Note 13 - Stock-based Compensation - Summary of Options Outstanding (Details) - USD ($)	3 Months Ended	12 Months Ended
	Mar. 31, 2020	Dec. 31, 2019
Options outstanding, shares (in shares)	785,529	1,163,381
Options outstanding, weighted average exercise price (in dollars per share)	$ 71.24	$ 71.16
Options granted, shares (in shares)	57,313	53,316
Options granted, weighted average exercise price (in dollars per share)	$ 84.46	$ 77.89
Options exercised, shares (in shares)	(7,615)	(346,168)
Options exercised, weighted average exercise price (in dollars per share)	$ 65.37	$ 71.57
Options cancelled, shares (in shares)		(85,000)
Options cancelled, weighted average exercise price (in dollars per share)		$ 72.94
Options outstanding, shares (in shares)	835,227	785,529
Options outstanding, weighted average exercise price (in dollars per share)	$ 72.20	$ 71.24
Options outstanding, aggregate intrinsic value	$ 1,331,000
Options exercisable, shares (in shares)	196,414
Options exercisable, weighted average exercise price (in dollars per share)	$ 68.31
Options exercisable, aggregate intrinsic value	$ 1,001,000