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Stock-based Compensation Expense (Tables)
 12 Months Ended
Dec. 31, 2018
Share-based Compensation [Abstract]  
Fixed-Price Stock Options
The following table summarizes information about the Company’s fixed-price stock options as of and for the year ended December 31, 2018:
 
Shares
 
Weighted-Average Exercise Price
 
Weighted-Average Remaining Contractual Term (Years)
 
Aggregate Intrinsic Value
(in thousands)
Outstanding at December 31, 2017
1,401,718

 
$
56.10

 
5.1
 
$
100,632

Granted
63,259

 
118.75

 
 
 
 
Exercised
(94,031
)
 
38.64

 
 
 
 
Forfeited
(54,213
)
 
75.72

 
 
 
 
Outstanding at December 31, 2018
1,316,733

 
$
59.55

 
4.3
 
$
26,438

Exercisable at December 31, 2018
911,011

 
$
54.28

 
3.2
 
$
20,872
Weighted-Average Assumptions used to Estimate Fair Value of Each Option Granted
The calculation used the following weighted-average assumptions:
 
Year Ended December 31,
 
2018
 
2017
 
2016
Volatility
29.92
%
 
30.34
%
 
27.69
%
Risk-free interest rate
2.36
%
 
1.34
%
 
0.91
%
The fair value of each option granted during the years ended December 31, 2018, 2017 and 2016 was estimated on the date of grant using the Black-Scholes option-pricing model with the following weighted-average assumptions:
 
Year Ended December 31,
 
2018
 
2017
 
2016
Dividend yield
1.44
%
 
1.56
%
 
1.84
%
Volatility
32.48
%
 
32.70
%
 
33.08
%
Average expected life (years)
6

 
6

 
6

Risk-free interest rate
3.06
%
 
2.51
%
 
1.96
%
Fair value of options granted
$
37.35

 
$
27.99

 
$
16.06
Activity in Performance Unit Awards
The following table summarizes activity in performance unit awards as of and for the year ended December 31, 2018:
 
Shares
 
Weighted-Average Grant Date Fair Value Per Share
Nonvested, beginning of period
433,003

 
$
75.55

Granted
70,274

 
155.65

Vested
(164,303
)
 
61.72

Forfeited
(21,537
)
 
120.49

Nonvested, end of period
317,437

 
97.39
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, 2018:
 
Shares
 
Weighted-Average Grant Date Fair Value Per Share
Nonvested, beginning of period
223,338

 
$
71.95

Granted
104,513

 
104.36

Vested
(48,178
)
 
66.93

Forfeited
(22,155
)
 
78.98

Nonvested, end of period
257,518

 
85.44