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Investments - Summary of Gross Unrealized Losses and Fair Value of Available-For-Sale Investment Securities in Unrealized Loss Position (Detail) - USD ($) $ in Thousands | Jun. 30, 2019 | Dec. 31, 2018
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Schedule of Available-for-sale Securities [Line Items] |  |
Less Than 12 Months, Estimated Fair Value | $ 4,000 | $ 242,961
Less Than 12 Months, Unrealized Losses | (8) | (208)
12 Months or Greater, Estimated Fair Value | 26,079 | 44,637
12 Months or Greater, Unrealized Losses | (14) | (214)
Total, Estimated Fair Value | 30,079 | 287,598
Total, Unrealized Losses | (22) | (422)
Corporate debt securities |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Less Than 12 Months, Estimated Fair Value | 4,000 | 91,265
Less Than 12 Months, Unrealized Losses | (8) | (130)
12 Months or Greater, Estimated Fair Value | 26,079 | 44,637
12 Months or Greater, Unrealized Losses | (14) | (214)
Total, Estimated Fair Value | 30,079 | 135,902
Total, Unrealized Losses | $ (22) | (344)
Commercial paper |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Less Than 12 Months, Estimated Fair Value |  | 151,696
Less Than 12 Months, Unrealized Losses |  | (78)
Total, Estimated Fair Value |  | 151,696
Total, Unrealized Losses |  | $ (78)

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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 twelve months or longer. + 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 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 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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