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Cash Equivalents, Restricted Cash and Short-Term Investments Classified as Available-for-sale Securities (Detail) - USD ($) $ in Thousands | Sep. 30, 2018 | Dec. 31, 2017
--- | --- | ---
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost | $ 129,158 | $ 37,638
Gross Unrealized Gains |  | 3
Gross Unrealized Losses | (24) | (3)
Estimated Fair Value | 129,134 | 37,638
Money Market Funds | Cash Equivalents |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost | 22,213 | 14,334
Estimated Fair Value | 22,213 | 14,334
Money Market Funds | Restricted cash (money market funds) |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost | 50 | 5,330
Gross Unrealized Gains |  | 3
Estimated Fair Value | 50 | 5,333
U.S. Government Agency Securities |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost | 93,246 | 16,474
Gross Unrealized Losses | (23) | (3)
Estimated Fair Value | 93,223 | 16,471
Cash and Cash Equivalents |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Estimated Fair Value | 35,861 | 14,334
Short-Term Investments |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Estimated Fair Value | 93,223 | 17,971
Agency Securities |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost | 13,649 | 1,500
Gross Unrealized Losses | (1) |
Estimated Fair Value | $ 13,648 | $ 1,500

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- Definition Amount before tax of unrealized gain in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563 + Details
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