Stockholders' Equity - Summary of Common Stock Reserved for Future Issuance (Details) - shares shares in Thousands	Dec. 31, 2017	Dec. 31, 2016
Class of Stock [Line Items]
Common stock reserved for future issuance (shares)	4,561	6,315
Shares subject to outstanding warrants to purchase common stock
Class of Stock [Line Items]
Common stock reserved for future issuance (shares)	38	1,975
Shares subject to outstanding common stock options
Class of Stock [Line Items]
Common stock reserved for future issuance (shares)	3,651	3,388
Employee Stock [Member]
Class of Stock [Line Items]
Common stock reserved for future issuance (shares)	872	952