Note 8 - Equity Incentive Plan - Summary of Stock Option Activity (Details) - $ / shares	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Issuance of shares of common stock, shares (in shares)	7,202	8,763
Outstanding, shares (in shares)	132,389
Outstanding, weighted average exercise price (in dollars per share)	$ 40.88
Stock Options and Stock Appreciation Rights [Member]
Outstanding, shares (in shares)	273,039	277,820
Outstanding, weighted average exercise price (in dollars per share)	$ 41.19	$ 40.11
Shares Granted (in shares)	0	0
Granted, Weighted Average Exercise Price (in dollars per share)	$ 0	$ 0
Issuance of shares of common stock, shares (in shares)	7,654	9,013
Exercised, Weighted Average Exercise Price (in dollars per share)	$ 33.82	$ 35.13
Shares Forfeited or expired (in shares)	4,300	0
Forfeited or expired, Weighted Average Exercise Price (in dollars per share)	$ 39.46	$ 0
Outstanding, shares (in shares)	261,085	268,807
Outstanding, weighted average exercise price (in dollars per share)	$ 41.43	$ 40.28
Options and SARs exercisable, shares (in shares)	142,692	109,897
Options and SARs exercisable, weighted average exercise price (in dollars per share)	$ 40.40	$ 39.44