D. EARNINGS PER SHARE (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Numerator:
Net income	$ 5,716	$ 19,375	$ 29,071	$ 19,789	$ 14,225	$ 17,736	$ 603	$ (1,330)	$ 73,951	$ 31,234	$ 10,793
Weighted average shares used to compute net income per share
Basic									19,097,355	17,201,731	15,626,753
Effective of dilutive options and restricted stock units									1,042,000	511,000	906,000
Diluted									20,139,105	17,712,928	16,532,850
Net income per share
Basic	$ 0.29	$ 1.00	$ 1.52	$ 1.06	$ 0.81	$ 1.03	$ 0.04	$ (0.08)	$ 3.87	$ 1.82	$ 0.69
Diluted	$ 0.28	$ 0.95	$ 1.43	$ 1.00	$ 0.77	$ 0.97	$ 0.03	$ (0.08)	$ 3.67	$ 1.76	$ 0.65