NET LOSS PER COMMON SHARE Net Loss Per Common Share - Schedule of Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended	9 Months Ended
Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Numerator:
Net Income (Loss), basic	$ 55,036	$ (12,620)	$ (38,872)	$ (73,561)
Net Income (Loss), diluted	$ 55,973	$ (12,620)	$ (38,872)	$ (73,561)
Denominator:
Weighted average number of shares outstanding, basic	179,289	177,481	178,873	176,767
Dilutive Securities, Effect on Basic Earnings Per Share [Abstract]
Weighted-average common shares outstanding, diluted	185,924	177,481	178,873	176,767
Net Income (Loss) per common share, basic (in dollars per share)	$ 0.31	$ (0.07)	$ (0.22)	$ (0.42)
Net Income (Loss) per common share, diluted (in dollars per share)	$ 0.30	$ (0.07)	$ (0.22)	$ (0.42)
Common stock issuable under the 2024 Notes
Numerator:
Add: Interest on 2024 notes	$ 937	$ 0	$ 0	$ 0
Dilutive Securities, Effect on Basic Earnings Per Share [Abstract]
Effect of dilutive securities	3,970	0	0	0
Options to purchase common stock
Dilutive Securities, Effect on Basic Earnings Per Share [Abstract]
Effect of dilutive securities	1,560	0	0	0
Unvested restricted stock units
Dilutive Securities, Effect on Basic Earnings Per Share [Abstract]
Effect of dilutive securities	669	0	0	0
Common stock potentially issuable for ESPP purchases
Dilutive Securities, Effect on Basic Earnings Per Share [Abstract]
Effect of dilutive securities	231	0	0	0
Common stock held by the NQDC
Dilutive Securities, Effect on Basic Earnings Per Share [Abstract]
Effect of dilutive securities	205	0	0	0

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