Fair Value of Financial Instruments - Financial Assets Carried at Fair Value (Detail) - Money Market Funds - USD ($) $ in Thousands |
Jun. 30, 2019 |
Dec. 31, 2018 |
---|---|---|
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | ||
Money market funds | $ 491,413 | $ 485,872 |
(Level 1) | ||
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | ||
Money market funds | 491,413 | 485,872 |
(Level 2) | ||
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | ||
Money market funds | 0 | 0 |
(Level 3) | ||
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | ||
Money market funds | $ 0 | $ 0 |