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Investments - Summary of Gross Unrealized Losses and Fair Value of Available-For-Sale Investments in Unrealized Loss Position (Detail) - USD ($) $ in Thousands | Jun. 30, 2018 | Dec. 31, 2017
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Schedule of Available-for-sale Securities [Line Items] |  |
Less Than 12 Months, Estimated Fair Value | $ 160,369 | $ 263,414
Less Than 12 Months, Unrealized Losses | (516) | (408)
Total, Estimated Fair Value | 160,369 | 263,414
Total, Unrealized Losses | (516) | (408)
U.S. Treasury notes |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Less Than 12 Months, Estimated Fair Value |  | 32,964
Less Than 12 Months, Unrealized Losses |  | (12)
Total, Estimated Fair Value |  | 32,964
Total, Unrealized Losses |  | (12)
Government sponsored enterprise securities |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Less Than 12 Months, Estimated Fair Value |  | 10,072
Less Than 12 Months, Unrealized Losses |  | (10)
Total, Estimated Fair Value |  | 10,072
Total, Unrealized Losses |  | (10)
Corporate debt securities |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Less Than 12 Months, Estimated Fair Value | 136,308 | 129,820
Less Than 12 Months, Unrealized Losses | (502) | (321)
Total, Estimated Fair Value | 136,308 | 129,820
Total, Unrealized Losses | (502) | (321)
Commercial paper |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Less Than 12 Months, Estimated Fair Value | 24,061 | 90,558
Less Than 12 Months, Unrealized Losses | (14) | (65)
Total, Estimated Fair Value | 24,061 | 90,558
Total, Unrealized Losses | $ (14) | $ (65)

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- Definition Amount of accumulated unrealized loss on investments in debt and equity securities classified as available-for-sale in a continuous loss position. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 7 -URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27337-111563 + Details
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- Definition This item represents the aggregate fair value of investments in debt and equity securities in an unrealized loss position which are categorized neither as held-to-maturity nor trading securities. + References No definition available. + Details
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- Definition Amount of accumulated unrealized loss on investments in debt and equity securities classified as available-for-sale that have been in a continuous loss position for less than twelve months. + References No definition available. + Details
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