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Stock-Based Compensation Plans
 6 Months Ended
Jun. 30, 2014
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]  
Stock-Based Compensation Plans
Stock-Based Compensation Plans
Stock Option Awards
The following table presents a summary of our stock option award activity for the six months ended June 30, 2014:
 
  Number of Shares(a)
 
Weighted Average Exercise Price
Outstanding at December 31, 2013
5,147,837

 
$
40.08

Granted
446,310

 
85.02

Exercised
(583,773
)
 
31.21

Forfeited, canceled or expired
(20,761
)
 
49.67

Outstanding at June 30, 2014
4,989,613

 
45.09

(a) 
Includes an immaterial number of stock appreciation rights.
The grant date fair value of stock option awards granted during the six months ended June 30, 2014 was $25.37 per share. The fair value of stock options granted to our employees is estimated on the date of the grant using the Black Scholes option-pricing model, which employs various assumptions. Beginning in the first quarter of 2014, the assumption for expected volatility of our stock price was refined to reflect a weighting of 50 percent of MPC’s common stock implied volatility and 50 percent of MPC's common stock historical volatility.

Restricted Stock Awards
The following table presents a summary of restricted stock award activity for the six months ended June 30, 2014:
 
Shares of Restricted Stock ("RS")
 
Restricted Stock Units ("RSU")
 
Number of Shares
 
Weighted Average Grant Date Fair Value
 
Number of Units
 
Weighted Average Grant Date Fair Value
Outstanding at December 31, 2013
624,122

 
$
61.11

 
385,079

 
$
33.96

Granted
232,594

 
88.19

 
12,889

 
89.16

RS's Vested/RSU's Issued
(264,470
)
 
58.48

 
(398
)
 
60.11

Forfeited
(16,446
)
 
65.72

 
(133
)
 
76.10

Outstanding at June 30, 2014
575,800

 
73.13

 
397,437

 
35.71


Performance Unit Awards
The following table presents a summary of the activity for performance unit awards to be settled in shares for the six months ended June 30, 2014:
 
Number of Units
Outstanding at December 31, 2013
3,822,500

Granted
2,033,700

Settled
—

Canceled
—

Outstanding at June 30, 2014
5,856,200


The performance unit awards granted in 2014 have a grant date fair value of $1.02 per unit, as calculated using a Monte Carlo valuation model.
MPLX Awards
During the six months ended June 30, 2014, MPLX granted equity-based compensation awards under the MPLX LP 2012 Incentive Compensation Plan. The compensation expense for these awards is not material to our consolidated financial statements.