Summary of Significant Accounting Policies - Schedule of Anti-dilutive Securities excluded from Computation of Earnings per Share (Details) - shares shares in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive securities (shares)	18,500	18,094	18,003
Options to purchase shares of common stock
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive securities (shares)	3,797	3,531	2,590
Shares of common stock issuable upon the vesting of RSUs
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive securities (shares)	1,129	1,070	613
Warrants
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive securities (shares)	1,008	1,008	7,382
Shares of common stock under employee stock purchase plan
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive securities (shares)	81	0	36
2022 Convertible Notes | Convertible Debt Securities
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive securities (shares)	11,695	11,695	0
2019 Convertible Notes | Convertible Debt Securities
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive securities (shares)	790	790	7,382