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Equity-Based Compensation Plans
 6 Months Ended
Jun. 30, 2021
Share-based Payment Arrangement [Abstract]  
Equity-Based Compensation Plans Equity-Based Compensation Plans
Equity Plans
On January 1, 2021, an additional 6,794 shares of our Class A common stock were reserved for issuance pursuant to the automatic increase provisions of the 2015 Equity Incentive Plan. As of June 30, 2021, 31,401 shares were available for issuance as future awards under this plan.
On January 1, 2021, an additional 1,000 shares of our Class A common stock were reserved for issuance pursuant to the automatic increase provisions of the 2015 Employee Stock Purchase Plan (the ESPP). As of June 30, 2021, 4,790 shares were available for issuance under the ESPP.
Equity Plan Activity
We have granted stock options at exercise prices equal to the fair market value of our Class A common stock on the grant date. We have granted both stock options and restricted stock awards (RSUs) vesting solely upon the continued service of the recipient as well as performance-based awards (PSUs) with vesting based on either (i) our achievement of financial targets or (ii) our relative total stockholder return (TSR) as compared to an index of public internet companies. We recognize the accounting grant date fair value of equity-based awards as compensation expense over the required service period of each award, taking into account the probability of our achievement of associated performance targets. Compensation expense for TSR-based PSUs is recognized regardless of whether the TSR market condition is satisfied.
The following table summarizes stock option activity:
Number of
Shares of Class A Common Stock (#)		Weighted-
Average
Exercise
Price Per Share ($)
Outstanding at December 31, 2020
3,428 42.79 
Exercised(646)40.20 
Forfeited(209)68.29 
Outstanding at June 30, 2021
2,573 41.37 
Vested at June 30, 2021
2,130 35.72 
The following table summarizes stock award activity:
Number of
Shares of Class A Common Stock (#)
Outstanding at December 31, 20206,133 
Granted: RSUs3,355 
Granted: TSR-based PSUs411 
Vested(1,816)
Forfeited(824)
Outstanding at June 30, 2021(1)
7,259 
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(1)Includes financial-based PSUs for which performance targets have not yet been established, and which are not yet considered granted for accounting purposes. The balance of outstanding awards is comprised of the following:
Number of
Shares of Class A Common Stock (#)		Weighted-Average Grant-Date Fair Value Per Share ($)
RSUs6,483 77.18
TSR-based PSUs612 106.66
Financial-based PSUs granted for accounting purposes83 78.62
Financial-based PSUs not yet granted for accounting purposes81 N/A
Outstanding at June 30, 20217,259 
As of June 30, 2021, total unrecognized compensation expense related to non-vested stock options and stock awards was $8.7 million and $403.0 million, respectively, with expected remaining weighted-average recognition periods of 1.8 years and 2.7 years, respectively. Such amounts exclude PSUs not yet considered granted for accounting purposes.