Preferred Stock and Stockholders' Equity - Summary of Common Stock Reserved for Future Issuance (Details) In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Class of Stock [Line Items]
Common stock, shares reserved for future issuance	37,971	33,643
Warrant [Member]
Class of Stock [Line Items]
Common stock, shares reserved for future issuance	16,232	16,189
Stock Options [Member]
Class of Stock [Line Items]
Common stock, shares reserved for future issuance	16,913	14,859
Employee Stock [Member]
Class of Stock [Line Items]
Common stock, shares reserved for future issuance	4,826	2,595