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Investments Measured at Fair Value on Recurring Basis (Detail) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
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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- 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
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-Name Accounting Standards Codification
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-Section S99
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