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Share-Based Compensation (Tables)
 12 Months Ended
Dec. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]  
Stock Option Activity
The following table summarizes stock option activity under our share-based plans for the year ended December 31, 2017:

 
 
Shares
 
Weighted Average
Exercise Price
 
Weighted Average
Remaining
Contractual Term
(Years)
 
Aggregate
Intrinsic Value
Balance at December 31, 2016
 
2,550,883

 
$
34.70

 
 
 
 
Granted
 
108,275

 
116.99

 
 
 
 

Less: Exercised
 
364,984

 
26.88

 
 
 
 

           Forfeited
 
6,775

 
94.20

 
 
 
 

Balance at December 31, 2017
 
2,287,399

 
$
39.67

 
3.85
 
$
205,820,051

 
 
 
 
 
 
 
 
 
Exercisable at December 31, 2017
 
1,650,386

 
$
26.33

 
2.55
 
$
170,526,117
Stock options outstanding and exercisable by exercise price range
The following table presents information about stock options outstanding and exercisable at December 31, 2017:

 
 
Outstanding
Stock Options
 
Exercisable
Stock Options
Range of Exercise Prices
 
Shares
 
Weighted Average
Remaining
Contractual Term
(Years)
 
Weighted Average Exercise Price
 
Shares
 
Weighted Average Exercise Price
$ 18.00 to $ 20.34
 
885,493

 
1.36
 
$
19.54

 
885,493

 
$
19.54

$ 20.35 to $ 45.61
 
811,756

 
3.91
 
33.75

 
695,031

 
31.76

$ 45.62 to $ 117.04
 
590,150

 
7.51
 
78.02

 
69,862

 
58.30

 
 
2,287,399

 
3.85
 
$
39.67

 
1,650,386

 
$
26.33
Summary of cash proceeds and tax benefits realized from stock option exercise.
The following table summarizes the cash proceeds and tax benefits realized from the exercise of stock options:

 
 
Year Ended December 31,
(in thousands, except share amounts)
 
2017
 
2016
 
2015
Options exercised
 
364,984

 
343,237

 
543,028

Cash proceeds
 
$
9,809

 
$
10,340

 
$
17,137

Intrinsic value of options exercised
 
$
33,302

 
$
21,094

 
$
22,676

Tax benefits realized
 
$
12,809

 
$
7,891

 
$
8,326
Summary of assumptions for estimated fair value of employee stock option awards at grant date
We estimated the fair value of employee stock option awards at the grant date based on the assumptions summarized in the following table:
 
 
 
Year Ended December 31,
(Weighted average)
 
2017
 
2016
 
2015
Expected volatility
 
26.6
%
 
 
29.7
%
 
 
33.8
%
 
Expected term
 
7.3

years
 
7.1

years
 
7.2

years
Risk-free interest rate
 
2.44
%
 
 
1.75
%
 
 
1.95
%
 
Expected dividend yield
 
1.5
%
 
 
1.5
%
 
 
1.5
%
 
Grant date fair value
 
$
32.00

 
 
$
22.86

 
 
$
22.57

 
Summary of restricted share actvity
The table below presents restricted stock awards activity under our share-based plans for the year ended December 31, 2017:

 
 
Shares
 
Weighted Average
Grant Date Fair Value
Balance unvested at December 31, 2016
 
285,019

 
$
64.33

Granted (at market price) (1)
 
95,479

 
115.38

Less: Vested
 
79,224

 
52.73

Forfeited
 
3,750

 
80.69

Balance unvested at December 31, 2017
 
297,524

 
$
83.36
Total restricted stock awards that vested for the past three years and related fair value
The table below presents the total number of restricted stock awards that vested for the past three years and the related fair value of those awards (in thousands, except share amounts):

 
 
2017
 
2016
 
2015
Restricted stock awards - shares vested
 
79,224

 
95,420

 
147,619

Fair value of restricted stock awards vested
 
$
9,260

 
$
7,960

 
$
10,182
Number of shares purchased by employees under the employee stock purchase plan
For the two six month offering periods in 2017, the one six month offering period in 2016, and the two six month offering periods in 2015, our employees purchased the following aggregate number of shares:

2017
 
2016
 
2015
16,610

 
8,649

 
22,555
Restricted [Member]  
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]  
Summary of share-based compensation expense and recognized tax benefits [Table Text Block]
The following table presents the total share-based compensation expense for restricted stock awards for the past three years (in thousands):

 
 
2017
 
2016
 
2015
Restricted stock awards share-based compensation expense
 
$
8,547

 
$
5,993

 
$
5,513
Options [Member]  
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]  
Summary of share-based compensation expense and recognized tax benefits [Table Text Block]
The following table presents the total share-based compensation expense for stock option awards and the related recognized tax benefits for the past three years (in thousands):

 
 
2017
 
2016
 
2015
Option grant share-based compensation expense
 
$
3,553

 
$
3,735

 
$
3,688

Option grant recognized tax benefits
 
888

 
1,409

 
1,331