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Stock Options and Other Share-Based Compensation (Tables)
 12 Months Ended
Dec. 31, 2015
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]  
Fair market values of stock options and stock appreciation rights granted
The fair market values of stock options and stock appreciation rights granted in 2015, 2014 and 2013 were measured 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

 
Expected term in years1
6.1



6.0


6.0


Volatility2
21.9

%

30.3

%
31.3

%
Risk-free interest rate based on zero coupon U.S. treasury note
1.4

%

1.9

%
1.2

%
Dividend yield
3.6

%

3.3

%
3.3

%
Weighted-average fair value per option granted
$
13.89



$
25.86


$
24.48


1    Expected term is based on historical exercise and postvesting cancellation data.
2    Volatility rate is based on historical stock prices over an appropriate period, generally equal to the expected term.
Summary of option activity
A summary of option activity during 2015 is presented below:
 
Shares (Thousands)

Weighted-Average
 Exercise Price
 
 
Averaged Remaining Contractual Term (Years)
Aggregate Intrinsic Value
 
Outstanding at January 1, 2015
78,341

 
$
93.59

 

 

Granted
22,126

 
$
103.71

 

 

Exercised
(3,104
)
 
$
62.06

 

 

Forfeited
(3,071
)
 
$
103.70

 

 

Outstanding at December 31, 2015
94,292

 
$
96.67

 
5.83
 
$
467

Exercisable at December 31, 2015
65,657

 
$
91.85

 
4.61
 
$
467