Stockholders' Equity, Employee Stock Purchase Plan (Details) - Employee Stock Purchase Plan ("ESPP") [Member] - shares	3 Months Ended	12 Months Ended
	Jun. 30, 2011	Dec. 31, 2017	Dec. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares reserved for issuance (in shares)	1,000,000
Term for employee stock purchase plan		10 years
Offering period of employee stock purchase plan		6 months
Minimum percentage of fair market value of the specified conditions used to determine purchase price of ESPP	85.00%
Employee stock purchase plan, shares issued (in shares)		28,018	42,268
Employee stock purchase plan, available for grant (in shares)		110,234