Employee benefit plan	12 Months Ended
Dec. 31, 2017
Employee benefit plan
Employee benefit plan	15. Employee benefit plan We have a defined contribution savings plan in accordance with Section 401(k) of the Internal Revenue Code. This plan covers substantially all employees who meet the IRS requirements and allows participants to contribute a portion of their annual compensation on a pre-tax basis. Prior to January 1, 2016, the Company's matching contributions to the plan were made at the discretion of the board of directors and vesting in the Company's matching contributions were based on four years of continuous credited service. Effective January 1, 2016, the plan was amended to provide for company matching contributions that are paid each pay period and employees are immediately vested in all of the Company's matching contributions regardless of the employee's length of service with the Company. Employer contributions of $891, $819 and $511 were made to the plan by us in 2017, 2016 and 2015, respectively.

15. Employee benefit plan

We have a defined contribution savings plan in accordance with Section 401(k) of the Internal Revenue Code. This plan covers substantially all employees who meet the IRS requirements and allows participants to contribute a portion of their annual compensation on a pre-tax basis. Prior to January 1, 2016, the Company's matching contributions to the plan were made at the discretion of the board of directors and vesting in the Company's matching contributions were based on four years of continuous credited service. Effective January 1, 2016, the plan was amended to provide for company matching contributions that are paid each pay period and employees are immediately vested in all of the Company's matching contributions regardless of the employee's length of service with the Company. Employer contributions of $891, $819 and $511 were made to the plan by us in 2017, 2016 and 2015, respectively.