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, 2019	Dec. 31, 2018	Dec. 31, 2017
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive securities (shares)	17,250	18,500	18,094
Options to purchase shares of common stock
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive securities (shares)	3,976	3,797	3,531
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)	1,579	1,129	1,070
Warrants
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive securities (shares)	0	1,008	1,008
Shares of common stock under employee stock purchase plan
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive securities (shares)	0	81	0
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	11,695
2019 Convertible Notes | Convertible Debt Securities
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive securities (shares)	0	790	790