Note 11 - Stockholders' Equity - Stock Option Plan (Details) - $ / shares	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Beginning balance, number outstanding (in shares)	387,930
Exercise of stock options (in shares)	0	34,000
Ending balance, number outstanding (in shares)	407,930	387,930
Number of shares exercisable (in shares)	257,930	207,930
The 2001 Non-qualified Stock Option Plan [Member]
Beginning balance, number outstanding (in shares)	22,930	59,730
Granted during period (in shares)
Exercise of stock options (in shares)		36,800
Canceled during period (in shares)
Ending balance, number outstanding (in shares)	22,930	22,930
Number of shares exercisable (in shares)	22,930	22,930
Beginning balance, weighted average exercise price (in dollars per share)	$ 5.36	$ 4.51
Granted during period, weighted average exercise price (in dollars per share)
Exercised during period, weighted average exercise price (in dollars per share)		3.97
Canceled during period, weighted average exercise price (in dollars per share)
Ending balance, weighted average exercise price (in dollars per share)	5.36	5.36
Number exercisable, weighted average exercise price (in dollars per share)	$ 5.36	$ 5.36
The 2007 Share Incentive Plan [Member]
Beginning balance, number outstanding (in shares)	365,000	225,000
Granted during period (in shares)		140,000
Exercise of stock options (in shares)
Canceled during period (in shares)
Ending balance, number outstanding (in shares)	365,000	365,000
Number of shares exercisable (in shares)	235,000	185,000
Beginning balance, weighted average exercise price (in dollars per share)	$ 12.35	$ 9.43
Granted during period, weighted average exercise price (in dollars per share)		10.82
Exercised during period, weighted average exercise price (in dollars per share)
Canceled during period, weighted average exercise price (in dollars per share)
Ending balance, weighted average exercise price (in dollars per share)	12.35	12.35
Number exercisable, weighted average exercise price (in dollars per share)	$ 13.15	$ 13.76
The 2016 Share Incentive Plan [Member]
Beginning balance, number outstanding (in shares)
Granted during period (in shares)	20,000
Exercise of stock options (in shares)
Canceled during period (in shares)
Ending balance, number outstanding (in shares)	20,000
Number of shares exercisable (in shares)
Beginning balance, weighted average exercise price (in dollars per share)
Granted during period, weighted average exercise price (in dollars per share)	8.07
Exercised during period, weighted average exercise price (in dollars per share)
Canceled during period, weighted average exercise price (in dollars per share)
Ending balance, weighted average exercise price (in dollars per share)	8.07
Number exercisable, weighted average exercise price (in dollars per share)