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, 2017	Dec. 31, 2016	Dec. 31, 2015
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive securities (shares)	18,094	17,967	9,168
Options to purchase shares of common stock
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive securities (shares)	3,531	2,590	1,619
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)	1,070	613	167
Warrants
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive securities (shares)	1,008	7,382	0
2022 Convertible Notes | Convertible Debt Securities
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive securities (shares)	11,695	0	0
2019 Convertible Notes | Convertible Debt Securities
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive securities (shares)	790	7,382	7,382