Note 8 - Net Loss Per Share - Net Loss 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
Net income (loss)	$ (26,293)	$ (873)	$ (1,572)	$ (2,032)	$ 22,880	$ 6,188	$ 1,947	$ (1,022)	$ (30,770)	$ 29,993	$ 2,577
Weighted average shares of common stock outstanding used in computing net income (loss) per share, basic (in shares)	13,932	13,851	13,709	13,587	13,744	13,973	13,935	13,840	13,771	13,873	13,225
Dilutive effect of incremental shares and share equivalents (in shares)										855	528
Weighted average shares of common stock outstanding used in computing net income (loss) per share, diluted (in shares)	13,932	13,851	13,709	13,587	14,569	14,767	14,629	13,840	13,771	14,728	13,753
Net income (loss) per share—basic (in dollars per share)	$ (1.89)	$ (0.06)	$ (0.11)	$ (0.15)	$ 1.66	$ 0.44	$ 0.14	$ (0.07)	$ (2.23)	$ 2.16	$ 0.19
Net income (loss) per share—diluted (in dollars per share)	$ (1.89)	$ (0.06)	$ (0.11)	$ (0.15)	$ 1.57	$ 0.42	$ 0.13	$ (0.07)	$ (2.23)	$ 2.04	$ 0.19