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Cash Equivalents, Restricted Cash and Short-Term Investments Classified as Available-for-sale Securities (Detail) - USD ($) $ in Thousands | Dec. 31, 2018 | Dec. 31, 2017
--- | --- | ---
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost | $ 119,057 | $ 37,637
Gross Unrealized Gains | 2 | 3
Gross Unrealized Losses | (6) | (2)
Estimated Fair Value | 119,053 | 37,638
Restricted cash |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost | 50 | 5,330
Gross Unrealized Gains |  | 3
Estimated Fair Value | 50 | 5,333
Money Market Funds |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost |  | 14,334
Estimated Fair Value |  | 14,334
Money Market Funds | Cash Equivalents |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost | 29,090 |
Estimated Fair Value | 29,090 |
U.S. Government Agency Securities |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost | 74,144 | 16,473
Gross Unrealized Gains | 1 |
Gross Unrealized Losses | (6) | (2)
Estimated Fair Value | 74,139 | 16,471
Cash and Cash Equivalents |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Estimated Fair Value | 30,089 | 14,334
Short-Term Investments |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Estimated Fair Value | 88,914 | 17,971
Agency Securities |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost | 14,774 | 1,500
Gross Unrealized Gains | 1 |
Estimated Fair Value | 14,775 | $ 1,500
Agency Securities | Cash Equivalents |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost | 999 |
Estimated Fair Value | $ 999 |

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- Definition Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading. + References No definition available. + Details
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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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- Definition Amount before tax of unrealized loss in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale. + References No definition available. + Details
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- Definition This item represents the cost of debt and equity securities, which are categorized neither as held-to-maturity nor trading, net of adjustments including accretion, amortization, collection of cash, previous other-than-temporary impairments recognized in earnings (less any cumulative-effect adjustments recognized, as defined), and fair value hedge accounting adjustments, if any. + References No definition available. + Details
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