Common Stock, Common Stock Warrants and Stock Option Plan - 2015 Employee Stock Purchase Plan (Details) - USD ($)		1 Months Ended	12 Months Ended
	Jul. 29, 2016	Sep. 30, 2015	Dec. 31, 2017	Dec. 31, 2016
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Common stock, capital shares reserved for future issuance (in shares)			3,316,184	3,401,403
Employee Stock [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Common stock, capital shares reserved for future issuance (in shares)		150,000	275,587	231,355
Employee stock purchase plan description			The number of shares reserved for issuance under our 2015 Employee Stock Purchase Plan will increase automatically on the 1st day of January following the first offering date by the number of shares equal to 1% of the total outstanding shares of our common stock as of the immediately preceding December 31 (rounded to the nearest whole share).
Percentage threshold of outstanding shares increased annually under the plan (as a percent)		1.00%
Common stock shares authorized for issuance (in shares)		2,250,000
Payroll deduction (as a percent)			15.00%
Percentage of fair market value of common stock (as a percent)			85.00%
Shares issued (in shares)	68,392		110,711
Maximum fair market value, that would permit employee to purchase common stock under plan			$ 25,000
Maximum number of shares issued each participants (in shares)			2,500