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Stock-based Compensation - (Tables)
 12 Months Ended
Dec. 31, 2015
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]  
Summary of stock-based compensation costs
Stock-based compensation costs are as follows for the years ended December 31:

 
2015
 
2014
 
2013
Stock-based compensation cost
$
120

 
$
122

 
$
115

Tax benefit
(28
)
 
(26
)
 
(24
)
Stock-based compensation cost, net of tax
$
92

 
$
96

 
$
91
Weighted average assumptions used in the option pricing model for options granted
The fair value of each stock option granted is estimated using the Black-Scholes option pricing model. The following table presents the weighted average assumptions used in the option pricing model for options granted. The expected life of the options represents the period of time the options are expected to be outstanding. The expected life is based on our historical exercise trends and post-vest termination data incorporated into a forward-looking stock price model. The expected volatility is based on our implied volatility, which is the volatility forecast that is implied by the prices of actively traded options to purchase our stock observed in the market. The risk-free interest rate is based on the observed U.S. Treasury yield curve in effect at the time the options were granted. The dividend yield is based on our history of dividend payouts.

 
 
2014
 
2013
Expected life (years)
 
4.6

 
5.2

Risk-free interest rate
 
1.5
%
 
1.3
%
Volatility
 
31.9
%
 
36.0
%
Dividend yield
 
1.0
%
 
1.3
%
Weighted average fair value per share at grant date
 
$
16.81

 
$
13.79
Summary of stock options outstanding and related information
The following table presents the changes in stock options outstanding and related information (in thousands, except per option prices):

 
Number of
Options
 
Weighted Average
Exercise Price
Per Option
Outstanding at December 31, 2014
 
9,737

 
 
 
$
53.80

 
Granted
 
—

 
 
 
—

 
Exercised
 
(873
)
 
 
 
44.25

 
Forfeited
 
(124
)
 
 
 
53.90

 
Expired
 
(138
)
 
 
 
67.85

 
Outstanding at December 31, 2015
 
8,602

 
 
 
$
54.56

 
Exercisable at December 31, 2015
 
7,363

 
 
 
$
54.46

 
Summary of restricted stock awards
The following table presents the combined changes of RSAs and RSUs and related information (in thousands, except per award/unit prices):

 
Number of
Awards and Units
 
Weighted Average
Grant Date Fair
Value Per Award/Unit
Unvested balance at December 31, 2014
2,732

 
$
57.88

Granted
2,314

 
57.37

Vested
(1,299
)
 
55.09

Forfeited
(391
)
 
54.62

Unvested balance at December 31, 2015
3,356

 
$
58.99
Assumptions used in estimating fair value of shares granted under the employee stock purchase plan
Compensation cost for the years ended December 31, was calculated using the Black-Scholes option pricing model with the following assumptions:

 
2015
 
2014
 
2013
Expected life (years)
0.5

 
0.5

 
0.5

Risk-free interest rate
0.1
%
 
0.03
%
 
0.1
%
Volatility
30.9
%
 
24.7
%
 
30.3
%
Dividend yield
1.2
%
 
1.0
%
 
1.4
%
Fair value per share of the 15% cash discount
$
8.79

 
$
9.72

 
$
6.45

Fair value per share of the look-back provision
4.97

 
4.39

 
3.58

Total weighted average fair value per share at grant date
$
13.76

 
$
14.11

 
$
10.03