Note 5 - Equity Incentive Plan - Summary of Stock Option Activity (Details) - $ / shares	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Issuance of shares of common stock, shares (in shares)	7,151	7,202
Outstanding, shares (in shares)	162,833
Outstanding, weighted average exercise price (in dollars per share)	$ 46.22
Stock Options and Stock Appreciation Rights [Member]
Outstanding, shares (in shares)	284,591	273,039
Outstanding, weighted average exercise price (in dollars per share)	$ 43.71	$ 41.19
Shares Granted (in shares)	24,999	0
Granted, Weighted Average Exercise Price (in dollars per share)	$ 59.02	$ 0
Issuance of shares of common stock, shares (in shares)	28,801	7,654
Exercised, Weighted Average Exercise Price (in dollars per share)	$ 40.02	$ 33.82
Shares Forfeited or expired (in shares)	0	4,300
Forfeited or expired, Weighted Average Exercise Price (in dollars per share)	$ 0	$ 39.46
Outstanding, shares (in shares)	280,789	261,085
Outstanding, weighted average exercise price (in dollars per share)	$ 45.45	$ 41.43
Options and SARs exercisable, shares (in shares)	141,993	142,692
Options and SARs exercisable, weighted average exercise price (in dollars per share)	$ 41.00	$ 40.40