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STOCK-BASED COMPENSATION | 9 Months Ended
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Sep. 30, 2018
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 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 |
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 |
Dividend yield was based on our current dividend yield as of the option grant date. 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.

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- Definition The entire disclosure for compensation-related costs for equity-based compensation, which may include disclosure of policies, compensation plan details, allocation of equity compensation, incentive distributions, equity-based arrangements to obtain goods and services, deferred compensation arrangements, employee stock ownership plan details and employee stock purchase plan details. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -URI http://asc.fasb.org/topic&trid=2228938 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 505 -SubTopic 50 -URI http://asc.fasb.org/subtopic&trid=2208855 + Details
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