401(k) Plan (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Retirement Benefits [Abstract]
Percentage of employer's contribution under 401(k) plan matching up to 6 percent of participant contributions subject to maximum percentage of eligible compensation, effective from 2004 plan year and thereafter	50.00%
Maximum percentage of participant contributions eligible for Employer contribution match, towards defined contribution plan	6.00%
Match expense	$ 1.1	$ 1.0	$ 0.9