Stock-Based Compensation Plans
 3 Months Ended
Mar. 31, 2018
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 three months ended March 31, 2018:
 
  Number of Shares
 
Weighted Average Exercise Price
Outstanding at December 31, 2017
8,465,398

 
$
33.74

Granted
529,197

 
64.79

Exercised
(477,086
)
 
24.68

Forfeited or expired
(4,512
)
 
49.94

Outstanding at March 31, 2018
8,512,997

 
36.17


The grant date fair value of stock option awards granted during the three months ended March 31, 2018 was $17.37 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.

Restricted Stock Awards
The following table presents a summary of restricted stock award activity for the three months ended March 31, 2018:
 
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, 2017
1,188,662

 
$
45.07

 
285,164

 
$
29.95

Granted
94,592

 
64.79

 
6,924

 
67.75

RS’s Vested/RSU’s Issued
(123,918
)
 
45.06

 
—

 
—

Forfeited
(24,639
)
 
44.62

 
—

 
—

Outstanding at March 31, 2018
1,134,697

 
46.72

 
292,088

 
30.85


Performance Unit Awards
The following table presents a summary of the activity for performance unit awards to be settled in shares for the three months ended March 31, 2018:
 
Number of Units
 
Weighted Average Grant Date Fair Value
Outstanding at December 31, 2017
6,851,542

 
$
0.81

Granted
3,830,000

 
0.83

Vested
(2,052,959
)
 
0.95

Forfeited
(10,000
)
 
0.92

Outstanding at March 31, 2018
8,618,583

 
0.79


The performance unit awards granted during the three months ended March 31, 2018 have a grant date fair value of $0.83 per unit, as calculated using a Monte Carlo valuation model.
MPLX Awards
During the three months ended March 31, 2018, 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.