Retirement Plan (Details) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Retirement Plan
Maximum employee compensation to be deferred per year	$ 17,000
Maximum employer matching contribution as a percentage of annual compensation	3.00%
Maximum employee salary that the employer will match	$ 200,000
Company's total contribution under 401 (k) plan	$ 132,000	$ 143,000	$ 134,000