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STOCK-BASED COMPENSATION (Tables)
3 Months Ended
Mar. 31, 2017
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]  
Stock-based compensation expense
Stock-based compensation expense was as follows:
 
Three Months Ended
March 31,
 
2017
 
2016
 
($ in millions)
Stock-based compensation
$
7.3

 
$
4.4

Mark-to-market adjustments
2.6

 
0.4

Total expense
$
9.9

 
$
4.8

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