Common Stock, Common Stock Warrants and Stock Option Plan - Stock Reserved for Future Issuance (Details) - shares	Dec. 31, 2017	Dec. 31, 2016	Sep. 30, 2015
Class of Stock [Line Items]
Common stock, capital shares reserved for future issuance (in shares)	3,316,184	3,401,403
Common Stock Warrants [Member]
Class of Stock [Line Items]
Warrants to purchase common stock (in shares)	389,423	411,514
Stock Options [Member]
Class of Stock [Line Items]
Common stock, capital shares reserved for future issuance (in shares)	1,560,453	991,895
Restricted Stock Units (RSUs) [Member]
Class of Stock [Line Items]
Common stock, capital shares reserved for future issuance (in shares)	509,894	490,954
Shares available for future grants (in shares)	580,827	1,275,685
Employee Stock [Member]
Class of Stock [Line Items]
Common stock, capital shares reserved for future issuance (in shares)	275,587	231,355	150,000