Convertible Preferred Stock and Stockholders' Equity - Summary of Reserved Shares of Common Stock (Details) - shares	Dec. 31, 2018	Dec. 31, 2017
Class of Stock [Line Items]
Reserved shares of common stock	53,465,467	92,322,345
Stock Award Plans [Member]
Class of Stock [Line Items]
Reserved shares of common stock	4,438,503	5,573,046
Employee Stock Purchase Plan ("ESPP") [Member]
Class of Stock [Line Items]
Reserved shares of common stock	51,251	125,618
Series A Convertible Preferred Stock Series [Member]
Class of Stock [Line Items]
Reserved shares of common stock	47,844,562	47,844,562
Warrant [Member]
Class of Stock [Line Items]
Reserved shares of common stock	1,131,151	38,779,119