Note 8 - Stock-based Compensation - Stock Option Activity (Details)	6 Months Ended
	Jun. 30, 2017 $ / shares shares
Options outstanding, shares (in shares) | shares	569,639
Options outstanding, weighted- average exercise price (in dollars per share) | $ / shares	$ 45
Granted, shares (in shares) | shares	43,095
Granted, weighted- average exercise price (in dollars per share) | $ / shares	$ 58.35
Exercised, shares (in shares) | shares	(88,114)
Exercised, weighted- average exercise price (in dollars per share) | $ / shares	$ 31.71
Forfeited, shares (in shares) | shares	(16,943)
Forfeited, weighted- average exercise price (in dollars per share) | $ / shares	$ 58.17
Options outstanding, shares (in shares) | shares	507,677
Options outstanding, weighted- average exercise price (in dollars per share) | $ / shares	$ 48
Options exercisable, shares (in shares) | shares	318,140
Options exercisable, weighted- average exercise price (in dollars per share) | $ / shares	$ 40.42