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Fair Value Measurements - Fair Value Measurements of Cash Equivalents and Available-For-Sale Investment Securities (Detail) - USD ($)
$ in Thousands
Dec. 31, 2017
Dec. 31, 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Investments, fair value disclosure $ 333,541 $ 523,433
Money market fund    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Investments, fair value disclosure 38,057 129,292
Quoted Prices in Active Markets for Identical Assets (Level 1)    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Investments, fair value disclosure 71,021 211,779
Quoted Prices in Active Markets for Identical Assets (Level 1) | Money market fund    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Investments, fair value disclosure 38,057 129,292
Significant Other Observable Inputs (Level 2)    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Investments, fair value disclosure 262,520 311,654
U.S. Treasury notes    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Investments, fair value disclosure 32,964 82,487
U.S. Treasury notes | Quoted Prices in Active Markets for Identical Assets (Level 1)    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Investments, fair value disclosure 32,964 82,487
Government sponsored enterprise securities    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Investments, fair value disclosure 10,072 88,773
Government sponsored enterprise securities | Significant Other Observable Inputs (Level 2)    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Investments, fair value disclosure 10,072 88,773
Corporate debt securities    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Investments, fair value disclosure 154,396 82,857
Corporate debt securities | Significant Other Observable Inputs (Level 2)    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Investments, fair value disclosure 154,396 82,857
Commercial paper    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Investments, fair value disclosure 98,052 140,024
Commercial paper | Significant Other Observable Inputs (Level 2)    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Investments, fair value disclosure $ 98,052 $ 140,024