Stock Plans (Tables)

12 Months Ended

Dec. 31, 2018

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, 2017 (2,206,415 shares exercisable)	3,231,493		$	42.42
Granted	264,982		$	111.45
Exercised	(892,344	)	$	19.35
Forfeited/Canceled	(41,561	)	$	72.88
Balance at December 31, 2018	2,562,570		$	57.10	5.9	$	118,448
Exercisable at December 31, 2018	1,637,801		$	39.64	4.5	$	102,752
Vested and expected to vest at December 31, 2018	2,203,542		$	49.53	5.4	$	116,461

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 2018, 2017 and 2016, 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,
	2018			2017			2016
Volatility	29.8		%	28.8		%	33.3		%
Risk-free interest rate - weighted average	2.8		%	2.2		%	2.0		%
Risk-free interest rate - range	(a)			(a)			1.29% to 2.01%
Dividend yield	—		%	—		%	—		%
Assumed forfeitures	8.0		%	8.0		%	8.0		%
Expected lives	5.6 years			5.5 years			5.5 years
Weighted-average fair value (per share)	$	37.16		$	28.59		$	25.29

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, 2017	432,336		$	72.09
Granted	103,304		$	107.88
Vested	(150,339	)	$	62.83
Forfeited	(13,460	)	$	72.00
Nonvested at December 31, 2018	371,841		$	85.78

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, 2018, 2017 and 2016 and the assumptions used to calculate the fair value using the Black-Scholes-Merton option-pricing model:

	Year Ended December 31,
	2018			2017			2016
Volatility - weighted average	30.1		%	18.4		%	36.3		%
Volatility - range	23.5% to 36.7%			14.6% to 27.2%			20.0% to 50.1%
Risk-free interest rate - weighted average	2.01		%	0.89		%	0.24		%
Risk-free interest rate - range	1.73% to 2.45%			0.51% to 1.39%			0.22% to 0.29%
Dividend yield	—		%	—		%	—		%
Expected lives	3 months			3 months			3 months
Weighted-average fair value (per share)	$	17.22		$	15.81		$	14.42