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Note 9 - Stock-based Compensation (Tables)

6 Months Ended

Jun. 30, 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			57,901				128.14
Exercised			(28,991	)			58.78
Forfeited			(8,547	)			101.65
Options outstanding at June 30, 2021			249,894			$	98.81
Exercisable at June 30, 2021			116,452			$	81.38

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

		Six Months Ended June 30,
		2021				2020
Risk-free interest rate			0.80 - 1.12%				0.50 - 1.47%
Expected life (years)			6.25				6.25
Expected volatility			45.28 - 45.53%				42.40 - 43.83%
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			178,398				133.67
Restrictions lapsed			(113,315	)			91.10
Forfeited			(14,525	)			105.72
Restricted stock at June 30, 2021			357,757			$	118.41

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			(4,184	)			99.59
Performance stock at June 30, 2021			30,850			$	160.19

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

		Six Months Ended June 30,
		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]

		Six Months Ended June 30,
		2021				2020
Risk-free interest rate			0.06 - 0.12%				0.17 - 1.59%
Expected life (months)			6.00				6.00
Expected volatility			50.85 - 65.53%				42.63 - 59.99%
Expected dividend yield			0%				0%

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