Fair Value Measurements (Details) - Money market funds - Fair Value, Measurements, Recurring - USD ($) $ in Thousands |
Sep. 30, 2018 |
Dec. 31, 2017 |
|---|---|---|
| Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | ||
| Money market funds | $ 201,264 | $ 25,146 |
| Level 1 | ||
| Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | ||
| Money market funds | 201,264 | 25,146 |
| 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 |