Stock and employee benefit plans - Stock option activity (Details) - Outstanding options - USD ($)
|
12 Months Ended |
Dec. 31, 2019 |
Dec. 31, 2018 |
| Shares |
|
|
| Outstanding at the beginning of the period (in shares) |
893,000
|
|
| Granted (in shares) |
678,710
|
657,375
|
| Exercised (in shares) |
(5,000)
|
|
| Canceled (in shares) |
(244,000)
|
|
| Outstanding at the end of the period (in shares) |
1,323,000
|
893,000
|
| Exercisable at the end of the period (in shares) |
522,000
|
|
| Weighted - Average Exercise Price |
|
|
| Outstanding at the beginning of the period (in dollars per share) |
$ 18.79
|
|
| Granted (in dollars per share) |
4.36
|
|
| Exercised (in dollars per share) |
4.32
|
|
| Canceled (in dollars per share) |
17.64
|
|
| Outstanding at the end of the period (in dollars per share) |
11.65
|
$ 18.79
|
| Exercisable at the end of the period (in dollars per share) |
$ 20.09
|
|
| Weighted- Average Remaining Contractual Term |
|
|
| Outstanding at the end of the period |
8 years
|
|
| Exercisable at the end of the period |
6 years 7 months 6 days
|
|
| Aggregate Intrinsic Value |
|
|
| Outstanding at the beginning of the period (in dollars) |
|
|
| Outstanding at the end of the period (in dollars) |
0
|
|
| Exercisable at the end of the period (in dollars) |
$ 0
|
|