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Stock-Based Compensation Expense and Employee Benefit Plans
 6 Months Ended
Jun. 30, 2021
Compensation Related Costs [Abstract]  
Stock-Based Compensation Expense and Employee Benefit Plans Stock-Based Compensation Expense and Employee Benefit Plans
Stock-Based Compensation Expense
The following table summarizes the components of stock-based compensation expense in the condensed consolidated statements of operations (in thousands):
 Three Months Ended
June 30,		Six Months Ended
June 30,
 2021202020212020
Cost of revenue$3,804 $4,736 $6,803 $10,243 
Research and development5,291 4,714 10,199 10,810 
Sales and marketing4,010 2,234 8,095 6,124 
General and administrative6,028 6,947 11,246 14,473 
$19,133 $18,631 $36,343 $41,650 
Stock Option and Stock Award Activity
The following table summarizes the stock option activity under our stock plans during the reporting period:
 Outstanding Options
 Number of
Shares		Weighted
Average
Exercise
Price		Remaining
Contractual
Life (Years)		Aggregate
Intrinsic
Value
   (in thousands)
Balances at December 31, 202015,354,271 $21.27 6.0$129,855 
Granted— 
Exercised(2,470,262)24.67 
Cancelled(630,306)13.58 
Balances at June 30, 202112,253,703 20.98 5.896,586 
Vested and expected to vest at June 30, 202112,046,166 21.18 5.892,978 
Exercisable at June 30, 20218,619,596 25.28 5.137,719 

Stock Options - During the three months ended June 30, 2021 and 2020, we recognized $3.4 million and $4.9 million of stock-based compensation expense for stock options, respectively. We did not grant options in the three and six months ended June 30, 2021. We granted 200,000 options of Class A common stock during the three and six months ended June 30, 2020 and the weighted average grant-date fair value of those options was $7.30 per share.
As of June 30, 2021 and 2020, we had unrecognized compensation expense related to unvested stock options of $12.6 million and $31.0 million, respectively. This expense is expected to be recognized over the remaining weighted-average period of 1.3 years and 2.2 years, respectively. Cash received from stock options exercised totaled $60.9 million and $1.0 million for the six months ended June 30, 2021 and 2020, respectively.
A summary of our stock awards activity and related information is as follows:
Number of
Awards
Outstanding		Weighted
Average Grant
Date Fair
Value
Unvested Balance at December 31, 20206,418,788 $13.71 
Granted5,147,861 24.56 
Vested(1,951,664)16.94 
Forfeited(745,708)13.43 
Unvested Balance at June 30, 20218,869,277 19.32 
Stock Awards - The estimated fair value of restricted stock units ("RSUs ") and performance stock units ("PSUs") is based on the fair value of our Class A common stock on the date of grant. For the three months ended June 30, 2021 and 2020, we recognized $14.1 million and $10.5 million of stock-based compensation expense for stock awards, respectively.
As of June 30, 2021 and 2020, we had $141.2 million and $35.3 million of unrecognized stock-based compensation expense related to unvested stock awards, expected to be recognized over a weighted average period of 2.6 years and 1.2 years, respectively.
During 2020 and 2021, we granted PSUs to certain executive officers and employees that only vest upon the achievement of certain specific financial or operational performance criteria. Stock-based compensation expense associated with these PSUs is recognized over the service period as we evaluate the probability of the achievement of the performance conditions.
In May 2021, we issued RSUs and PSUs to our Chief Executive Officer. The RSUs vest over four years. Some of the PSUs can be earned based on achieving certain financial performance goals while the remaining are earned based upon achieving certain stock price goals. The PSUs will be subject to a two-year post-vest holding period in which the award holder will be restricted from selling any shares (net of shares settled for taxes). As of June 30, 2021, the unamortized compensation expense for the RSUs and PSUs was $30.0 million. Actual compensation expense is dependent on the performance of the PSUs that vest based upon a performance condition. We estimated the fair value of the PSUs that vest based on a market condition on the date of grant using a Monte Carlo simulation with the following assumptions: (i) expected volatility of 71.2%, (ii) risk-free interest rate of 1.6%, and (iii) no expected dividend yield.
The following table presents the stock activity and the total number of shares available for grant under our stock plans as of June 30, 2021:
 Plan Shares Available
for Grant
  
Balances at December 31, 202020,233,754 
Added to plan7,675,984 
Granted(5,147,681)
Cancelled1,376,014 
Expired(171,843)
Balances at June 30, 202123,966,228 
2018 Employee Stock Purchase Plan
During the six months ended June 30, 2021 and 2020, we recognized $2.2 million and $4.7 million of stock-based compensation expense for the 2018 ESPP, respectively. We issued 977,508 shares in the six months ended June 30, 2021. During the six months ended June 30, 2021, we added an additional 1,902,572 shares and there were 3,512,465 shares available for issuance as of June 30, 2021.
As of June 30, 2021 and 2020, we had $2.1 million and $3.7 million of unrecognized stock-based compensation expense, expected to be recognized over a weighted average period of 0.2 years and 0.6 years, respectively.