Stock-based Compensation Expense (Tables)

12 Months Ended

Dec. 31, 2017

The following table summarizes information about the Company’s fixed-price stock options as of and for the year ended December 31, 2017:



	Shares		Weighted-Average Exercise Price		Weighted-Average Remaining Contractual Term (Years)	Aggregate Intrinsic Value (in thousands)
Outstanding at December 31, 2016	1,542,244		$	51.85	5.5	$	52,798
Granted	82,204		92.93
Exercised	(210,432	)	39.15
Forfeited	(12,298	)	59.05
Outstanding at December 31, 2017	1,401,718		$	56.10	5.1	$	100,632
Exercisable at December 31, 2017	768,436		$	50.35	3.3	$	59,585

Weighted-Average Assumptions used to Estimate Fair Value of Each Option Granted

The fair value of each option granted during the years ended December 31, 2017, 2016 and 2015 was estimated on the date of grant using the Black-Scholes option-pricing model with the following weighted-average assumptions:



	Year Ended December 31,
	2017			2016			2015
Dividend yield	1.56		%	1.84		%	1.80		%
Volatility	32.70		%	33.08		%	32.92		%
Average expected life (years)	6			6			6
Risk-free interest rate	2.51		%	1.96		%	2.17		%
Fair value of options granted	$	27.99		$	16.06		$	16.04

Activity in Performance Unit Awards

The following table summarizes activity in performance unit awards as of and for the year ended December 31, 2017:



	Shares		Weighted-Average Grant Date Fair Value Per Share
Nonvested, beginning of period	494,691		$	66.42
Granted	82,396		116.43
Vested	(131,374	)	67.59
Forfeited	(12,710	)	67.36
Nonvested, end of period	433,003		75.55

Activity in Non-Performance Based Restricted Stock Awards

The following table summarizes activity in non-performance based restricted stock and restricted stock unit awards as of and for the year ended December 31, 2017:



	Shares		Weighted-Average Grant Date Fair Value Per Share
Nonvested, beginning of period	180,656		$	57.99
Granted	84,528		97.49
Vested	(32,675	)	63.45
Forfeited	(9,171	)	60.17
Nonvested, end of period	223,338		71.95