Note 6 - Share-based Compensation - Stock Option Activity (Details)	3 Months Ended
	Mar. 31, 2018 USD ($) $ / shares shares
Balance (in shares) | shares	24,000
Balance (in dollars per share) | $ / shares	$ 24.15
Options granted (in shares) | shares
Options granted (in dollars per share) | $ / shares
Options exercised (in shares) | shares
Options exercised (in dollars per share) | $ / shares
Options canceled (in shares) | shares
Options canceled (in dollars per share) | $ / shares
Balance (in shares) | shares	24,000
Balance (in dollars per share) | $ / shares	$ 24.15
Balance (Year)
Balance | $
Exercisable, March 31, 2018 (in shares) | shares	24,000
Exercisable, March 31, 2018 (in dollars per share) | $ / shares	$ 24.15
Exercisable, March 31, 2018 (Year)	2 years
Exercisable, March 31, 2018 | $