Note 11 - Summary of Outstanding Stock Options (Details) - USD ($)	6 Months Ended	12 Months Ended
	Jun. 30, 2016	Dec. 31, 2015
Options outstanding, shares (in shares)	621,390	954,678
Options outstanding, weighted average exercise price (in dollars per share)	$ 48.15	$ 46.92
Options outstanding, aggregate intrinsic value
Share-based Compensation Arrangement by Share-based Payment Award, Options, Grants in Period, Gross	57,120	56,210
Weighted Average Exercise Price - Options Granted (in dollars per share)	$ 62.53	$ 61.47
Number of Shares - Options Exercised (in shares)	(480,275)	(389,498)
Weighted Average Exercise Price - Options Exercised (in dollars per share)	$ 46.81	$ 47.06
Options outstanding, shares (in shares)	197,579	621,390
Options outstanding, weighted average exercise price (in dollars per share)	$ 55.55	$ 48.15
Options outstanding, aggregate intrinsic value	$ 1,816,000
Number of Shares - Options Forfeited (in shares)	(656)
Weighted Average Exercise Price - Options Forfeited (in dollars per share)	$ 46.69
Options exercisable at March 31, 2015, shares (in shares)	185,459
Options exercisable at March 31, 2015, weighted average exercise price (in dollars per share)	$ 55.15
Options exercisable at March 31, 2015, aggregate intrinsic value	$ 1,778,000