Note 13 - Stock-based Compensation - Information about Stock Options Outstanding (Details) - $ / shares	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Options outstanding, vested and expected to vest, number outstanding (in shares)	276,266	252,616	399,055	569,639
Options outstanding, vested and expected to vest, weighted average exercise price (in dollars per share)	$ 72.38	$ 64.71	$ 51.13	$ 45.00
Exercise Price Range One [Member]
Range of exercise prices, lower limit (in dollars per share)	7.76
Range of exercise prices, upper limit (in dollars per share)	$ 31.43
Options outstanding, vested and expected to vest, number outstanding (in shares)	23,883
Options outstanding, vested and expected to vest, weighted average remaining contractual life (Year)	1 year 11 months 23 days
Options outstanding, vested and expected to vest, weighted average exercise price (in dollars per share)	$ 25.04
Options exercisable, number exercisable (in shares)	23,883
Options exercisable, weighted average exercise price (in dollars per share)	$ 25.04
Exercise Price Range Two [Member]
Range of exercise prices, lower limit (in dollars per share)	31.44
Range of exercise prices, upper limit (in dollars per share)	$ 66.87
Options outstanding, vested and expected to vest, number outstanding (in shares)	125,022
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)	$ 58.67
Options exercisable, number exercisable (in shares)	85,656
Options exercisable, weighted average exercise price (in dollars per share)	$ 58.81
Exercise Price Range Three [Member]
Range of exercise prices, lower limit (in dollars per share)	66.88
Range of exercise prices, upper limit (in dollars per share)	$ 96.20
Options outstanding, vested and expected to vest, number outstanding (in shares)	45,255
Options outstanding, vested and expected to vest, weighted average remaining contractual life (Year)	4 years 5 months 26 days
Options outstanding, vested and expected to vest, weighted average exercise price (in dollars per share)	$ 70.98
Options exercisable, number exercisable (in shares)	33,445
Options exercisable, weighted average exercise price (in dollars per share)	$ 70.41
Exercise Price Range Four [Member]
Range of exercise prices, lower limit (in dollars per share)	96.21
Range of exercise prices, upper limit (in dollars per share)	$ 123.10
Options outstanding, vested and expected to vest, number outstanding (in shares)	82,106
Options outstanding, vested and expected to vest, weighted average remaining contractual life (Year)	8 years 8 months 4 days
Options outstanding, vested and expected to vest, weighted average exercise price (in dollars per share)	$ 107.79
Options exercisable, number exercisable (in shares)	9,439
Options exercisable, weighted average exercise price (in dollars per share)	$ 110.36