401(k) Plan (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Retirement Benefits [Abstract]
Employee contribution limit	92.00%
Percent of eligibile compensation maximum company match	25.00%
Company match	4.00%
Company matching contributions	$ 0.6	$ 0.5	$ 0.4