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Investments - Investments Classified as Short-Term Available-for-Sale Investments (Detail) - USD ($) $ in Thousands | Mar. 31, 2019 | Dec. 31, 2018
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Schedule of Available-for-sale Securities [Line Items] |  |
Fair value of available-for-sale investments | $ 44,326 | $ 73,826
Time Deposits and Certificates of Deposit [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Fair value of available-for-sale investments | 101 | 102
Bankers' Acceptance Drafts [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Fair value of available-for-sale investments | 1,722 | 989
Asset-Backed Securities [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Fair value of available-for-sale investments |  | 9,113
Commercial Paper [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Fair value of available-for-sale investments | 33,509 | 19,359
Corporate Obligations [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Fair value of available-for-sale investments | $ 8,994 | 9,352
U.S. Treasury Obligations [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Fair value of available-for-sale investments |  | 13,298
U.S. Agency Obligations [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Fair value of available-for-sale investments |  | $ 21,613

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- Definition Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading. + References No definition available. + Details
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