Fair Value Measurements - Schedule of Reconciliation of Changes In Fair Value of Convertible Debentures (Detail) - Convertible Debentures [Member] $ in Thousands |
6 Months Ended |
---|---|
Jun. 30, 2019
USD ($)
| |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] | |
Balance, December 31, 2018 | $ 6,970 |
Fair value adjustment | 4,440 |
Balance, June 30, 2019 | $ 11,410 |