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STOCK-BASED COMPENSATION (Tables)
6 Months Ended
Jun. 30, 2013
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]  
Stock-based compensation expense
Stock-based compensation expense was as follows:
 
Three Months Ended
June 30,
 
Six Months Ended
June 30,
 
2013
 
2012
 
2013
 
2012
 
($ in millions)
Stock-based compensation
$
3.5

 
$
1.9

 
$
6.2

 
$
3.8

Mark-to-market adjustments
(0.5
)
 
(0.4
)
 
1.3

 
0.5

Total expense
$
3.0

 
$
1.5

 
$
7.5

 
$
4.3

Schedule of fair value of stock options granted
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
2013
 
2012
Dividend yield
3.44
%
 
3.65
%
Risk-free interest rate
1.35
%
 
1.36
%
Expected volatility
43
%
 
43
%
Expected life (years)
7.0

 
7.0

Grant fair value (per option)
$
7.05

 
$
6.55

Exercise price
$
23.28

 
$
21.92

Shares granted
621,000

 
480,250