Stock-based Compensation Expense (Tables)

12 Months Ended

Dec. 31, 2015

The following table summarizes information about the Company’s fixed-price stock options as of and for the year ended December 31, 2015:



	Shares		Weighted-Average Exercise Price		Weighted-Average Remaining Contractual Term (Years)	Aggregate Intrinsic Value (in thousands)
Outstanding at December 31, 2014	1,484,243		$	50.30	6.5	$	17,887
Granted	313,803		55.74
Exercised	(18,000	)	28.72
Forfeited	(98,519	)	62.98
Expired	(4,600	)	66.14
Outstanding at December 31, 2015	1,676,927		$	50.76	6.1	$	14,152
Exercisable at December 31, 2015	998,952		$	43.95	4.5	$	14,048

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, 2015, 2014 and 2013 was estimated on the date of grant using the Black-Scholes option-pricing model with the following weighted-average assumptions:



	Year Ended December 31,
	2015			2014			2013
Dividend yield	1.80		%	1.71		%	1.58		%
Volatility	32.92		%	33.03		%	33.55		%
Average expected life (years)	6			6			6
Risk-free interest rate	2.17		%	2.94		%	2.18		%
Fair value of options granted	$	16.04		$	19.56		$	19.73

Activity in Performance Unit Awards

The following table summarizes activity in performance unit awards as of and for the year ended December 31, 2015:



	Shares		Weighted-Average Grant Date Fair Value Per Share
Nonvested, beginning of period	456,018		$	66.21
Granted	214,610		55.34
Vested	(43,177	)	65.39
Forfeited	(130,246	)	64.50
Nonvested, end of period	497,205		62.04

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, 2015:



	Shares		Weighted-Average Grant Date Fair Value Per Share
Nonvested, beginning of period	105,288		$	61.34
Granted	61,156		56.64
Vested	(39,073	)	61.97
Forfeited	(9,250	)	62.37
Nonvested, end of period	118,121		58.62