Stock-Based Compensation (Tables)

12 Months Ended

Dec. 31, 2017

Stock-Based Compensation

Summary of stock option activity

The following table summarizes stock option activity (in thousands, except per share data):

		Weighted		Weighted Average
		Average		Remaining	Aggregate
	Number of	Exercise		Contractual	Intrinsic
	Shares	Price		Life (Years)	Value (a)
Outstanding at December 31, 2016	2,577	$	22.01	8.79	$	66,466
Granted	1,103	$	78.59
Exercised	(268)	$	14.93
Cancelled or forfeited	(245)	$	36.02
Outstanding at December 31, 2017	3,167	$	41.30	8.35	$	219,656
Exercisable at December 31, 2017	1,158	$	20.96	7.70	$	103,843
Exercisable and expected to vest at December 31, 2017	3,167	$	41.30	8.35	$	219,656

(a) The aggregate intrinsic value is calculated as the difference between the exercise price of the underlying options and the estimated fair value of the common stock for the options that were in-the-money at December 31, 2016 and 2017.

Summary of stock-based compensation expense by classification in the Statement of Operations and Comprehensive Loss

	Year ended		Year ended		Year ended
	December 31,		December 31,		December 31,
	2017		2016		2015
Research and development	$	15,383	$	5,988	$	1,294
General and administrative		15,502		9,668		3,956
	$	30,885	$	15,656	$	5,250

Summary of weighted-average assumptions used in determining fair value of stock options

	Year ended		Year ended		Year ended
	December 31,		December 31,		December 31,
	2017		2016		2015
Expected volatility	86.34	%	91.84	%	90.00	%
Risk-free interest rate	1.94	%	1.45	%	1.57	%
Expected term (in years)	6.08		6.08		6.08
Expected dividend yield	0.00	%	0.00	%	0.00	%

Summary of service-based restricted stock units activity

		Weighted
		Average
		Grant Date
	Number	Fair Value
	of Shares	Per Share
Outstanding at December 31, 2016	58	$	34.90
Granted	2		99.26
Vested	(20)	$	37.60
Forfeited and cancelled	(13)	$	34.90
Outstanding at December 31, 2017	27	$	38.79