Stock-based Compensation - Employee Stock Purchase Plans (Details) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
1995 Employee Stock Purchase Plan | Maximum
Class of Stock
Common stock shares authorized for issuance under stock purchase plan	500,000
2017 Employee Stock Purchase Plan
Class of Stock
Maximum percentage of total combined voting power or value of the stock of the Company after the grant	5.00%
Common shares available for grant	456,961
2017 Employee Stock Purchase Plan | Maximum
Class of Stock
Common stock shares authorized for issuance under stock purchase plan	500,000
Employee Stock Purchase Plans
Class of Stock
Minimum percentage in payroll of deduction base salary to acquire shares of common stock	1.00%
Maximum percentage in payroll of deduction base salary to acquire shares of common stock	10.00%
Percentage of fair market value of common stock for the ESPP option price	85.00%
Annual maximum a participant may purchase under the employee stock purchase plan	$ 25,000
Common stock share issued	174,972	121,460	27,951
Defined Contribution Plan, Cost	$ 200,000	$ 100,000
Employee Stock Purchase Plans | Maximum
Class of Stock
Subsequent offering period as established by the compensation committee	12 months
Defined Contribution Plan, Cost			$ 100,000