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STOCK-BASED COMPENSATION
 6 Months Ended
Jun. 30, 2015
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]  
STOCK-BASED COMPENSATION
STOCK-BASED COMPENSATION

Stock-based compensation granted includes stock options, performance stock awards, restricted stock awards and deferred directors’ compensation.  Stock-based compensation expense was as follows:
 
Three Months Ended
June 30,
 
Six Months Ended
June 30,
 
2015
 
2014
 
2015
 
2014
 
($ in millions)
Stock-based compensation
$
3.9

 
$
2.1

 
$
6.3

 
$
5.0

Mark-to-market adjustments
(3.1
)
 
(0.2
)
 
2.2

 
(1.1
)
Total expense
$
0.8

 
$
1.9

 
$
8.5

 
$
3.9



The fair value of each stock option granted, which typically vests ratably over three years, but not less than one year, was estimated on the date of grant, using the Black-Scholes option-pricing model with the following weighted-average assumptions:
Grant date
2015
 
2014
Dividend yield
2.92
%
 
3.13
%
Risk-free interest rate
1.69
%
 
2.13
%
Expected volatility
34
%
 
42
%
Expected life (years)
6.0

 
7.0

Weighted average grant fair value (per option)
$
6.80

 
$
8.34

Weighted average exercise price
$
27.40

 
$
25.69

Shares granted
776,750

 
624,200



Dividend yield for 2015 and 2014 was based on a historical average.  Risk-free interest rate was based on zero coupon U.S. Treasury securities rates for the expected life of the options.  Expected volatility was based on our historical stock price movements, as we believe that historical experience is the best available indicator of the expected volatility.  Expected life of the option grant was based on historical exercise and cancellation patterns, as we believe that historical experience is the best estimate of future exercise patterns.