Note 15 - Stock-based Compensation - Summary of Options Outstanding (Details) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Options outstanding, shares (in shares)	1,239,407	177,959	621,390
Options outstanding, weighted average exercise price (in dollars per share)	$ 71.19	$ 55.48	$ 48.15
Options granted, shares (in shares)	110,265	1,125,443	56,291
Options granted, weighted average exercise price (in dollars per share)	$ 61.39	$ 72.96	$ 62.54
Options exercised, shares (in shares)	(68,291)	(48,995)	(499,066)
Options exercised, weighted average exercise price (in dollars per share)	$ 54.31	$ 51.25	$ 47.16
Options cancelled, shares (in shares)	(118,000)	(15,000)	(656)
Options cancelled, weighted average exercise price (in dollars per share)	$ 72.11	$ 72.94	$ 46.69
Options outstanding, shares (in shares)	1,163,381	1,239,407	177,959
Options outstanding, weighted average exercise price (in dollars per share)	$ 71.16	$ 71.19	$ 55.48
Options outstanding, aggregate intrinsic value	$ 8,483,000
Options exercisable, shares (in shares)	185,881
Options exercisable, weighted average exercise price (in dollars per share)	$ 64.41
Options exercisable, aggregate intrinsic value	$ 2,609,000