Note 9 - Stock-based Compensation (Tables)

6 Months Ended

Jun. 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		(7,640	)		55.93
Forfeited		(9,641	)		81.88
Options outstanding at June 30, 2019		289,043		$	72.00
Exercisable at June 30, 2019		158,686		$	58.13

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

	Six Months Ended June 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%	41.95 - 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		(95,818	)		79.32
Forfeited		(14,891	)		83.73
Restricted stock at June 30, 2019		327,679		$	89.11

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		(2,375	)		105.37
Performance stock at June 30, 2019		71,199		$	84.14

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

	Six Months Ended June 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%