Common Stock Reserved for Future Issuance (Details) - shares	Sep. 30, 2019	Dec. 31, 2018
Common Stock Reserved for Future Issuance
Shares reserved for future issuance (in shares)	7,617,536	5,589,766
Common stock warrants
Common Stock Reserved for Future Issuance
Shares reserved for future issuance (in shares)	6,193,749	4,202,449
Common stock options outstanding
Common Stock Reserved for Future Issuance
Shares reserved for future issuance (in shares)	843,183	433,389
Common stock options available for future issuance
Common Stock Reserved for Future Issuance
Shares reserved for future issuance (in shares)	468,123	877,917
Employee stock purchase plan
Common Stock Reserved for Future Issuance
Shares reserved for future issuance (in shares)	112,481	76,011