Note 12 - Stock-based Compensation - Information about Stock Options Outstanding (Details) - $ / shares	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Options outstanding, vested and expected to vest, number outstanding (in shares)	229,531	276,266	252,616	399,055
Options outstanding, vested and expected to vest, weighted average exercise price (in dollars per share)	$ 86.46	$ 72.38	$ 64.71	$ 51.13
Exercise Price Range One [Member]
Range of exercise prices, lower limit (in dollars per share)
Options outstanding, vested and expected to vest, number outstanding (in shares)	5,964
Range of exercise prices, upper limit (in dollars per share)
Options outstanding, vested and expected to vest, weighted average remaining contractual life (Year)	1 year 4 months 6 days
Options outstanding, vested and expected to vest, weighted average exercise price (in dollars per share)	$ 30.58
Options exercisable, number exercisable (in shares)	5,964
Options exercisable, weighted average exercise price (in dollars per share)	$ 30.58
Exercise Price Range Two [Member]
Range of exercise prices, lower limit (in dollars per share)
Options outstanding, vested and expected to vest, number outstanding (in shares)	78,482
Range of exercise prices, upper limit (in dollars per share)
Options outstanding, vested and expected to vest, weighted average remaining contractual life (Year)	4 years 9 months 21 days
Options outstanding, vested and expected to vest, weighted average exercise price (in dollars per share)	$ 57.51
Options exercisable, number exercisable (in shares)	56,032
Options exercisable, weighted average exercise price (in dollars per share)	$ 57.18
Exercise Price Range Three [Member]
Range of exercise prices, lower limit (in dollars per share)
Options outstanding, vested and expected to vest, number outstanding (in shares)	10,591
Range of exercise prices, upper limit (in dollars per share)
Options outstanding, vested and expected to vest, weighted average remaining contractual life (Year)	4 years 5 months 1 day
Options outstanding, vested and expected to vest, weighted average exercise price (in dollars per share)	$ 74.05
Options exercisable, number exercisable (in shares)	9,343
Options exercisable, weighted average exercise price (in dollars per share)	$ 71.09
Exercise Price Range Four [Member]
Range of exercise prices, lower limit (in dollars per share)
Options outstanding, vested and expected to vest, number outstanding (in shares)	134,494
Range of exercise prices, upper limit (in dollars per share)
Options outstanding, vested and expected to vest, weighted average remaining contractual life (Year)	8 years 5 months 12 days
Options outstanding, vested and expected to vest, weighted average exercise price (in dollars per share)	$ 106.81
Options exercisable, number exercisable (in shares)	22,979
Options exercisable, weighted average exercise price (in dollars per share)	$ 108.41