Note 8 - Stock-based Compensation - Stock Option Activity (Details)	3 Months Ended
	Mar. 31, 2020 $ / shares shares
Options outstanding (in shares) | shares	276,266
Options outstanding, weighted-average exercise price (in dollars per share) | $ / shares	$ 72.38
Granted (in shares) | shares	33,192
Granted, weighted-average exercise price (in dollars per share) | $ / shares	$ 96.72
Exercised (in shares) | shares	(2,001)
Exercised, weighted-average exercise price (in dollars per share) | $ / shares	$ 67.53
Forfeited (in shares) | shares	(848)
Forfeited, weighted-average exercise price (in dollars per share) | $ / shares	$ 110.38
Options outstanding (in shares) | shares	306,609
Options outstanding, weighted-average exercise price (in dollars per share) | $ / shares	$ 74.94
Options exercisable (in shares) | shares	187,326
Options exercisable, weighted-average exercise price (in dollars per share) | $ / shares	$ 62.48