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Stock-based Compensation (Tables)

12 Months Ended

Dec. 31, 2018

Disclosure of Compensation Related Costs, Share-based Payments [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,
			2018						2017						2016
Research and development			$	7,919					$	7,388					$	5,778
General and administrative			8,601						7,356						6,387
Total stock-based compensation expense			$	16,520					$	14,744					$	12,165

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,
			2018						2017						2016
Expected dividend yield			0%						0%						0%
Expected volatility			68% - 72%						67% - 68%						64% - 69%
Risk-free interest rate			2.4% - 3.1%						1.9% - 2.3%						1.2% - 2.4%
Expected term			6.25 years						6.25 years						6.25 years

Schedule of Stock Option and Restricted Stock Unit Activity

The following table summarizes stock option activity for 2018:

			Shares						Weighted-Average Exercise Price						Weighted-Average Remaining Contractual Term (Years)						Aggregate Intrinsic Value (in thousands)
Outstanding, December 31, 2017			4,504,642						$	19.79					7.0
Granted			1,269,844						27.18
Options exercised			(285,432)						4.22
Forfeited or expired			(215,090)						24.43
Outstanding, December 31, 2018			5,273,964						22.23						6.8						$	7,969
December 31, 2018:
Exercisable			3,544,139						20.60						5.9						7,969
Vested and expected to vest			5,086,009						22.10						6.7						7,969

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Tabular disclosure of the allocation of equity-based compensation costs to a given line item on the balance sheet and income statement for the period. This may include the reporting line for the costs and the amount capitalized and expensed.

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Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (h)(1)
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Tabular disclosure of the significant assumptions used during the year to estimate the fair value of stock options, including, but not limited to: (a) expected term of share options and similar instruments, (b) expected volatility of the entity's shares, (c) expected dividends, (d) risk-free rate(s), and (e) discount for post-vesting restrictions.

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