UCSB License Agreement - Additional Information (Details) - USD ($)		3 Months Ended		6 Months Ended
	Apr. 02, 2019	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018		Dec. 31, 2013
License Agreement [Line Items]
Annual minimum royalty obligations		$ 150,000		$ 150,000
License termination period				60 days
Common stock, shares issued		45,403,838		45,403,838		45,083,209
Research and development		$ 30,835,000	$ 25,553,000	$ 67,211,000	$ 48,011,000
Accrued liabilities		22,469,000		22,469,000		$ 26,724,000	[1]
AbbVie | Discovery Agreement | Second Target
License Agreement [Line Items]
Additional sublicense fees				800,000
Milestone payment receivable				10,000,000.0
UCSB
License Agreement [Line Items]
Common stock, shares issued	150,000							157,332
Fair value of common stock price per share	$ 10.68
Payment of upfront fees	$ 1,000,000.0
Annual license maintenance fees	$ 750,000	800,000		800,000
License payment term	2031
Percentage of remaining maintenance fees outstanding	50.00%
Fair value of common stock issued	$ 1,600,000
Milestone and minimum annual royalty provision paid		4,400,000	$ 500,000	4,200,000	$ 500,000
Accrued liabilities		$ 900,000		$ 900,000		$ 3,200,000
UCSB | Sublicense and Maintenance Fees
License Agreement [Line Items]
Research and development	$ 3,400,000

[1] The condensed balance sheet as of December 31, 2018 was derived from the audited financial statements included in the Company's Annual Report on Form 10-K for the year ended December 31, 2018.