License, Collaboration and Other Significant Agreements - International Collaboration and License Agreement with Otsuka Pharmaceutical Co. Ltd. (Details)		3 Months Ended			9 Months Ended
	Apr. 25, 2017 USD ($)	Sep. 30, 2020 USD ($)	Sep. 30, 2019 USD ($)	Mar. 31, 2017 USD ($)	Sep. 30, 2020 USD ($) performanceObligation	Sep. 30, 2019 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Recognized revenue		$ 59,988,000	$ 91,977,000		$ 238,608,000	$ 265,446,000
Deferred revenue		51,694,000			51,694,000
Short-term deferred revenue		18,034,000			18,034,000		$ 39,830,000
Deferred revenue, net of current portion		33,660,000			33,660,000		33,120,000
Accounts payable
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Deferred revenue		4,446,000	0		4,446,000	0	0	$ 13,492,000
Prepaid expenses and other current assets
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Accounts receivable		1,248,000			1,248,000		0
Accounts receivable
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Accounts receivable		2,761,000	4,204,000		2,761,000	4,204,000	15,822,000	$ 1,587,000
License, collaboration and other revenue
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Recognized revenue		25,596,000	61,973,000		$ 144,311,000	183,242,000
Otsuka Pharmaceutical Company. Ltd. | Otsuka International Agreement
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Estimated current global development costs subsequent to March 31, 2017				$ 224,700,000
Milestone or royalty payments received	$ 0
License agreement, notice period for termination	12 months
Number of performance obligations | performanceObligation					3
Transaction price					$ 297,900,000
Reimbursement for Otsuka's share of costs previously incurred					200,000
Deferred revenue		15,566,000			15,566,000
Short-term deferred revenue		7,362,000			7,362,000
Deferred revenue, net of current portion		8,204,000			8,204,000
Otsuka Pharmaceutical Company. Ltd. | Otsuka International Agreement | Accounts payable
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Deferred revenue		1,400,000			1,400,000
Otsuka Pharmaceutical Company. Ltd. | Otsuka International Agreement | Prepaid expenses and other current assets
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Accounts receivable		400,000			400,000
Otsuka Pharmaceutical Company. Ltd. | Otsuka International Agreement | Accounts receivable
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Accounts receivable							$ 4,000,000.0
Otsuka Pharmaceutical Company. Ltd. | Otsuka International Agreement | License, collaboration and other revenue
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Recognized revenue		7,243,000	$ 20,220,000		39,445,000	$ 64,643,000
Otsuka Pharmaceutical Company. Ltd. | Otsuka International Agreement | Global Development Plan
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Reimbursement for Otsuka's share of costs previously incurred				$ 200,000
Otsuka Pharmaceutical Company. Ltd. | Up Front Non Refundable And Non Creditable | Otsuka International Agreement
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Upfront cash payment received	$ 73,000,000.0				73,000,000.0
Otsuka Pharmaceutical Company. Ltd. | Regulatory Milestone Payments | Otsuka International Agreement | Maximum
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Potential milestone revenue (up to)		52,000,000.0			52,000,000.0
Otsuka Pharmaceutical Company. Ltd. | Commercial Milestone Payments | Otsuka International Agreement | Maximum
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Potential milestone revenue (up to)		$ 525,000,000.0			$ 525,000,000.0