Note 16 - Stock-based Compensation - Summary of Options Outstanding (Details) - USD ($)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Options outstanding, shares (in shares)	809,529	1,163,381	1,239,407
Options outstanding, weighted average exercise price (in dollars per share)	$ 71.24	$ 71.16	$ 71.19
Options granted, shares (in shares)	104,057	77,316	110,265
Options granted, weighted average exercise price (in dollars per share)	$ 73.98	$ 77.89	$ 61.39
Options exercised, shares (in shares)	(43,630)	(346,168)	(68,291)
Options exercised, weighted average exercise price (in dollars per share)	$ 63.37	$ 71.57	$ 54.31
Options cancelled, shares (in shares)	(3,000)	(85,000)	(118,000)
Options cancelled, weighted average exercise price (in dollars per share)	$ 72.94	$ 72.94	$ 72.11
Options outstanding, shares (in shares)	866,956	809,529	1,163,381
Options outstanding, weighted average exercise price (in dollars per share)	$ 72.11	$ 71.24	$ 71.16
Options outstanding, aggregate intrinsic value	$ 572,790
Options exercisable, shares (in shares)	215,456
Options exercisable, weighted average exercise price (in dollars per share)	$ 68.35
Options exercisable, aggregate intrinsic value	$ 572,790