Stock-Based Compensation (Tables)

12 Months Ended

Dec. 31, 2020

Share-based Payment Arrangement [Abstract]

Schedule of Option and Appreciation Rights Activity

The following is a summary of stock options and stock appreciation rights activity under the 2015 Plan and the 2005 Plan for the year ended December 31, 2020:

	2015 Plan			2005 Plan
	Stock Options	Weighted- Average Exercise Price		Stock Options	Weighted- Average Exercise Price
Outstanding at January 1, 2020	3,388,750	$	35.64	501,780	$	64.19
Granted	3,363,766	$	38.01	—	$	—
Exercised	(1,025,025)	$	31.39	(264,265)	$	45.89
Canceled	(307,028)	$	33.51	(23,329)	$	51.92
Outstanding at December 31, 2020	5,420,463	$	38.05	214,186	$	88.11
Shares exercisable at December 31, 2020	878,488	$	71.27	214,186	$	88.11
Shares available for grant at December 31, 2020	2,473,916

Schedule of Assumptions Used in Estimation of Fair Value of Stock

	2020	2019	2018
Weighted average Black-Scholes fair value of stock options and SARs granted	$80.48	$4.98	$34.80
Risk-free interest rate	0.2%-1.5%	1.5%-2.6%	2.3%-3.1%
Dividend yield	—%	—%	—%
Volatility	116.0%-152.2%	105.4%-134.1%	93.3%-115.6%
Expected term (in years)	3.9-7.6	3.9-7.5	4.1-7.5

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:

	2020	2019	2018
Range of Black-Scholes fair values of ESPP shares granted	$2.57-$92.67	$2.57-$35.00	$7.20-$70.64
Risk-free interest rate	0.2%-2.6%	1.2%-2.6%	0.7%-2.2%
Dividend yield	—%	—%	—%
Volatility	66.6%-189.7%	52.2%-171.6%	52.2%-203.8%
Expected term (in years)	0.5-2.0	0.5-2.0	0.5-2.0

Schedule of Restricted Stock Units Activity

The following is a summary of restricted stock units activity for the year ended December 31, 2020:

	Number of Shares	Per Share Weighted- Average Fair Value
Outstanding and Unvested at January 1, 2020	1,102,311	$	5.95
Restricted stock units granted	837,896	94.74
Restricted stock units vested	(840,812)	9.37
Restricted stock units forfeited	(54,415)	40.37
Outstanding and Unvested at December 31, 2020	1,044,980	$	72.59

Schedule of Stock-Based Compensation Expense

The Company recorded stock-based compensation expense for awards issued under the above mentioned plans in the consolidated statements of operations as follows (in thousands):

	Year Ended December 31,
	2020		2019		2018
Research and development	$	55,955	$	8,436	$	10,575
General and administrative	72,080		8,612		7,739
Total stock-based compensation expense	$	128,035	$	17,048	$	18,314