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Note 9 - Stock-based Compensation (Tables)

3 Months Ended

Mar. 31, 2018

Notes Tables

Share-based Compensation, Stock Options, Activity [Table Text Block]

						Weighted-
						Average
		Stock Options				Exercise Price
Options outstanding at December 31, 2017			399,397			$	51.14
Granted			22,450				105.75
Exercised			(81,964	)			35.14
Forfeited			(10,822	)			61.94
Options outstanding at March 31, 2018			329,061			$	58.50
Exercisable at March 31, 2018			193,438			$	48.70

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

		Three Months Ended March 31,
		2018				2017
Risk-free interest rate			2.52%				2.24%
Expected life (years)			6.25				6.50
Expected volatility			42.22%				44.68%
Expected dividend yield			0%				0%

Schedule of Share-based Compensation, Restricted Stock and Restricted Stock Units Activity [Table Text Block]

						Weighted-
						Average
						Grant Date
		Restricted				Fair Value
		Stock				Per Share
Restricted stock at December 31, 2017			334,608			$	63.29
Granted			20,976				105.75
Restrictions lapsed			(22,616	)			62.94
Forfeited			(13,536	)			61.33
Restricted stock at March 31, 2018			319,432			$	66.18

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, 2017			25,707			$	58.35
Granted			20,006				105.75
Restrictions lapsed			-				-
Performance assumption change			6,427				58.35
Forfeited			-				-
Performance stock at March 31, 2018			52,140			$	76.54

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

		Three Months Ended March 31,
		2018				2017
Risk-free interest rate			1.48%				0.59%
Expected life (months)			6.00				6.00
Expected volatility			24.49%				39.51%
Expected dividend yield			0%				0%

X

- Definition

Tabular disclosure of the changes in outstanding nonvested performance-based units.

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