NET INCOME (LOSS) PER SHARE - Schedule of Basic and Diluted Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Basic and diluted net income (loss) per share
Net income (loss)	$ (24,138)	$ 48,270	$ (21,048)	$ 33,824	$ (33,104)	$ (15,992)	$ (26,659)	$ (26,741)	$ 36,908	$ (102,496)	$ (88,720)
Denominator (in shares)									63,794	62,702	61,297
Basic net income (loss) per share (in dollars per share)	$ (0.38)	$ 0.76	$ (0.33)	$ 0.53	$ (0.52)	$ (0.25)	$ (0.43)	$ (0.43)	$ 0.58	$ (1.63)	$ (1.45)
Add effect of diluted securities:
Stock options and equivalents (in shares)									414	0	0
Denominator for diluted income (loss) per share (in shares)									64,208	62,702	61,297
Diluted net income (loss) per share (in dollars per share)	$ (0.38)	$ 0.65	$ (0.33)	$ 0.48	$ (0.52)	$ (0.25)	$ (0.43)	$ (0.43)	$ 0.57	$ (1.63)	$ (1.45)