Employee 401 (k) Plan - Narrative (Details) - USD ($) $ in Millions	1 Months Ended	12 Months Ended
	Apr. 30, 2005	Dec. 31, 2017	Dec. 31, 2015
Defined Benefit Plan Disclosure [Line Items]
Employer matching contributions, vesting schedule	5 years
Employer 401(k) contributions			$ 0.4
Profit Sharing Plan
Defined Benefit Plan Disclosure [Line Items]
Employer matching contributions, vesting schedule		5 years