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Schedule of Fair Value Other Financial Instruments Measured (Detail) - Fair Value, Measurements, Recurring - USD ($) $ in Thousands | Dec. 31, 2018 | Dec. 31, 2017
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Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fair value of assets | $ 327,542 | $ 399,322
Quoted Prices In Active Markets (Level 1) |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fair value of assets | 327,542 | 399,322
Money Market Cash Equivalents |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fair value of assets | 327,542 | 399,322
Money Market Cash Equivalents | Quoted Prices In Active Markets (Level 1) |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fair value of assets | $ 327,542 | $ 399,322

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- Definition Fair value portion of probable future economic benefits obtained or controlled by an entity as a result of past transactions or events. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=99377789&loc=d3e19207-110258 + Details
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