NET INCOME (LOSS) PER SHARE - RECONCILIATION OF NUMERATORS AND DENOMINATORS USED IN COMPUTING BASIC AND DILUTED NET INCOME (LOSS) 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)	$ (1,387)	$ (4,640)	$ (21,023)	$ 57,485
Denominator:
Weighted-average common stock outstanding, basic (in shares)	31,711	30,780	31,461	30,340
Dilutive effect of potential common shares, Stock options, restricted stock units, restricted stock awards and employee stock purchase plan (in shares)	0	0	0	994
Dilutive effect of potential common shares, Total shares, diluted (in shares)	31,711	30,780	31,461	31,334
Basic net income (loss) per share (in dollars per share)	$ (0.04)	$ (0.15)	$ (0.67)	$ 1.89
Diluted net income (loss) per share (in dollars per share)	$ (0.04)	$ (0.15)	$ (0.67)	$ 1.83