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Equity and Stock-based Compensation (Tables)

12 Months Ended

Dec. 31, 2018

Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]

Summary of Common Stock Reserved for Future Issuance

Shares of Common Stock outstanding, shares of stock plans outstanding and shares reserved for future issuance related to stock options and RSUs grant and the Company’s Employee Stock Purchase Plan (“ESPP”) purchases are as follows (in thousands):

	December 31,
	2018		2017
Common stock outstanding		85,432		82,498
Stock options outstanding		10,430		11,550
RSUs outstanding		1,428		1,562
Common stock warrants outstanding		4		4
Shares reserved for future stock options and RSUs grant		6,041		4,190
Shares reserved for future ESPP offering		2,618		2,024
Total shares of common stock reserved		105,953		101,828

Summary of Stock Option Transactions

Stock option transactions, including forfeited options granted under the 2014 Plan as well as prior plans, are summarized below:

	Shares (In thousands)			Weighted Average Exercise per Share		Weighted Average Remaining Contractual Life (In Years)		Aggregate Intrinsic Value (In thousands)
Outstanding at December 31, 2017		11,550		$	14.82
Granted		1,307			53.64
Exercised		(2,271	)		11.20
Expired		(7	)		5.82
Forfeited		(149	)		30.55
Outstanding at December 31, 2018		10,430			20.25		5.47	$	281,436
Vested and expected to vest, December 31, 2018		10,430			20.25		5.47		281,436
Exercisable at December 31, 2018		7,644		$	13.83		4.47	$	248,206

Summary of RSU Activity

The following table summarizes RSU activity:

	Shares (In thousands)			Fair Value at Grant
Unvested at December 31, 2017		1,562		$	24.75
Granted		706			53.69
Vested		(727	)		24.70
Forfeited		(113	)		35.12
Unvested at December 31, 2018		1,428		$	38.26

Schedule of Allocated Stock-Based Compensation Expense

Stock-based compensation expense allocated to research and development and general and administrative expense for the years ended December 31, 2018, 2017 and 2016 was as follows (in thousands):

	Years Ended December 31,
	2018		2017		2016
Research and development	$	30,491	$	21,807	$	19,070
General and administrative		21,651		15,732		13,062
Total stock-based compensation expense	$	52,142	$	37,539	$	32,132

Schedule of Assumptions used to Estimate Fair Value of Stock Options Granted and Employee Stock Purchase Plans

The assumptions used to estimate the fair value of stock options granted and ESPPs using the Black-Scholes option valuation model were as follows:

	Years Ended December 31,
	2018			2017			2016
Stock Options
Expected term (in years)		5.4			5.7			5.3
Expected volatility		67.9	%		71.5	%		69.9	%
Risk-free interest rate		2.7	%		2.2	%		1.4	%
Expected dividend yield		—			—			—
Weighted average estimated fair value	$	32.12		$	16.96		$	11.49

ESPPs
Expected term (in years)	0.5 - 2.0			0.5 - 2.0			0.5 - 2.0
Expected volatility	47.3 - 75.3		%	52.8 - 77.2		%	61.9 - 80.7		%
Risk-free interest rate	0.8 - 2.9		%	0.5 - 1.6		%	0.2 - 1.0		%
Expected dividend yield		—			—			—
Weighted average estimated fair value	$	16.27		$	9.41		$	9.94

Schedule of Warrants to Purchase Shares of Common Stock

The following warrants to purchase shares of Common Stock were issued in connection with certain facility and equipment lease financing arrangements and are outstanding at December 31, 2018:

Year of Issuance	Number of Shares		Exercise Price per Share		Reason for Issuance	Expiration Date
2000		4,430	$	15.00	Issued in connection with lease agreement	Five years after initial public offering or upon merger or sale of the Company’s assets, whichever occurs first
		4,430

- Definition

Tabular disclosure of warrants or rights issued. Warrants and rights outstanding are derivative securities that give the holder the right to purchase securities (usually equity) from the issuer at a specific price within a certain time frame. Warrants are often included in a new debt issue to entice investors by a higher return potential. The main difference between warrants and call options is that warrants are issued and guaranteed by the company, whereas options are exchange instruments and are not issued by the company. Also, the lifetime of a warrant is often measured in years, while the lifetime of a typical option is measured in months. Disclose the title of issue of securities called for by warrants and rights outstanding, the aggregate amount of securities called for by warrants and rights outstanding, the date from which the warrants or rights are exercisable, and the price at which the warrant or right is exercisable.

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