Note 5 - Stock-based Compensation - Stock Options (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2017	Mar. 31, 2017	Jun. 30, 2017	Jun. 30, 2016	Dec. 31, 2016	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016
Balance, Shares (in shares)	64,630	82,670	82,670
Balance, Weighted-Average Exercise Price (in dollars per share)	$ 23.37	$ 23.37	$ 23.37
Balance, Weighted-Average Exercise Price (in dollars per share)	23.37	23.37	23.37		$ 23.37
Balance, Weighted-Average Exercise Price (in dollars per share)	$ 23.37	$ 23.37	$ 23.37		$ 23.37	$ 23.37	$ 23.37	$ 23.37
Balance, Weighted-Average Remaining Contractual Life (Year)	255 days	328 days			1 year 36 days
Balance, Aggregate Intrinsic Value						$ 658	$ 925	$ 1,211
Exercised, Shares (in shares)	(19,500)	(18,040)	(37,540)	(2,110)
Exercised, Weighted-Average Exercise Price (in dollars per share)	$ 23.37	$ 23.37
Balance, Shares (in shares)	64,630	82,670	82,670		82,670	45,130	64,630	82,670
Balance, Shares (in shares)	45,130	64,630	45,130		82,670
Exercisable, Shares (in shares)						45,130
Exercisable, Weighted-Average Exercise Price (in dollars per share)						$ 23.37
Exercisable, Weighted-Average Remaining Contractual Life (Year)			255 days
Exercisable, Aggregate Intrinsic Value						$ 658