Note 9 - Stock-based Compensation (Tables)

9 Months Ended

Sep. 30, 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		53,708			105.81
Exercised		(11,921	)		56.25
Forfeited		(10,833	)		83.74
Options outstanding at September 30, 2019		283,570		$	72.12
Exercisable at September 30, 2019		154,539		$	58.18

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

	Nine Months Ended September 30,
	2019	2018
Risk-free interest rate	2.35 - 2.58%	2.52 - 3.07%
Expected life (years)	6.25	6.25
Expected volatility	42.52 - 42.74%	41.68 - 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		114,467			106.41
Restrictions lapsed		(95,884	)		79.32
Forfeited		(20,299	)		86.54
Restricted stock at September 30, 2019		322,205		$	89.02

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		(12,854	)		58.35
Forfeited		(2,375	)		105.37
Performance stock at September 30, 2019		58,345		$	89.82

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

	Nine Months Ended September 30,
	2019	2018
Risk-free interest rate	2.33 - 2.35%	1.48 - 2.06%
Expected life (months)	6.00	6.00
Expected volatility	37.36 - 53.57%	24.49 - 31.50%
Expected dividend yield	0%	0%