Note 7 - Equity Award Plans - Summary of Activity Under the 2006 Plan and 2013 Plan (Details) - USD ($) shares in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2019	Dec. 31, 2018
Number of Shares, Options Outstanding, Balance (in shares)	436
Weighted Average Exercise Price Per Share, Options Outstanding, Balance (in dollars per share)	$ 29.01
Weighted Average Remaining Contractual Term, Options Outstanding (Year)	6 years 219 days	6 years 288 days
Aggregate Intrinsic Value, Options Outstanding, Balance
Number of Shares, Options Forfeited and Cancelled (in shares)	(6)
Weighted Average Exercise Price Per Share, Options forfeited and cancelled (in dollars per share)	$ 63.58
Number of Shares, Options Outstanding, Balance (in shares)	430	436
Weighted Average Exercise Price Per Share, Options Outstanding, Balance (in dollars per share)	$ 28.49	$ 29.01
Number of Shares, Options Exercisable, Balance (in shares)	310
Weighted Average Exercise Price Per Share, Options exercisable, Balance (in dollars per share)	$ 35.71
Weighted Average Remaining Contractual Term, Options exercisable (Year)	5 years 302 days
Number of Shares, Options Vested, Balance (in shares)	310
Weighted Average Exercise Price Per Share, Options vested, Balance (in dollars per share)	$ 35.70
Weighted Average Remaining Contractual Term, Options vested (Year)	5 years 302 days
Options vested and expected to vest (in shares)	419
Weighted Average Exercise Price Per Share, Options vested and expected to vest, Balance (in dollars per share)	$ 28.93
Weighted Average Remaining Contractual Term, Options vested and expected to vest (Year)	6 years 200 days
Aggregate Intrinsic Value, Options vested and expected to vest, Balance