Note 8 - Income Taxes - Summary of Net Operating Losses and Tax Credit Carryforwards (Details)	12 Months Ended
	Dec. 31, 2017 USD ($)
Domestic Tax Authority [Member]
Net operating losses, federal	$ 34,700,000
Tax credits, federal	$ 6,109,000
Domestic Tax Authority [Member] | Earliest Tax Year [Member]
Net operating losses, federal, expiration years	2029
Tax credits, federal, expiration years	2024
Domestic Tax Authority [Member] | Latest Tax Year [Member]
Net operating losses, federal, expiration years	2035
Tax credits, federal, expiration years	2036
State and Local Jurisdiction [Member]
Net operating losses, federal	$ 20,773,000
Tax credits, federal	$ 6,872,000
State and Local Jurisdiction [Member] | Earliest Tax Year [Member]
Net operating losses, federal, expiration years	2029
State and Local Jurisdiction [Member] | Latest Tax Year [Member]
Net operating losses, federal, expiration years	2035