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Investments Measured at Fair Value on Recurring Basis (Detail) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Carrying Value	$ 757,233	$ 725,227
Short-term investments
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Carrying Value	670,625	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	166,637	141,714
Short-term investments | Certificates of deposit
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Carrying Value	463
Short-term investments | Commercial paper
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Carrying Value	135,197	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	112,961	20,840
Short-term investments | Corporate debt securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Carrying Value	255,367	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	3,206	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	45,209	64,837
Long-term investments | Corporate debt securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Carrying Value	159,584	151,191
Long-term investments | Restricted equity securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Carrying Value	48,915
Investments Including Cash Equivalents And Restricted Cash
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Carrying Value	929,039	872,418
Cash and Cash Equivalents
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Carrying Value	171,806	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	167,100	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	166,637	141,714
Quoted Prices in Active Markets for Identical Assets (Level 1) | Short-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	463
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	3,206	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	171,806	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	171,806	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	708,318	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	503,525	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	135,197	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	112,961	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	255,367	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	45,209	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	159,584	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	708,318	$ 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	48,915
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	48,915
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	$ 48,915

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Fair value portion of probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.

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-Publisher FASB
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-SubTopic 10
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