Stock-Based Compensation (Tables)

12 Months Ended

Dec. 31, 2018

Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]

Summary of Stock Option and Related Information

The following table summarizes stock option activity and related information for the periods presented below:

	Shares Subject to Outstanding Options			Weighted Average Exercise Price Per Option		Weighted Average Remaining Contractual Term (in years)		Aggregate Intrinsic Value (inthousands)
Balance at December 31, 2016		2,141,868		$	6.42		8.4	$	14,963
Options granted		1,735,875			15.41
Options exercised		(333,822	)		4.81
Options canceled		(579,372	)		8.11
Balance at December 31, 2017		2,964,549			11.54		8.3		90,780
Options granted		1,370,381			52.53
Options exercised		(489,153	)		10.61
Options canceled		(144,316	)		22.67
Balance at December 31, 2018		3,701,461			26.40		8.0		88,524
Options outstanding and exercisable as of    December 31, 2018		1,519,637			14.35		7.3		52,979

Summary of RSU Activity and Related Information

The following table summarizes RSU activity and related information for the period presented below:

	Shares Subject to Outstanding Awards			Weighted Average Grant Date Fair Value		Weighted Average Remaining Contractual Term (in years)		Aggregate Intrinsic Value (inthousands)
Balance at December 31, 2017		—		$	—		—	$	—
RSUs awarded		171,766			53.27
RSUs released		(3,250	)		48.93
RSUs forfeited		(5,570	)		52.83
Balance at December 31, 2018		162,946			53.37		2.0		7,962

Schedule of Stock-Based Compensation Expense, Net of Forfeitures

Stock-based compensation expense, net of forfeitures, as applicable for the years ended December 31, 2018, 2017 and 2016, is reflected in the statements of operations and comprehensive loss as follows (in thousands):

	Year Ended December 31,
	2018		2017		2016
Research and development	$	8,144	$	2,752	$	1,252
General and administrative		11,197		3,386		1,561
Total stock-based compensation	$	19,341	$	6,138	$	2,813

Schedule of Assumptions for Black-Scholes Option-Pricing Model Used in Determining Fair Value of Time-Based and Performance-Based Options Granted to Employees

The following table illustrates the assumptions for the Black-Scholes option-pricing model used in determining the fair value of time-based and performance-based options granted to employees:

	Year Ended December 31,
	2018	2017	2016
Risk-free interest rate	2.54%-2.98%	1.92%-2.27%	1.05%-2.10%
Expected life (in years)	5.3-6.1	5.3-6.1	5.3-6.1
Volatility	69%-75%	71%-75%	71%-73%
Dividend yield	0%	0%	0%