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STOCK-BASED COMPENSATION (Tables)
 9 Months Ended
Sep. 30, 2017
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,
 
2017
 
2016
 
2017
 
2016
 
($ in millions)
Stock-based compensation
$
3.0

 
$
3.8

 
$
14.5

 
$
9.8

Mark-to-market adjustments
2.6

 
(1.4
)
 
3.9

 
1.0

Total expense
$
5.6

 
$
2.4

 
$
18.4

 
$
10.8
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
2017
 
2016
Dividend yield
2.69
%
 
6.09
%
Risk-free interest rate
2.06
%
 
1.35
%
Expected volatility
34
%
 
32
%
Expected life (years)
6.0

 
6.0

Weighted-average grant fair value (per option)
$
7.78

 
$
1.90

Weighted-average exercise price
$
29.75

 
$
13.14

Shares granted
1,572,000

 
1,670,400