Note 8 - Stock-based Compensation - Stock Option Activity (Details)	9 Months Ended
	Sep. 30, 2017 $ / shares shares
Options outstanding (in shares) | shares	569,639
Options outstanding, weighted- average exercise price (in dollars per share) | $ / shares	$ 45
Granted (in shares) | shares	43,095
Granted, weighted- average exercise price (in dollars per share) | $ / shares	$ 58.35
Exercised (in shares) | shares	(136,483)
Exercised, weighted- average exercise price (in dollars per share) | $ / shares	$ 34.82
Forfeited (in shares) | shares	(16,943)
Forfeited, weighted- average exercise price (in dollars per share) | $ / shares	$ 58.17
Options outstanding (in shares) | shares	459,308
Options outstanding, weighted- average exercise price (in dollars per share) | $ / shares	$ 48.79
Options exercisable (in shares) | shares	270,705
Options exercisable, weighted- average exercise price (in dollars per share) | $ / shares	$ 40.54