Note 9 - Stock-based Compensation - Stock Option Activity (Details)	9 Months Ended
	Sep. 30, 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	53,708
Granted, weighted-average exercise price (in dollars per share) | $ / shares	$ 105.81
Exercised (in shares) | shares	(11,921)
Exercised, weighted-average exercise price (in dollars per share) | $ / shares	$ 56.25
Forfeited (in shares) | shares	(10,833)
Forfeited, weighted-average exercise price (in dollars per share) | $ / shares	$ 83.74
Options outstanding (in shares) | shares	283,570
Options outstanding, weighted-average exercise price (in dollars per share) | $ / shares	$ 72.12
Options exercisable (in shares) | shares	154,539
Options exercisable, weighted-average exercise price (in dollars per share) | $ / shares	$ 58.18