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Stock-Based Compensation
 3 Months Ended
Mar. 31, 2021
Share-based Payment Arrangement [Abstract]  
Stock-Based Compensation Stock-Based Compensation
Stock Options
The 2015 Stock Incentive Plan, as amended (“2015 Plan”), was approved at the Company's annual meeting of stockholders in June 2015. Under the 2015 Plan, equity awards may be granted to officers, directors, employees and consultants of and advisors to the Company and any present or future subsidiary.
The 2015 Plan authorizes the issuance of up to 10.9 million shares of common stock under equity awards granted under the 2015 Plan. All such shares authorized for issuance under the 2015 Plan have been reserved. The 2015 Plan will expire on March 4, 2025.
The Amended and Restated 2005 Stock Incentive Plan (“2005 Plan”) expired in February 2015 and no new awards may be made under such plan, although awards will continue to be outstanding in accordance with their terms.
The 2015 Plan permits and the 2005 Plan permitted the grant of stock options (including incentive stock options), restricted stock, stock appreciation rights and restricted stock units. In addition, under the 2015 Plan, unrestricted stock, stock units and performance awards may be granted. Stock options and stock appreciation rights generally have a maximum term of ten years and may be or were granted with an exercise price that is no less than 100% of the fair market value of the Company's common stock at the time of grant. Grants of stock options are generally subject to vesting over periods ranging from one to four years.
Stock Options and Stock Appreciation Rights
The following is a summary of stock options and SARs activity under the 2015 Plan and 2005 Plan for the three months ended March 31, 2021:
2015 Plan2005 Plan
Stock
Options and SARs		Weighted-Average
Exercise
Price		Stock
Options		Weighted-Average
Exercise
Price
Outstanding at January 1, 20215,420,463 $38.05 214,186 $88.11 
Granted32,958 $132.14 — $— 
Exercised(437,054)$51.42 (35,401)$106.49 
Canceled(6,270)$27.65 — $— 
Outstanding at March 31, 20215,010,097 $37.52 178,785 $84.47 
Shares exercisable at March 31, 2021590,006 $72.46 178,785 $84.47 
Shares available for grant at March 31, 20212,381,759 
The fair value of stock options granted under the 2015 Plan was estimated at the date of grant using the Black-Scholes option-pricing model with the following assumptions:
Three Months Ended
March 31,
20212020
Weighted average Black-Scholes fair value of stock options granted$116.26$5.39
Risk-free interest rate
0.5%-0.9%
0.6%-1.5%
Dividend yield—%—%
Volatility
124.7%-140.3%
133.6%-142.6%
Expected term (in years)
4.1-5.3
3.9
The total aggregate intrinsic value and weighted-average remaining contractual term of stock options and SARs outstanding under the 2015 Plan and 2005 Plan as of March 31, 2021 was approximately $738 million and 8.4 years, respectively. The total aggregate intrinsic value and weighted-average remaining contractual term of stock options and SARs exercisable under the 2015 Plan and 2005 Plan as of March 31, 2021 was $81.6 million and 5.3 years, respectively. The aggregate intrinsic value represents the total intrinsic value (the difference between the Company's closing stock price on the last trading day of the period and the exercise price, multiplied by the number of in-the-money stock options and SARs) that would have been received by the holders had all stock option and SAR holders exercised their stock options and stock appreciation rights on March 31, 2021. This amount is subject to change based on changes to the closing price of the Company's common stock. The aggregate intrinsic value of stock options and vesting of restricted stock awards for the three months ended March 31, 2021 and 2020 was $81.5 million and $0.2 million, respectively.
Employee Stock Purchase Plan
The Employee Stock Purchase Plan, as amended (the “ESPP”), was approved at the Company's annual meeting of stockholders in June 2013. The ESPP currently authorizes an aggregate of 600,000 shares of common stock to be purchased. The ESPP allows employees to purchase shares of common stock of the Company at each purchase date through payroll deductions of up to a maximum of 15% of their compensation, at 85% of the lesser of the market price of the shares at the time of purchase or the market price on the beginning date of an option period (or, if later, the date during the option period when the employee was first eligible to participate). As of March 31, 2021, there were 212,897 shares available for issuance under the ESPP.
The ESPP is considered compensatory for financial reporting purposes. As such, the fair value of ESPP shares was estimated at the date of grant using the Black-Scholes option-pricing model with the following assumptions:
Three Months Ended
March 31,
20212020
Range of Black-Scholes fair values of ESPP shares granted
$128.70-$238.85
$2.57-$35.00
Risk-free interest rate0.1%
1.5%-2.6%
Dividend yield—%—%
Volatility
120.4%-159.4%
66.6%-154.4%
Expected term (in years)
0.5-2.0
0.5-2.0
Restricted Stock Units
The following is a summary of restricted stock units activity for the three months ended March 31, 2021:
Number of
Shares		Per Share
Weighted-
Average
Fair Value
Outstanding and Unvested at January 1, 20201,044,980 $72.59 
Restricted stock units granted69,732 $181.24 
Restricted stock units vested(26,097)$68.44 
Restricted stock units forfeited(4,263)$142.88 
Outstanding and Unvested at March 31, 20211,084,352 $79.39 
The Company recorded all stock-based compensation expense in the consolidated statements of operations as follows (in thousands):
Three Months Ended
March 31,
20212020
Research and development$23,790 $1,908 
General and administrative29,270 2,057 
Total stock-based compensation expense$53,060 $3,965 
As of March 31, 2021, there was approximately $276 million of total unrecognized compensation expense related to unvested stock options, SARs, restricted stock units and the ESPP. The increase in unrecognized compensation expense is primarily due to the significant increase in the Company's common stock price in 2020. This unrecognized non-cash compensation expense is expected to be recognized over a weighted-average period of 1.1 years, and will be allocated between research and development and general and administrative expenses accordingly. This estimate does not include the impact of other possible stock-based awards that may be made during future periods and awards that require approval by the stockholders.