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Stock-based Compensation (Tables)

12 Months Ended

Dec. 31, 2020

Share-based Payment Arrangement [Abstract]

Schedule of Stock-Based Compensation Expense

The following stock-based compensation amounts were recognized for the periods indicated (in thousands):

			Year Ended December 31,
			2020						2019						2018
Research and development			$	10,833					$	10,023					$	7,919
General and administrative			9,843						9,548						8,601
Total stock-based compensation expense			$	20,676					$	19,571					$	16,520

Schedule of Employee Stock Options Award Valuation Assumptions

The fair value of each option award is estimated on the date of grant using the Black-Scholes option-pricing model using the assumptions in the following table:

			Year Ended December 31,
			2020						2019						2018
Expected dividend yield			0%						0%						0%
Expected volatility			67% - 109%						74% - 76%						68% - 72%
Risk-free interest rate			0.4% - 1.8%						1.4% - 2.6%						2.4% - 3.1%
Expected term			6.25 years						6.25 years						6.25 years

Schedule of Stock Option Activity

The following table summarizes stock option activity for 2020:

			Shares						Weighted-Average Exercise Price						Weighted-Average Remaining Contractual Term (Years)						Aggregate Intrinsic Value (in thousands)
Outstanding, December 31, 2019			6,706,994						$	22.33					6.9
Granted			1,851,327						15.11
Exercised			(504,445)						10.56
Forfeited or expired			(795,523)						20.72
Outstanding, December 31, 2020			7,258,353						21.48						6.8						$	27,409
December 31, 2020:
Exercisable			4,660,921						22.92						5.8						13,906
Vested and expected to vest			6,973,272						21.57						6.7						26,011

Schedule of RSU Activity

The following table summarizes RSU activity for 2020:

			Shares						Weighted-Average Grant Date Fair Value
Outstanding, December 31, 2019			452,500						$	15.32
Granted			15,000						23.68
Exercised			(210,950)						15.32
Forfeited or expired			(47,300)						15.32
Outstanding, December 31, 2020			209,250						15.92

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Tabular disclosure of allocation of amount expensed and capitalized for award under share-based payment arrangement to statement of income or comprehensive income and statement of financial position. Includes, but is not limited to, corresponding line item in financial statement.

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Tabular disclosure for stock option plans. Includes, but is not limited to, outstanding awards at beginning and end of year, grants, exercises, forfeitures, and weighted-average grant date fair value.

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