401(k) plan (Details) - USD ($) $ in Millions |
12 Months Ended | ||
---|---|---|---|
Dec. 31, 2018 |
Dec. 31, 2017 |
Dec. 31, 2016 |
|
Retirement Benefits [Abstract] | |||
Matching contribution (as a percent of total salary) | 92.00% | ||
Percentage of employee's base salary, matched by employer | 6.00% | ||
Expense recorded | $ 2.2 | $ 1.6 | $ 1.1 |