Note 8 - Stock-based Compensation - Stock Option Activity (Details)	9 Months Ended
	Sep. 30, 2016 $ / shares shares
Options outstanding, beginning of period (in shares) | shares	766,042
Options outstanding, beginning of period, weighted-average exercise price (in dollars per share) | $ / shares	$ 36.52
Granted (in shares) | shares	113,645
Granted, weighted-average exercise price (in dollars per share) | $ / shares	$ 58.12
Exercised (in shares) | shares	(173,781)
Exercised, weighted-average exercise price (in dollars per share) | $ / shares	$ 18.42
Forfeited (in shares) | shares	(69,965)
Forfeited, weighted-average exercise price (in dollars per share) | $ / shares	$ 61.37
Options outstanding, end of period (in shares) | shares	635,941
Options outstanding, end of period, weighted-average exercise price (in dollars per share) | $ / shares	$ 42.59
Options exercisable (in shares) | shares	368,052
Options exercisable, weighted-average exercise price (in dollars per share) | $ / shares	$ 31.13