Note 13 - Stock-based Compensation - Summary of Options Outstanding (Details)	6 Months Ended	12 Months Ended
	Jun. 30, 2019 USD ($) $ / shares shares	Dec. 31, 2018 $ / shares shares
Options outstanding, shares (in shares) | shares	1,163,381	1,239,407
Options outstanding, weighted average exercise price (in dollars per share) | $ / shares	$ 71.16	$ 71.19
Options granted, shares (in shares) | shares	53,796	110,265
Options granted, weighted average exercise price (in dollars per share) | $ / shares	$ 77.89	$ 61.39
Options exercised, shares (in shares) | shares	(332,852)	(68,291)
Options exercised, weighted average exercise price (in dollars per share) | $ / shares	$ 71.56	$ 54.31
Options cancelled, shares (in shares) | shares	(60,000)	(118,000)
Options cancelled, weighted average exercise price (in dollars per share) | $ / shares	$ 72.94	$ 72.11
Options outstanding, shares (in shares) | shares	824,325	1,163,381
Options outstanding, weighted average exercise price (in dollars per share) | $ / shares	$ 71.31	$ 71.16
Options outstanding, aggregate intrinsic value | $	$ 8,115,000
Options exercisable, shares (in shares) | shares	209,029
Options exercisable, weighted average exercise price (in dollars per share) | $ / shares	$ 68.05
Options exercisable, aggregate intrinsic value | $	$ 2,739,000