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Fair Value Measurements - Fair Values of Financial Assets Measured on Recurring Basis (Detail) - USD ($) $ in Thousands | Sep. 30, 2018 | Dec. 31, 2017
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Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Investments | $ 126,611 | $ 72,566
Corporate bonds [Member] |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Investments | 6,009 | 53,943
Bond and mutual funds [Member] |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Investments | 10,446 | 18,402
Equity investments [Member] |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Investments | 223 | 221
Certificate of deposits [Member] |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Investments | 109,933 |
Fair Value, Measurements, Recurring [Member] |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Total fair value of financial assets | 621,742 | 642,508
Fair Value, Measurements, Recurring [Member] | Corporate bonds [Member] |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Investments | 6,009 | 53,943
Fair Value, Measurements, Recurring [Member] | Bond and mutual funds [Member] |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Investments | 10,446 | 18,402
Fair Value, Measurements, Recurring [Member] | Equity investments [Member] |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Investments | 223 | 221
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 | 495,131 | 569,942
Fair Value, Measurements, Recurring [Member] | Certificate of deposits [Member] |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Investments | 109,933 |
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 | 505,800 | 588,565
Fair Value, Measurements, Recurring [Member] | Level 1 [Member] | Corporate bonds [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] | Bond and mutual funds [Member] |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Investments | 10,446 | 18,402
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 | 223 | 221
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 | 495,131 | 569,942
Fair Value, Measurements, Recurring [Member] | Level 1 [Member] | Certificate of deposits [Member] |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Investments | 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 | 115,942 | 53,943
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | Corporate bonds [Member] |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Investments | 6,009 | 53,943
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] | Certificate of deposits [Member] |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Investments | 109,933 |
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] | Corporate bonds [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] | Certificate of deposits [Member] |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Investments | $ 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=99377789&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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