Fair Value Measurements - Fair Value of Warrants (Details) |
6 Months Ended |
|---|---|
|
Jun. 30, 2018
$ / shares
| |
| Tranche one of warrants issued with the Amazon.com, Inc transaction agreement | |
| Fair Value | |
| Fair value per warrant | $ 1.15 |
| Tranche two of warrants issued with the Amazon.com, Inc. Transaction Agreement | |
| Fair Value | |
| As of vesting date - second tranche, first installment | 2.16 |
| As of period end - second tranche | 1.71 |
| Tranche one of warrants issued with the Walmart Stores Inc transaction agreement | |
| Fair Value | |
| Fair value per warrant | 1.88 |
| Tranche two of warrants issued with the Walmart Stores, Inc. Transaction Agreement | |
| Fair Value | |
| As of period end - second tranche | $ 1.56 |