Collaboration and License Agreement - Additional Information (Details)	1 Months Ended	3 Months Ended		12 Months Ended
	Nov. 30, 2017 USD ($) Target CandidateorProduct	Mar. 31, 2019 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2018 USD ($)	Jan. 01, 2019 USD ($)	Jan. 01, 2018 USD ($)
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Collaboration agreement revenue recognized		$ 270,000	$ 1,129,000	$ 5,322,000
Novartis
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Upfront, nonrefundable payment received	$ 35,000,000
Allocated to collaboration agreement	33,300,000
Collaborative agreement target fee	$ 5,000,000
Collaboration agreement, research term	5 years
Novartis | Collaborative Arrangement
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Collaboration agreement revenue recognized		300,000	$ 1,100,000
Deferred revenue		31,400,000		31,500,000	$ 100,000	$ 400,000
Novartis | Collaborative Arrangement | Reimbursable Research and Development Costs
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Accounts receivable		200,000		$ 800,000
Novartis | ASC 606
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Transaction price		$ 33,300,000
Novartis | Maximum
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Number of targets selected | Target	2
Collaboration agreement, reimbursement of manufacturing activities during research term	$ 17,000,000
Number of targets during research term | Target	2
Collaboration agreement number of candidates or products for develop and commercialize | CandidateorProduct	2
Collaboration agreement eligible milestone payments eligible to receive	$ 530,000,000
Collaboration agreement eligible development milestone payments eligible to receive	180,000,000
Collaboration agreement eligible regulatory milestone payments eligible to receive	170,000,000
Collaboration agreement eligible commercial milestone payments eligible to receive	180,000,000
Novartis | Series B Preferred Stock
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Upfront, nonrefundable payment received	1,700,000
Proceeds from issuance of shares of preferred stock	10,000,000
Preferred stock estimated fair value	$ 11,700,000