Note 9 - Stock-based Compensation - Stock Option Activity (Details)	3 Months Ended
	Mar. 31, 2019 $ / shares shares
Options outstanding (in shares) | shares	252,616
Options outstanding, weighted-average exercise price (in dollars per share) | $ / shares	$ 64.71
Granted (in shares) | shares	30,046
Granted, weighted-average exercise price (in dollars per share) | $ / shares	$ 104.99
Exercised (in shares) | shares	(3,010)
Exercised, weighted-average exercise price (in dollars per share) | $ / shares	$ 57.85
Forfeited (in shares) | shares	(680)
Forfeited, weighted-average exercise price (in dollars per share) | $ / shares	$ 96.20
Options outstanding (in shares) | shares	278,972
Options outstanding, weighted-average exercise price (in dollars per share) | $ / shares	$ 69.04
Options exercisable (in shares) | shares	157,602
Options exercisable, weighted-average exercise price (in dollars per share) | $ / shares	$ 56.70