | 25,925 | 13,750
Corporate Obligations [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Investments, Cost | 66,632 | 77,788
Investments, Gross Unrealized Gains | 59 | 58
Investments, Gross Unrealized (Losses) | (28) | (25)
Investments, Estimated Fair Value | 66,663 | 77,821
Municipal Bonds [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Investments, Cost | 2,620 | 1,970
Investments, Gross Unrealized (Losses) | (5) |
Investments, Estimated Fair Value | 2,615 | 1,970
U.S. Treasury Obligations [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Investments, Cost | 31,418 | 28,054
Investments, Gross Unrealized Gains | 16 | 24
Investments, Estimated Fair Value | 31,434 | 28,078
U.S. Agency Obligations [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Investments, Cost | 21,341 | 35,728
Investments, Gross Unrealized Gains | 5 | 10
Investments, Gross Unrealized (Losses) | (1) |
Investments, Estimated Fair Value | 21,345 | 35,738
Group Insurance Contracts [Member] | Long Term Investments [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Investments, Cost | 6,051 | 6,006
Investments, Gross Unrealized Gains | 25 | 249
Investments, Estimated Fair Value | $ 6,076 | $ 6,255

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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://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 This item represents the cost of debt and equity securities, which are categorized neither as held-to-maturity nor trading, net of adjustments including accretion, amortization, collection of cash, previous other-than-temporary impairments recognized in earnings (less any cumulative-effect adjustments recognized, as defined), and fair value hedge accounting adjustments, if any. + References No definition available. + Details
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