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Stock Options and Other Share-Based Compensation (Tables)
 12 Months Ended
Dec. 31, 2012
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 2012, 2011 and 2010 were measured on the date of grant using the Black-Scholes option-pricing model, with the following weighted-average assumptions:
 
Year ended December 31
 
2012

 
 
2011

 
2010

 
Stock Options
 
 
 
 
 
 

Expected term in years1
6.0



6.2


6.1


Volatility2
31.7

%

31.0

%
30.8

%
Risk-free interest rate based on zero coupon U.S. treasury note
1.1

%

2.6

%
2.9

%
Dividend yield
3.2

%

3.6

%
3.9

%
Weighted-average fair value per option granted
$
23.35



$
21.24


$
16.28


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 2012 is presented below:
 
 
 
 
 
 
 
 
 
 
 
Weighted-

 
Average
 
 
 
 
 
Average

 
Remaining
 
Aggregate

 
Shares

 
Exercise

 
Contractual
 
Intrinsic

 
(Thousands)

 
Price

 
Term (Years)
 
Value

Outstanding at
January 1, 2012
72,348

 
$
73.71

 

 

Granted
12,455

 
$
107.73

 

 

Exercised
(12,024
)
 
$
62.13

 

 

Forfeited
(884
)
 
$
96.78

 

 

Outstanding at
December 31, 2012
71,895

 
$
81.26

 
6.3
 
$
1,933

Exercisable at
December 31, 2012
47,060

 
$
72.82

 
5.2
 
$
1,662