Stock-Based Compensation (Tables)

6 Months Ended

Jun. 30, 2018

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

The following table summarizes the Company’s stock option activity and related information for the 2011 Plan, the 2000 Plan and the Directors Plan for the six months ended June 30, 2018:



	Options		Weighted Average Exercise Price		Weighted Average Remaining Contractual Life	Aggregate Intrinsic value (in thousands)
Outstanding at December 31, 2017	983,512		$	40.62	4.0	$	40,137
Granted	82,580		77.54			1,737
Exercised	(293,034	)	22.94			22,165
Canceled	(16,137	)	65.32			537
Outstanding at June 30, 2018	756,921		47.84		4.9	38,406
Exercisable at June 30, 2018	579,744		42.09		4.1	32,750

Share-Based Fair Value of Each Option Grant Weighted-Average Assumptions

The fair value of each option grant is estimated on the date of grant using the Black-Scholes option pricing model with the following weighted-average assumptions:



	Six Months Ended June 30,
	2018		2017
Dividend yield	0.31	%	0.23	%
Risk-free interest rate	2.78	%	2.17	%
Stock price volatility	23.9	%	23.4	%
Expected life (years)	5.0		5.0

Restricted Stock Activity and Incentive Stock Awards Activity

The following table summarizes the restricted stock activity and related information for the 2011 Plan, the 2000 Plan and the Directors Plan, and incentive stock units activity for the 2011 Plan and the 2000 Plan with related information for the six months ended June 30, 2018:



	Restricted Stock and Units		Incentive Stock Units		Weighted Average Grant Date Fair Value
Outstanding at December 31, 2017	399,000		327,333		$	78.76
Granted	223,960		175,100		73.76
Vested	(133,092	)	(93,312	)	81.34
Adjustment for incentive stock awards expected to vest	—		26,864		75.69
Canceled	(14,509	)	(19,800	)	77.38
Outstanding at June 30, 2018	475,359		416,185		75.94