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Stock Incentive Plans (Tables)
 12 Months Ended
Dec. 31, 2014
Stock Incentive Plans  
Summary of the assumptions used to value the ESPP awards
The following table is a summary of the assumptions used to value the ESPP awards for each of the respective periods: 
 
2014
 
2013
 
2012
Dividend yield(1)
0.75
%
 
0.86
%
 
0.73
%
Expected volatility(2)
14.87
%
 
16.94
%
 
22.88
%
Risk-free interest rate(3)
0.08
%
 
0.10
%
 
0.10
%
Expected life (in years)(4)
0.5

 
0.5

 
0.5

Weighted-average grant date fair value
$
13.74

 
$
10.08

 
$
9.22


 
(1)
The dividend yield is calculated based on semi-annual dividends paid and the fair market value of the Company’s stock at the grant date.
(2)
The expected volatility is based on the historical volatility of the Company’s daily stock market prices over the previous six month period.
(3)
The risk-free interest rate is based on the Treasury constant maturity interest rate whose term is consistent with the expected term of ESPP options (i.e., 6 months).
(4)
The expected life is based on the semi-annual purchase period.
Summary of the restricted stock unit and restricted share award activity
The following table is a summary of the restricted stock unit and restricted share award activity for the year ended December 31, 2014. 
Units in thousands
Units
 
Weighted Average 
Grant Date 
Fair Value
Nonvested at beginning of year
3,021

 
$
38.56

Granted
2,708

 
$
73.60

Vested
(803
)
 
$
73.11

Forfeited
(249
)
 
$
57.58

Nonvested at end of year
4,677

 
$
51.90
Black-Scholes option pricing model, assumptions
The fair value of each stock option is estimated using the Black-Scholes option pricing model based on the following assumptions at the time of grant:
 
 
2014
 
2013
 
2012
Dividend yield(1)
1.47
%
 
1.65
%
 
1.44
%
Expected volatility(2)
19.92
%
 
30.96
%
 
32.49
%
Risk-free interest rate(3)
1.35
%
 
0.73
%
 
0.84
%
Expected life (in years)(4)
4.0

 
4.7

 
4.7

Weighted-average grant date fair value
$
11.04

 
$
12.50

 
$
11.12


                                    
(1)
The dividend yield is based on annual dividends paid and the fair market value of the Company’s stock at the grant date.
(2)
The expected volatility is estimated using the Company’s historical volatility over a period equal to the expected life of each option grant after adjustments for infrequent events such as stock splits.
(3)
The risk-free interest rate is selected based on yields from U.S. Treasury zero-coupon issues with a remaining term equal to the expected term of the options being valued.
(4)
The expected life represents the number of years the options are expected to be outstanding from grant date based on historical option holder exercise experience.
Summary of the Company's stock option activity
The following table is a summary of the Company’s stock option activity for the year ended December 31, 2014: 
Shares in thousands 
Shares
 
Weighted Average
Exercise Price
 
Weighted Average
Remaining 
Contractual
Term
 
Aggregate Intrinsic
Value
Outstanding at December 31, 2013
34,738

 
$
41.40

 

 


Granted
4,525

 
$
74.96

 

 


Exercised
(9,563
)
 
$
37.30

 

 


Forfeited
(1,202
)
 
$
50.15

 

 


Expired
(332
)
 
$
36.93

 

 


Outstanding at December 31, 2014
28,166

 
$
47.87

 
4.15
 
$
1,364,408,886

Exercisable at December 31, 2014
11,634

 
$
37.86

 
2.82
 
$
679,995,090

Vested at December 31, 2014 and expected
    to vest in the future
27,394

 
$
47.51

 
4.11
 
$
1,336,774,863