Note 13 - Stock-based Compensation - Summary of Options Outstanding (Details) - USD ($)	3 Months Ended	12 Months Ended
	Mar. 31, 2019	Dec. 31, 2018
Options outstanding, shares (in shares)	1,163,381	1,239,407
Options outstanding, weighted average exercise price (in dollars per share)	$ 71.16	$ 71.19
Options granted, shares (in shares)	8,475	110,265
Options granted, weighted average exercise price (in dollars per share)	$ 77.72	$ 61.39
Options exercised, shares (in shares)	(318,152)	(68,291)
Options exercised, weighted average exercise price (in dollars per share)	$ 72.36	$ 54.31
Options cancelled, shares (in shares)		(118,000)
Options cancelled, weighted average exercise price (in dollars per share)		$ 72.11
Options outstanding, shares (in shares)	853,704	1,163,381
Options outstanding, weighted average exercise price (in dollars per share)	$ 70.77	$ 71.16
Options outstanding, aggregate intrinsic value	$ 4,374,000
Options exercisable, shares (in shares)	178,729
Options exercisable, weighted average exercise price (in dollars per share)	$ 64.42
Options exercisable, aggregate intrinsic value	$ 2,048,000