Note 9 - Net Income (Loss) Per Share - Net Income (Loss) Per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ 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
Net income (loss)	$ 22,880	$ 6,188	$ 1,947	$ (1,022)	$ 4,213	$ 1,644	$ (1,229)	$ (2,051)	$ 29,993	$ 2,577	$ (4,440)
Basic (in shares)	13,744	13,973	13,935	13,840	13,591	13,163	13,131	13,010	13,873	13,225	13,960
Add: dilutive effect of potential common shares (in shares)									855	528
Weighted-average shares used in computing diluted net income per share (in shares)	14,569	14,767	14,629	13,840	14,201	13,544	13,131	13,010	14,728	13,753	13,960
Net income (loss) per share—basic (in dollars per share)	$ 1.66	$ 0.44	$ 0.14	$ (0.07)	$ 0.31	$ 0.12	$ (0.09)	$ (0.16)	$ 2.16	$ 0.19	$ (0.32)
Net income (loss) per share—diluted (in dollars per share)	$ 1.57	$ 0.42	$ 0.13	$ (0.07)	$ 0.30	$ 0.12	$ (0.09)	$ (0.16)	$ 2.04	$ 0.19	$ (0.32)