License, Collaboration and Other Significant Agreements - U.S. Collaboration and License Agreement with Otsuka Pharmaceutical Co. Ltd. (Details)	1 Months Ended		3 Months Ended			9 Months Ended
	Mar. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Sep. 30, 2020 USD ($)	Sep. 30, 2019 USD ($)	Jun. 30, 2019	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 U.S. Agreement
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Collaborative arrangements cost sharing payments						312,400,000
Percentage of increase in funding to fund current global development costs		52.50%			80.00%
Collaborative arrangements additional cost sharing payments amount						80,900,000
Milestone revenue						$ 0
Percentage of market penetration for license agreement						90.00%
License agreement, notice period for termination						12 months
Number of performance obligations | performanceObligation						3
Transaction price						$ 471,200,000
Reimbursement for Otsuka's share of costs previously incurred						33,800,000
Deferred revenue			31,449,000			31,449,000
Short-term deferred revenue			10,672,000			10,672,000
Deferred revenue, net of current portion			20,777,000			20,777,000
Cost share costs incurred			1,200,000	500,000
Reimbursable by Otsuka			500,000	200,000
Cost share costs by partner for license agreement			500,000	300,000
Reimbursable to Otsuka			300,000	200,000
Otsuka Pharmaceutical Company. Ltd. | Otsuka U.S. Agreement | Global Development Plan
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Reimbursement for Otsuka's share of costs previously incurred	$ 33,800,000
Collaborative arrangements additional cost sharing payments expected amount						$ 119,000,000.0
Otsuka Pharmaceutical Company. Ltd. | Otsuka U.S. Agreement | Minimum
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Reduction in percentage of future payments due upon current global development costs creditable						50.00%
Percentage of remaining creditable amount applied to future payments						50.00%
Otsuka Pharmaceutical Company. Ltd. | Otsuka U.S. Agreement | Minimum | Global Development Plan
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Collaborative arrangements cost sharing payments						$ 312,400,000
Otsuka Pharmaceutical Company. Ltd. | Otsuka U.S. Agreement | Accounts payable
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Deferred revenue			3,100,000			3,100,000
Otsuka Pharmaceutical Company. Ltd. | Otsuka U.S. Agreement | Prepaid expenses and other current assets
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Accounts receivable			900,000			900,000
Otsuka Pharmaceutical Company. Ltd. | Otsuka U.S. Agreement | Accounts receivable
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Accounts receivable								$ 8,900,000
Otsuka Pharmaceutical Company. Ltd. | Otsuka U.S. Agreement | License, collaboration and other revenue
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Recognized revenue			16,256,000	$ 39,718,000		80,721,000	$ 103,461,000
Otsuka Pharmaceutical Company. Ltd. | Otsuka U.S. Agreement | Up Front Non Refundable And Non Creditable
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Upfront cash payment received		$ 125,000,000.0				125,000,000.0
Otsuka Pharmaceutical Company. Ltd. | Otsuka U.S. Agreement | Regulatory Milestone Payments | Maximum
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Potential milestone revenue (up to)			65,000,000.0			65,000,000.0
Otsuka Pharmaceutical Company. Ltd. | Otsuka U.S. Agreement | Commercial Milestone Payments | Maximum
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Potential milestone revenue (up to)			$ 575,000,000.0			575,000,000.0
Otsuka Pharmaceutical Company. Ltd. | Otsuka U.S. Agreement | Development And Regulatory Milestones
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Milestone revenue						$ 0