Note 9 - Stock-based Compensation (Tables)

3 Months Ended

Mar. 31, 2019

Notes Tables

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

				Weighted-
				Average
	Stock Options			Exercise Price
Options outstanding at December 31, 2018		252,616		$	64.71
Granted		30,046			104.99
Exercised		(3,010	)		57.85
Forfeited		(680	)		96.20
Options outstanding at March 31, 2019		278,972		$	69.04
Exercisable at March 31, 2019		157,602		$	56.70

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

	Three Months Ended March 31,
	2019			2018
Risk-free interest rate		2.58	%		2.52	%
Expected life (years)		6.25			6.25
Expected volatility		42.74	%		42.22	%
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, 2018		323,921		$	79.85
Granted		27,476			105.03
Restrictions lapsed		(22,420	)		71.40
Forfeited		(3,692	)		108.51
Restricted stock at March 31, 2019		325,285		$	82.35

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, 2018		52,140	$	76.54
Granted		21,434		104.99
Restrictions lapsed		-		-
Performance change		-		-
Forfeited		-		-
Performance stock at March 31, 2019		73,574	$	84.83

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

	Three Months Ended March 31,
	2019			2018
Risk-free interest rate		2.33	%		1.48	%
Expected life (months)		6.00			6.00
Expected volatility		37.36	%		24.49	%
Expected dividend yield		0	%		0	%