Note 8 - Stock-based Compensation (Tables)

3 Months Ended

Mar. 31, 2020

Notes Tables

Share-based Payment Arrangement, Option, Activity [Table Text Block]

				Weighted-
				Average
	Stock Options			Exercise Price
Options outstanding at December 31, 2019		276,266		$	72.38
Granted		33,192			96.72
Exercised		(2,001	)		67.53
Forfeited		(848	)		110.38
Options outstanding at March 31, 2020		306,609		$	74.94
Exercisable at March 31, 2020		187,326		$	62.48

Schedule of Share-based Payment Award, Stock Options, Valuation Assumptions [Table Text Block]

	Three Months Ended March 31,
	2020	2019
Risk-free interest rate	1.47%	2.58%
Expected life (years)	6.25	6.25
Expected volatility	42.40%	42.74%
Expected dividend yield	0%	0%

Share-based Payment Arrangement, Restricted Stock and Restricted Stock Unit, Activity [Table Text Block]

				Weighted-
				Average
				Grant Date
	Restricted			Fair Value
	Stock			Per Share
Restricted stock at December 31, 2019		314,973		$	89.55
Granted		28,105			96.72
Restrictions lapsed		(25,598	)		79.27
Forfeited		(7,616	)		90.47
Restricted stock at March 31, 2020		309,864		$	91.03

Schedule of Nonvested Performance-based Units Activity [Table Text Block]

				Weighted-
				Average
				Grant Date
	Performance			Fair Value
	Stock			Per Share
Performance stock at December 31, 2019		9,383		$	58.35
Granted		19,956			118.66
Restrictions lapsed		(7,113	)		58.35
Performance change		-			-
Forfeited		(2,270	)		58.35
Performance stock at March 31, 2020		19,956		$	118.66

Schedule of Share-based Payment Award, Awards Other Than Stock Options, Valuation Assumptions [Table Text Block]

	Three Months Ended March 31,
	2020
Risk-free interest rate	1.41%
Expected life (years)	2.88
Expected volatility	38.70%
Expected dividend yield	0%

Schedule of Share-based Payment Award, Employee Stock Purchase Plan, Valuation Assumptions [Table Text Block]

	Three Months Ended March 31,
	2020	2019
Risk-free interest rate	1.59%	2.33%
Expected life (months)	6.00	6.00
Expected volatility	42.63%	37.36%
Expected dividend yield	0%	0%