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Stock Options and Other Share-Based Compensation (Tables)
 12 Months Ended
Dec. 31, 2016
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 2016, 2015 and 2014 were measured on the date of grant using the Black-Scholes option-pricing model, with the following weighted-average assumptions:
 
Year ended December 31
 
2016

 
 
2015

 
2014

 
Expected term in years1
6.3



6.1


6.0


Volatility2
21.7

%

21.9

%
30.3

%
Risk-free interest rate based on zero coupon U.S. treasury note
1.6

%

1.4

%
1.9

%
Dividend yield
4.5

%

3.6

%
3.3

%
Weighted-average fair value per option granted
$
9.53



$
13.89


$
25.86


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 2016 is presented below:
 
Shares (Thousands)

Weighted-Average
 Exercise Price
 
 
Averaged Remaining Contractual Term (Years)
Aggregate Intrinsic Value
 
Outstanding at January 1, 2016
94,292

 
$
96.67

 

 

Granted
30,913

 
$
83.31

 

 

Exercised
(8,589
)
 
$
75.57

 

 

Forfeited
(4,341
)
 
$
86.81

 

 

Outstanding at December 31, 2016
112,275

 
$
94.99

 
6.13
 
$
2,550

Exercisable at December 31, 2016
71,153

 
$
97.32

 
4.66
 
$
1,450