Note 12 - Income Taxes (Details Textual) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent	21.00%	34.00%	34.00%
Cumulative Change in Ownership, Percentage	50.00%
Cumulative Change in Ownership, Period	3 years
Valuation Allowance, Deferred Tax Asset, Increase (Decrease), Amount	$ 1,402	$ 8,158	$ 469
Unrecognized Tax Benefits that Would Impact Effective Tax Rate	1,203	1,610
Unrecognized Tax Benefits, Income Tax Penalties and Interest Expense, Total	0	0	$ 0
Tax Cuts and Jobs Act of 2017 Incomplete Accounting Change in Tax Rate Deferred Tax Asset Provisional Income Tax Benefit	18,696
Tax Cuts and Jobs Act of 2017 Incomplete Accounting Change in Tax Rate Deferred Tax Liability Provisional Income Tax Benefit	4,394
Tax Cuts and Jobs Act of 2017 Incomplete Accounting Change in Tax Rate Valuation Allowance Provisional Income Tax Benefit		$ 14,302
Federal Research and Development [Member]
Tax Credit Carryforward, Amount	$ 6,328
Tax Credit Carryforward, Expiration Year	2026
State Research and Development [Member]
Tax Credit Carryforward, Amount	$ 7,084
Domestic Tax Authority [Member]
Operating Loss Carryforwards, Total	$ 117,816
Operating Loss Carry Forwards, Expiration Year	2026
State and Local Jurisdiction [Member]
Operating Loss Carryforwards, Total	$ 98,993
Operating Loss Carry Forwards, Expiration Year	2022