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Fair Value Measurements - Fair Values of Financial Assets Measured on Recurring Basis (Detail) - USD ($) $ in Thousands | Jun. 30, 2019 | Dec. 31, 2018
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Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Investments | $ 597,089 | $ 587,964
US treasury notes and bills [Member] |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Investments | 476,154 | 467,336
Bond and mutual funds [Member] |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Investments | 10,677 | 10,484
Equity investments [Member] |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Investments | 237 | 234
Term deposit [Member] |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Investments | 80,000 | 80,000
Certificate of deposit [Member] |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Investments | 30,021 | 29,910
Fair Value, Measurements, Recurring [Member] |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Total fair value of financial assets | 630,893 | 638,439
Fair Value, Measurements, Recurring [Member] | US treasury notes and bills [Member] |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Investments | 476,154 | 517,270
Fair Value, Measurements, Recurring [Member] | Bond and mutual funds [Member] |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Investments | 10,677 | 10,484
Fair Value, Measurements, Recurring [Member] | Equity investments [Member] |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Investments | 237 | 234
Fair Value, Measurements, Recurring [Member] | Money market funds [Member] |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Assets fair value disclosure | 33,804 | 541
Fair Value, Measurements, Recurring [Member] | Term deposit [Member] |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Investments | 80,000 | 80,000
Fair Value, Measurements, Recurring [Member] | Certificate of deposit [Member] |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Investments | 30,021 | 29,910
Fair Value, Measurements, Recurring [Member] | Level 1 [Member] |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Total fair value of financial assets | 395,609 | 303,576
Fair Value, Measurements, Recurring [Member] | Level 1 [Member] | US treasury notes and bills [Member] |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Investments | 350,891 | 292,317
Fair Value, Measurements, Recurring [Member] | Level 1 [Member] | Bond and mutual funds [Member] |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Investments | 10,677 | 10,484
Fair Value, Measurements, Recurring [Member] | Level 1 [Member] | Equity investments [Member] |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Investments | 237 | 234
Fair Value, Measurements, Recurring [Member] | Level 1 [Member] | Money market funds [Member] |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Assets fair value disclosure | 33,804 | 541
Fair Value, Measurements, Recurring [Member] | Level 1 [Member] | Term deposit [Member] |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Investments | 0 | 0
Fair Value, Measurements, Recurring [Member] | Level 1 [Member] | Certificate of deposit [Member] |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Investments | 0 | 0
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Total fair value of financial assets | 235,284 | 334,863
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | US treasury notes and bills [Member] |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Investments | 125,263 | 224,953
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | Bond and mutual funds [Member] |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Investments | 0 | 0
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | Equity investments [Member] |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Investments | 0 | 0
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | Money market funds [Member] |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Assets fair value disclosure | 0 | 0
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | Term deposit [Member] |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Investments | 80,000 | 80,000
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | Certificate of deposit [Member] |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Investments | 30,021 | 29,910
Fair Value, Measurements, Recurring [Member] | Level 3 [Member] |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Total fair value of financial assets | 0 | 0
Fair Value, Measurements, Recurring [Member] | Level 3 [Member] | US treasury notes and bills [Member] |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Investments | 0 | 0
Fair Value, Measurements, Recurring [Member] | Level 3 [Member] | Bond and mutual funds [Member] |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Investments | 0 | 0
Fair Value, Measurements, Recurring [Member] | Level 3 [Member] | Equity investments [Member] |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Investments | 0 | 0
Fair Value, Measurements, Recurring [Member] | Level 3 [Member] | Money market funds [Member] |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Assets fair value disclosure | 0 | 0
Fair Value, Measurements, Recurring [Member] | Level 3 [Member] | Term deposit [Member] |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Investments | 0 | 0
Fair Value, Measurements, Recurring [Member] | Level 3 [Member] | Certificate of deposit [Member] |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Investments | $ 0 | $ 0

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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=117815213&loc=d3e19207-110258 + Details
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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 Fair value portion of currency on hand as well as demand deposits with banks or financial institutions. Includes other kinds of accounts that have the general characteristics of demand deposits. Also includes short-term, highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates. + References No definition available. + Details
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- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + Details
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