Note 9 - Stock-based Compensation - Stock Option Activity (Details)	6 Months Ended
	Jun. 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	(7,640)
Exercised, weighted-average exercise price (in dollars per share) | $ / shares	$ 55.93
Forfeited (in shares) | shares	(9,641)
Forfeited, weighted-average exercise price (in dollars per share) | $ / shares	$ 81.88
Options outstanding (in shares) | shares	289,043
Options outstanding, weighted-average exercise price (in dollars per share) | $ / shares	$ 72.00
Options exercisable (in shares) | shares	158,686
Options exercisable, weighted-average exercise price (in dollars per share) | $ / shares	$ 58.13