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Investments - Investments Classified as Short-Term Available-for-Sale Investments (Detail) - USD ($) $ in Thousands | Jun. 30, 2018 | Dec. 31, 2017
--- | --- | ---
Schedule of Available-for-sale Securities [Line Items] |  |
Fair value of available-for-sale investments | $ 203,686 | $ 209,434
Time Deposits and Certificates of Deposit [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Fair value of available-for-sale investments | 18,455 | 9,757
Bankers Acceptance Drafts [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Fair value of available-for-sale investments | 2,522 | 5,330
Asset-Backed Securities [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Fair value of available-for-sale investments | 34,727 | 36,990
Commercial Paper [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Fair value of available-for-sale investments | 25,925 | 13,750
Corporate Obligations [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Fair value of available-for-sale investments | 66,663 | 77,821
Municipal Bonds [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Fair value of available-for-sale investments | 2,615 | 1,970
U.S. Treasury Obligations [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Fair value of available-for-sale investments | 31,434 | 28,078
U.S. Agency Obligations [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Fair value of available-for-sale investments | $ 21,345 | $ 35,738

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