Note 6 - Stock Based Compensation - Stock Option Activity (Details) - USD ($)		6 Months Ended	12 Months Ended
		Jun. 30, 2018	Dec. 31, 2017
Granted, options outstanding (in shares)		0	0
2005 Stock Option Plan [Member]
Beginning balance, options outstanding (in shares)		187,000
Beginning balance, weighted average exercise price (in dollars per share)		$ 7.78
Granted, options outstanding (in shares)
Granted, weighted average exercise price (in dollars per share)
Exercised, options outstanding (in shares)		(18,300)
Exercised, weighted average exercise price (in dollars per share)		$ 7.83
Expired, options outstanding (in shares)		(31,500)
Expired, weighted average exercise price (in dollars per share)		$ (10.79)
Forfeited, options outstanding (in shares)
Forfeited, weighted average exercise price (in dollars per share)
Ending balance, options outstanding (in shares)		137,200	187,000
Ending balance, weighted average exercise price (in dollars per share)		$ 7.08	$ 7.78
Balance, aggregate intrinsic value	[1]	$ 958,186
Balance, weighted average contractual term (Year)		6 years 87 days
Exercisable, options outstanding (in shares)		80,000
Exercisable, weighted average exercise price (in dollars per share)		$ 7.21
Exercisable, aggregate intrinsic value	[1]	$ 548,474
Exercisable, weighted average contractual term (Year)		6 years 25 days

[1] The aggregate intrinsic value of a stock option in the table above represents the total pre-tax intrinsic value (the amount by which the current market value of the underlying stock exceeds the exercise price of the option) that would have been received by the option holders had all option holders exercised their options on June 30, 2018.