Stock-Based Compensation
 3 Months Ended
Mar. 31, 2014
Share-based Compensation [Abstract]  
Stock-Based Compensation
Stock-Based Compensation
A summary of the option activity under our equity incentive plan during the three months ended March 31, 2014 is presented below:
    
 
 
Options Outstanding
 
 
Shares
 
Weighted Average Exercise Price Per Share
 
Weighted Average Remaining Contractual Term
 
Aggregate Intrinsic Value
 
 
 
 
 
 
(in years)
 
(in thousands)
Balances at December 31, 2013
 
12,236,400

 
$
7.64

 

 
 
Options exercised
 
(1,615,549
)
 
3.39

 
 
 
 
Options forfeited
 
(72,570
)
 
13.38

 
 
 
 
Balances at March 31, 2014
 
10,548,281

 
$
8.25

 
7.4
 
$
715,450

Vested and expected to vest at March 31, 2014
 
10,251,945

 
$
8.13

 
7.4
 
$
696,610

Exercisable at March 31, 2014
 
4,887,401

 
$
4.72

 
6.2
 
$
348,767


We also grant Restricted Stock Unit ("RSU") awards to our employees and non-employee directors under the provisions of our equity incentive plan. The fair value of an RSU is determined by using the closing price of our Class A common stock on the New York Stock Exchange on the date of grant. An RSU award entitles the holder to receive shares of the Company’s Class A common stock as the award vests, which is generally based on length of service. The Company's unvested RSUs do not have nonforfeitable rights to dividends or dividend equivalents.
The following provides a summary of our RSU activity during the three months ended March 31, 2014:
 
 
Number of Shares Underlying Outstanding RSUs
 
Weighted-Average Grant-Date Fair Value per RSU
 
 
 
 
 
RSUs outstanding at December 31, 2013
 
574,350

 
$
64.06

RSUs granted
 
942,856

 
97.64

RSUs forfeited
 
(4,278
)
 
75.03

RSUs outstanding at March 31, 2014
 
1,512,928

 
$
84.95


    
Stock-based compensation expense is amortized using the straight-line method over the vesting period. As of March 31, 2014, total unrecognized compensation cost, adjusted for estimated forfeitures, related to stock options and unvested RSUs was approximately $139.8 million, which is expected to be recognized over a period of approximately 3.5 years.
    
The summary of shares available for issuance for equity based awards (including stock options and RSUs) for the three months ended March 31, 2014 are as follows:
 
 
Shares Available for Grant
Balances at December 31, 2013
 
5,313,281

Authorized
 
3,109,934

Granted
 
(942,856
)
Forfeited
 
76,848

Balances at March 31, 2014
 
7,557,207