Note 11 - Stock-based Compensation - Summary of Options Outstanding (Details) - Mercantile Plans [Member] - $ / shares	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Minimum exercise price (in dollars per share)	$ 22.15	$ 5.19	$ 5.19
Maximum exercise price (in dollars per share)	$ 36.22	$ 36.22	$ 36.22
Outstanding Number (in shares)	9,700
Average remaining option term (Year)	2 years 4 months 24 days	3 years 2 months 12 days	3 years 4 months 24 days
Outstanding Weighted Average Exercise Price (in dollars per share)	$ 32.83
Exercisable Number (in shares)	9,700
Exercisable Weighted Average Exercise Price (in dollars per share)	$ 32.83
Exercise Price Range 1 [Member]
Minimum exercise price (in dollars per share)	22.00
Maximum exercise price (in dollars per share)	$ 23.00
Outstanding Number (in shares)	1,000
Average remaining option term (Year)	10 months 24 days
Outstanding Weighted Average Exercise Price (in dollars per share)	$ 22.15
Exercisable Number (in shares)	1,000
Exercisable Weighted Average Exercise Price (in dollars per share)	$ 22.15
Exercise Price Range 2 [Member]
Minimum exercise price (in dollars per share)	27.00
Maximum exercise price (in dollars per share)	$ 28.00
Outstanding Number (in shares)	2,200
Average remaining option term (Year)	1 year 10 months 24 days
Outstanding Weighted Average Exercise Price (in dollars per share)	$ 27.66
Exercisable Number (in shares)	2,200
Exercisable Weighted Average Exercise Price (in dollars per share)	$ 27.66
Exercise Price Range 3 [Member]
Minimum exercise price (in dollars per share)	36.00
Maximum exercise price (in dollars per share)	$ 37.00
Outstanding Number (in shares)	6,500
Average remaining option term (Year)	2 years 10 months 24 days
Outstanding Weighted Average Exercise Price (in dollars per share)	$ 36.22
Exercisable Number (in shares)	6,500
Exercisable Weighted Average Exercise Price (in dollars per share)	$ 36.22