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Investments Measured at Fair Value 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] |  |
Carrying Value | $ 702,861 | $ 725,227
Short-term investments |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Carrying Value | 619,978 | 650,913
Short-term investments | Cash and money market fund |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Carrying Value | 140,985 | 141,714
Short-term investments | Commercial paper |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Carrying Value | 116,490 | 94,572
Short-term investments | Securities of government-sponsored entities |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Carrying Value | 59,424 | 20,840
Short-term investments | Corporate debt securities |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Carrying Value | 303,079 | 393,787
Long-term investments | Cash and money market fund |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Carrying Value | 1,500 | 1,500
Long-term investments | Certificates of deposit |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Carrying Value | 3,977 | 3,977
Long-term investments | Securities of government-sponsored entities |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Carrying Value | 30,289 | 64,837
Long-term investments | Corporate debt securities |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Carrying Value | 116,214 | 151,191
Long-term investments | Restricted equity securities |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Carrying Value | 77,365 |
Investments Including Cash Equivalents And Restricted Cash |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Carrying Value | 849,323 | 872,418
Cash and Cash Equivalents |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Carrying Value | 146,462 | 147,191
Quoted Prices in Active Markets for Identical Assets (Level 1) | Short-term investments | Fair Value Measurements Recurring |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Fair value of assets on recurring basis | 140,985 | 141,714
Quoted Prices in Active Markets for Identical Assets (Level 1) | Short-term investments | Cash and money market fund | Fair Value Measurements Recurring |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Fair value of assets on recurring basis | 140,985 | 141,714
Quoted Prices in Active Markets for Identical Assets (Level 1) | Long-term investments | Cash and money market fund | Fair Value Measurements Recurring |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Fair value of assets on recurring basis | 1,500 | 1,500
Quoted Prices in Active Markets for Identical Assets (Level 1) | Long-term investments | Certificates of deposit | Fair Value Measurements Recurring |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Fair value of assets on recurring basis | 3,977 | 3,977
Quoted Prices in Active Markets for Identical Assets (Level 1) | Investments Including Cash Equivalents And Restricted Cash | Fair Value Measurements Recurring |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Fair value of assets on recurring basis | 146,462 | 147,191
Quoted Prices in Active Markets for Identical Assets (Level 1) | Cash and Cash Equivalents | Fair Value Measurements Recurring |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Fair value of assets on recurring basis | 146,462 | 147,191
Significant Other Observable Inputs (Level 2) | Fair Value Measurements Recurring |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Fair value of assets on recurring basis | 625,496 | 725,227
Significant Other Observable Inputs (Level 2) | Short-term investments | Fair Value Measurements Recurring |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Fair value of assets on recurring basis | 478,993 | 509,199
Significant Other Observable Inputs (Level 2) | Short-term investments | Commercial paper | Fair Value Measurements Recurring |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Fair value of assets on recurring basis | 116,490 | 94,572
Significant Other Observable Inputs (Level 2) | Short-term investments | Securities of government-sponsored entities | Fair Value Measurements Recurring |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Fair value of assets on recurring basis | 59,424 | 20,840
Significant Other Observable Inputs (Level 2) | Short-term investments | Corporate debt securities | Fair Value Measurements Recurring |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Fair value of assets on recurring basis | 303,079 | 393,787
Significant Other Observable Inputs (Level 2) | Long-term investments | Securities of government-sponsored entities | Fair Value Measurements Recurring |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Fair value of assets on recurring basis | 30,289 | 64,837
Significant Other Observable Inputs (Level 2) | Long-term investments | Corporate debt securities | Fair Value Measurements Recurring |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Fair value of assets on recurring basis | 116,214 | 151,191
Significant Other Observable Inputs (Level 2) | Investments Including Cash Equivalents And Restricted Cash | Fair Value Measurements Recurring |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Fair value of assets on recurring basis | 625,496 | $ 725,227
Significant Unobservable Inputs (Level 3) | Fair Value Measurements Recurring |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Fair value of assets on recurring basis | 77,365 |
Significant Unobservable Inputs (Level 3) | Long-term investments | Restricted equity securities | Fair Value Measurements Recurring |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Fair value of assets on recurring basis | 77,365 |
Significant Unobservable Inputs (Level 3) | Investments Including Cash Equivalents And Restricted Cash | Fair Value Measurements Recurring |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Fair value of assets on recurring basis | $ 77,365 |

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