Fair Value Measurements - Level 3 Instruments Reconciliation (Details) - Warrants - USD ($) $ in Thousands |
12 Months Ended | |
|---|---|---|
Dec. 31, 2018 |
Dec. 31, 2017 |
|
| Reconciliations of the beginning and ending balances for liabilities measured at fair value on a recurring basis using significant unobservable inputs (i.e. Level 3) | ||
| Balance at the beginning of the period | $ 4,391 | $ 11,387 |
| Change in fair value of common stock warrants | (4,286) | 15,188 |
| Issuance of common stock warrants | 4,905 | |
| Exercise of common stock warrants | (27,089) | |
| Balance at the end of the period | $ 105 | $ 4,391 |