Note 13 - Income Taxes - Reconciliation of Statutory and Effective Tax Rates (Details)		12 Months Ended
		Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
U.S. statutory rate		21.00%	35.00%	35.00%
State taxes		4.70%	4.10%	4.10%
Research and development credits		(1.30%)	(1.40%)	(1.00%)
State credits		(1.00%)	(0.20%)	(0.20%)
Share-based compensation	[1]	(0.50%)	(1.40%)
Tax Act impact		(0.20%)	(13.90%)
Other		(0.20%)	(0.90%)	(1.10%)
Effective tax rate		22.50%	21.30%	36.80%

[1] With the adoption of ASU 2016-09 in 2017, excess tax benefits from equity awards are reflected within the provision for income taxes rather than within the consolidated balance sheet.