Stock Plans (Tables)

12 Months Ended

Dec. 31, 2019

Disclosure of Compensation Related Costs, Share-based Payments [Abstract]

Disclosure of Share-based Compensation Arrangements by Share-based Payment Award [Table Text Block]

Summary stock options activity is presented in the table below:

	Number of Shares		Weighted Average Exercise Price		Weighted Average Remaining Contractual Term (years)	Aggregate Intrinsic Value (thousands)
Balance at December 31, 2018 (1,637,801 shares exercisable)	2,562,570		$	57.10
Granted	795,274		$	145.92
Exercised	(295,420	)	$	44.22
Forfeited/Canceled	(46,287	)	$	89.67
Expired	(362	)
Balance at December 31, 2019	3,015,775		$	81.29	6.2	$	230,052
Exercisable at December 31, 2019	1,653,340		$	46.36	4.1	$	183,846
Vested and expected to vest at December 31, 2019	2,383,821		$	66.65	5.4	$	216,739

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

The following table provides the fair value of options granted under the SCP during 2019, 2018 and 2017, together with a description of the assumptions used to calculate the fair value using the Black-Scholes-Merton option-pricing model:

	Year ended December 31,
	2019			2018			2017
Volatility	29.3		%	29.8		%	28.8		%
Risk-free interest rate - weighted average	2.1		%	2.8		%	2.2		%
Risk-free interest rate - range	(a)			(a)			.022
Dividend yield	—		%	—		%	—		%
Assumed forfeitures	8.0		%	8.0		%	8.0		%
Expected lives	5.2 years			5.6 years			5.5 years
Weighted-average fair value (per share)	$	43.96		$	37.16		$	28.59

Schedule of Nonvested Restricted Stock Units Activity [Table Text Block]

Summary restricted stock activity is presented in the table below:

	Number of Shares		Weighted Average Grant Date Fair Value Per Share
Nonvested at December 31, 2018	371,841		$	85.78
Granted	254,631		$	145.93
Vested	(115,740	)	$	78.77
Forfeited	(16,784	)	$	92.44
Nonvested at December 31, 2019	493,948		$	118.20

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

The following table provides the weighted-average fair value of the ESPP stock purchase rights during the years ended December 31, 2019, 2018 and 2017 and the assumptions used to calculate the fair value using the Black-Scholes-Merton option-pricing model:

	Year Ended December 31,
	2019			2018			2017
Volatility - weighted average	24.3		%	30.1		%	18.4		%
Volatility - range	20.3% to 28.1%			23.5% to 36.7%			14.6% to 27.2%
Risk-free interest rate - weighted average	2.07		%	2.01		%	0.89		%
Risk-free interest rate - range	1.55% to 2.44%			1.73% to 2.45%			0.51% to 1.39%
Dividend yield	—		%	—		%	—		%
Expected lives	3 months			3 months			3 months
Weighted-average fair value (per share)	$	25.87		$	17.22		$	15.81