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STOCK-BASED COMPENSATION (Tables)
 9 Months Ended
Sep. 30, 2018
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]  
Stock-based compensation expense
Stock-based compensation expense was as follows:
 
Three Months Ended September 30,
 
Nine Months Ended September 30,
 
2018
 
2017
 
2018
 
2017
 
($ in millions)
Stock-based compensation
$
4.1

 
$
3.0

 
$
15.0

 
$
14.5

Mark-to-market adjustments
(1.4
)
 
2.6

 
(5.7
)
 
3.9

Total expense
$
2.7

 
$
5.6

 
$
9.3

 
$
18.4
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
2018
 
2017
Dividend yield
2.43
%
 
2.69
%
Risk-free interest rate
2.72
%
 
2.06
%
Expected volatility
32
%
 
34
%
Expected life (years)
6.0

 
6.0

Weighted-average grant fair value (per option)
$
8.89

 
$
7.78

Weighted-average exercise price
$
32.94

 
$
29.82

Shares granted
927,000

 
1,621,000