Note 8 - Stock-based Compensation (Tables)

6 Months Ended

Jun. 30, 2017

Notes Tables

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

				Weighted-
				Average
	Stock Options			Exercise Price
Options outstanding at December 31, 2016		569,639		$	45.00
Granted		43,095			58.35
Exercised		(88,114	)		31.71
Forfeited		(16,943	)		58.17
Options outstanding at June 30, 2017		507,677		$	48.00
Exercisable at June 30, 2017		318,140		$	40.42

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

	Six Months Ended June 30,
	2017		2016
Risk-free interest rate		2.24%		1.53	-	1.68%
Expected life (years)		6.50			6.50
Expected volatility		44.68%		44.38	-	45.93%
Expected dividend yield		0%			0%

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

	Restricted			Weighted- Average Grant Date Fair Value
	Stock			Per Share
Restricted stock at December 31, 2016		215,105		$	62.78
Granted		197,649			61.60
Restrictions lapsed		(55,139	)		63.61
Forfeited		(9,570	)		63.76
Restricted stock at June 30, 2017		348,045		$	61.95

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, 2016		-	$	-
Granted		25,707		58.35
Restrictions lapsed		-		-
Forfeited		-		-
Performance stock at June 30, 2017		25,707	$	58.35

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

	Six Months Ended June 30,
	2017				2016
Risk-free interest rate		0.59%	-	0.97%		0.39	-	0.56%
Expected life (months)			6.00				6.00
Expected volatility		34.51%	-	39.51%		29.41	-	49.13%
Expected dividend yield			0%				0%