Note 7 - Stock-based Compensation - Stock Option Activity (Details)	6 Months Ended
	Jun. 30, 2017 USD ($) $ / shares shares
Stock Option Plan 2001 [Member]
Options exercisable (in shares) | shares	60,093
Options exercisable, weighted average exercise price (in dollars per share)	$ 2.95
Options exercisable, weighted average remaining contractual term (Year)	1 year 255 days
Options exercisable, intrinsic value | $	$ 1,103,000
Options outstanding (in shares) | shares	81,893
Options outstanding, weighted average exercise price (in dollars per share)	$ 2.95
Options outstanding, weighted average exercise price (in dollars per share)	$ 2.95
Options outstanding, weighted average remaining contractual term (Year)	1 year 255 days
Options outstanding, intrinsic value | $	$ 1,103,000
Options exercised. (in shares) | shares	(21,800)
Options- exercised, weighted average exercise price (in dollars per share)	$ 2.95
Options exercised, weighted average exercise price (in dollars per share)	$ 2.95
Options outstanding (in shares) | shares	60,093
Stock Option Plan 2013 [Member]
Options exercisable (in shares) | shares	76,200
Options exercisable, weighted average exercise price (in dollars per share)	$ 7.08
Options exercisable, weighted average remaining contractual term (Year)	5 years 146 days
Options exercisable, intrinsic value | $	$ 1,084,000
Options outstanding (in shares) | shares	192,800
Options outstanding, weighted average exercise price (in dollars per share)	$ 7.60
Options outstanding, weighted average exercise price (in dollars per share)	$ 7.70
Options outstanding, weighted average remaining contractual term (Year)	5 years 328 days
Options outstanding, intrinsic value | $	$ 2,394,000
Options exercised. (in shares) | shares	(16,800)
Options- exercised, weighted average exercise price (in dollars per share)	$ 6.55
Options exercised, weighted average exercise price (in dollars per share)	$ 6.55
Options outstanding (in shares) | shares	176,000
Expected to vest (in shares) | shares	88,922
Expected to vest, weighted average exercise price (in dollars per share)	$ 8.18
Expected to vest after June 30, 2017 (Year)	6 years 73 days
Expected to vest after June 30, 2017 | $	$ 1,167,000