Preferred Stock and Stockholders' Equity - Summary of Common Stock Reserved for Future Issuance (Details) In Thousands, unless otherwise specified	Dec. 31, 2013	Dec. 31, 2012
Class of Stock [Line Items]
Common stock, shares reserved for future issuance	33,643	27,359
Warrant [Member]
Class of Stock [Line Items]
Common stock, shares reserved for future issuance	16,189	16,292
Stock Options [Member]
Class of Stock [Line Items]
Common stock, shares reserved for future issuance	14,859	9,901
Employee Stock [Member]
Class of Stock [Line Items]
Common stock, shares reserved for future issuance	2,595	1,166