Stockholders' Equity - Summary of Common Stock Reserved for Future Issuance (Details) - shares shares in Thousands	Dec. 31, 2018	Dec. 31, 2017
Class of Stock [Line Items]
Common stock reserved for future issuance (shares)	5,745	4,561
Shares subject to outstanding warrants to purchase common stock
Class of Stock [Line Items]
Common stock reserved for future issuance (shares)	28	38
Stock options and RSUs outstanding
Class of Stock [Line Items]
Common stock reserved for future issuance (shares)	4,033	3,651
Available for future issuance under employee equity plans
Class of Stock [Line Items]
Common stock reserved for future issuance (shares)	1,684	872