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Stock Compensation Plans
9 Months Ended
Sep. 30, 2018
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]  
STOCK COMPENSATION PLANS
STOCK COMPENSATION PLANS
Our Board of Directors granted 0.8 million restricted stock units and 0.2 million performance share units during the nine months ended September 30, 2018 under our stock compensation plans.
We measure the fair value of grants of restricted stock units and performance share units based primarily on the closing market price of a share of our common stock on the date of the grant, modified as appropriate to take into account the features of such grants. The weighted average fair value per share was $29.17 for restricted stock units and $29.04 for performance share units granted during the nine months ended September 30, 2018.
We recognized stock-based compensation expense of $6 million and $11 million during the three and nine months ended September 30, 2018, respectively. At September 30, 2018, unearned compensation cost related to the unvested portion of all stock-based awards was approximately $33 million and is expected to be recognized over the remaining vesting period of the respective grants, through the third quarter of 2022. We recognized stock-based compensation expense of $5 million and $17 million during the three and nine months ended September 30, 2017, respectively.