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Stock-based Compensation (Tables)

12 Months Ended

Dec. 31, 2019

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,
			2019						2018						2017
Research and development			$	10,023					$	7,919					$	7,388
General and administrative			9,548						8,601						7,356
Total stock-based compensation expense			$	19,571					$	16,520					$	14,744

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,
			2019						2018						2017
Expected dividend yield			0%						0%						0%
Expected volatility			74% - 76%						68% - 72%						67% - 68%
Risk-free interest rate			1.4% - 2.6%						2.4% - 3.1%						1.9% - 2.3%
Expected term			6.25 years						6.25 years						6.25 years

Schedule of Stock Option Activity

The following table summarizes stock option activity for 2019:

			Shares						Weighted-Average Exercise Price						Weighted-Average Remaining Contractual Term (Years)						Aggregate Intrinsic Value (in thousands)
Outstanding, December 31, 2018			5,273,964						$	22.23					6.8
Granted			1,946,770						20.81
Exercised			(219,045)						3.03
Forfeited or expired			(294,695)						24.72
Outstanding, December 31, 2019			6,706,994						22.33						6.9						$	4,749
December 31, 2019:
Exercisable			4,298,178						22.21						5.9						4,712
Vested and expected to vest			6,446,218						22.31						6.9						4,746

Schedule of RSU Activity

The following table summarizes RSU activity for 2019:

			Shares						Weighted-Average Grant Date Fair Value
Outstanding, December 31, 2018			—						—
Granted			467,600						$	15.32
Exercised			—						—
Forfeited or expired			(15,100)						15.32
Outstanding, December 31, 2019			452,500						15.32

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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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