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Note 9 - Stock-based Compensation (Tables)

3 Months Ended

Mar. 31, 2021

Notes Tables

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

						Weighted-
						Average
		Stock Options				Exercise Price
Options outstanding at December 31, 2020			229,531			$	86.46
Granted			25,642				169.32
Exercised			(28,991	)			58.78
Forfeited			(8,097	)			101.21
Options outstanding at March 31, 2021			218,085			$	99.34
Exercisable at March 31, 2021			100,932			$	77.31

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

		Three Months Ended March 31,
		2021		2020
Risk-free interest rate		0.80% - 0.96%		1.47%
Expected life (years)		6.25		6.25
Expected volatility		45.28% - 45.35%		42.40%
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, 2020			307,199			$	98.87
Granted			74,411				187.04
Restrictions lapsed			(30,038	)			84.32
Forfeited			(8,662	)			100.95
Restricted stock at March 31, 2021			342,910			$	119.21

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, 2020			19,956			$	118.66
Granted			15,078				203.64
Restrictions lapsed			-				-
Performance change			-				-
Forfeited			-				-
Performance stock at March 31, 2021			35,034			$	155.23

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

		Three Months Ended March 31,
		2021		2020
Risk-free interest rate		0.22%		1.41%
Expected life (years)		2.87		2.88
Expected volatility		51.40%		38.70%
Expected dividend yield		0%		0%

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

		Three Months Ended March 31,
		2021		2020
Risk-free interest rate		0.12%		1.59%
Expected life (months)		6.00		6.00
Expected volatility		50.85%		42.63%
Expected dividend yield		0%		0%

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