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Balance Sheet Components - Summary of Contractual Maturities Available-for-Sale Investments at Fair Value(Detail) - USD ($) $ in Thousands | Dec. 31, 2017 | Dec. 31, 2016
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Investments Schedule [Abstract] |  |
Within one year | $ 53,943 |
After one year through four years | 0 |
Total debt investments | 53,943 |
Bond and mutual funds | 18,402 |
Equity investments | 221 |
Total investments | $ 72,566 | $ 150,407

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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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