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Investments - Gross Unrealized Gains and (Losses) Aggregated by Investment Category (Detail) - USD ($) $ in Thousands | Dec. 31, 2018 | Dec. 31, 2017
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Schedule of Available-for-sale Securities [Line Items] |  |
Investments, Cost | $ 73,994 | $ 209,453
Investments, Gross Unrealized Gains | 7 | 108
Investments, Gross Unrealized (Losses) | (175) | (127)
Investments, Estimated Fair Value | 73,826 | 209,434
Time Deposits and Certificates of Deposit [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Investments, Cost | 102 | 9,756
Investments, Gross Unrealized Gains |  | 1
Investments, Estimated Fair Value | 102 | 9,757
Bankers' Acceptance Drafts [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Investments, Cost | 989 | 5,330
Investments, Estimated Fair Value | 989 | 5,330
Asset-Backed Securities [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Investments, Cost | 9,121 | 37,017
Investments, Gross Unrealized Gains | 1 | 15
Investments, Gross Unrealized (Losses) | (9) | (42)
Investments, Estimated Fair Value | 9,113 | 36,990
Commercial Paper [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Investments, Cost | 19,504 | 13,810
Investments, Gross Unrealized (Losses) | (145) | (60)
Investments, Estimated Fair Value | 19,359 | 13,750
Corporate Obligations [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Investments, Cost | 9,367 | 77,788
Investments, Gross Unrealized Gains |  | 58
Investments, Gross Unrealized (Losses) | (15) | (25)
Investments, Estimated Fair Value | 9,352 | 77,821
Municipal Bonds [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Investments, Cost |  | 1,970
Investments, Estimated Fair Value |  | 1,970
U.S. Treasury Obligations [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Investments, Cost | 13,294 | 28,054
Investments, Gross Unrealized Gains | 4 | 24
Investments, Estimated Fair Value | 13,298 | 28,078
U.S. Agency Obligations [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Investments, Cost | 21,617 | 35,728
Investments, Gross Unrealized Gains | 2 | 10
Investments, Gross Unrealized (Losses) | (6) |
Investments, Estimated Fair Value | 21,613 | 35,738
Group Insurance Contracts [Member] | Long Term Investments [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Investments, Cost | 5,546 | 6,006
Investments, Gross Unrealized Gains | 344 | 249
Investments, Estimated Fair Value | $ 5,890 | $ 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 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 No definition available. + 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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