Collaboration Agreement with Novartis - Additional Information (Details) - USD ($)		1 Months Ended			3 Months Ended									12 Months Ended
	Apr. 23, 2018	Mar. 31, 2018	Feb. 28, 2018	Jan. 31, 2016	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Jan. 31, 2020
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Preferred stock shares issued to related party						0				0				0	0
Proceeds from stock issued to related party in private placement															$ 11,500,000
Deferred revenue - related party, non-current						$ 33,676,000				$ 39,342,000				$ 33,676,000	39,342,000
Collaboration revenue - related party						$ 439,000	$ 344,000	$ 143,000	$ 14,434,000	$ 9,764,000	$ 1,730,000	$ 2,428,000	$ 45,495,000	$ 15,360,000	$ 59,417,000	$ 12,826,000
Type of Revenue [Extensible List]						us-gaap:CollaborativeArrangementMember	us-gaap:CollaborativeArrangementMember	us-gaap:CollaborativeArrangementMember	us-gaap:CollaborativeArrangementMember	us-gaap:CollaborativeArrangementMember	us-gaap:CollaborativeArrangementMember	us-gaap:CollaborativeArrangementMember	us-gaap:CollaborativeArrangementMember	us-gaap:CollaborativeArrangementMember	us-gaap:CollaborativeArrangementMember	us-gaap:CollaborativeArrangementMember
ASU 2014-09
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Collaboration revenue - related party															$ 59,417,000	$ 12,826,000
Novartis
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Payment for reimbursement of manufacturing costs incurred														$ 0	3,437,000
Novartis Collaboration
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Revenue performance obligation						$ 38,592,000								38,592,000
Novartis Collaboration | Novartis Institutes for Biomedical Research, Inc.
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Payment for reimbursement of manufacturing costs incurred														0	3,437,000	$ 0
Proceeds from collaboration arrangement														$ 0
Type of Revenue [Extensible List]														us-gaap:CollaborativeArrangementMember
Novartis Collaboration | Novartis Institutes for Biomedical Research, Inc. | Scenario, Forecast
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Collaboration revenue - related party					$ 38,592,000
Novartis Collaboration | Novartis Institutes for Biomedical Research, Inc. | ASU 2014-09
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Proceeds from collaboration arrangement		$ 5,000,000	$ 45,000,000
Revenue recognized under milestone payment														$ 4,882,000	$ 27,850,000
Deferred revenue - related party, non-current						12,268,000								12,268,000
Novartis Collaboration | Novartis Institutes for Biomedical Research, Inc. | Up Front Payment Arrangement
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Proceeds from collaboration arrangement				$ 70,000,000
Novartis Collaboration | Novartis Institutes for Biomedical Research, Inc. | Milestone Payment
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Potential milestones payment														525,000,000
Novartis Collaboration | Novartis Institutes for Biomedical Research, Inc. | Minimum
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Percentage of share holding				5.00%
Novartis Collaboration | Novartis Institutes for Biomedical Research, Inc. | Maximum | Milestone Payment
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Maximum aggregate amount of potential option purchase, option exercise and milestone payments to be currently received						$ 745,000,000								$ 745,000,000
Novartis Collaboration | Novartis Institutes for Biomedical Research, Inc. | Maximum | Milestone Payment | Subsequent Event
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Maximum aggregate amount of potential option purchase, option exercise and milestone payments to be currently received																	$ 525,000,000
Novartis Collaboration | Series A One Redeemable Convertible Preferred Stock | Novartis Institutes for Biomedical Research, Inc.
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Preferred stock shares issued to related party				2,000,000
Novartis Collaboration | Private Placement | Novartis Institutes for Biomedical Research, Inc.
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Common stock shares issued to related party	766,666			766,666
Share price				$ 15.00
Proceeds from stock issued to related party in private placement				$ 11,500,000