Share-Based Compensation (Tables)

12 Months Ended

Dec. 31, 2019

Share-based Payment Arrangement [Abstract]

Schedule of Stock Option Activity

The following table summarizes the Company's stock option activity and related information for the year ended December 31, 2019:

	Number of options		Weighted average exercise price		Weighted-Average Remaining Contractual Term (years)	Aggregate Intrinsic Value (millions)
Balance outstanding as of December 31, 2018	3,683,786		$	19.59
Granted	2,310,994		$	76.22
Exercised	(569,146	)	$	14.89
Canceled/forfeited	(137,657	)	$	48.31
Balance outstanding as of December 31, 2019	5,287,977		$	44.10	7.7	$	448.2
Options exercisable at December 31, 2019	2,339,904		$	21.11	6.1	$	252.1

Schedule of Share-based Compensation Expense

Total share-based compensation expense by statement of operations and comprehensive loss classification is presented below (in thousands):

	Year ended December 31,
	2019		2018		2017
Research and development expense	$	31,024	$	7,232	$	3,192
General and administrative expense	24,513		8,622		3,594
	$	55,537	$	15,854	$	6,786

Schedule of Assumptions Used in Fair Value Estimates

The assumptions used for the specified reporting periods and the resulting estimates of weighted-average estimated fair value per share of options granted during those periods are as follows:

	Year Ended December 31,
	2019	2018	2017
Risk-free interest rate	2.2%	2.6%	2.1%
Dividend yield	—%	—%	—%
Volatility factor	82.1%	94.3%	96.0%
Expected term (in years)	5.6	6.0	6.0
Weighted average estimated fair value per share	$52.03	$24.39	$4.17