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
	Dec. 31, 2016 USD ($)	Sep. 30, 2021 USD ($)	Sep. 30, 2020 USD ($)	Sep. 30, 2021 USD ($) performance_obligation	Sep. 30, 2020 USD ($)	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Recognized revenue		$ 48,756,000	$ 59,988,000	$ 153,973,000	$ 238,608,000
Deferred revenue		43,657,000		43,657,000
Short-term deferred revenue		15,692,000		15,692,000		$ 15,214,000
Deferred revenue, net of current portion		27,965,000		27,965,000		25,345,000
Accounts receivable
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Accounts receivable		21,413,000	2,761,000	21,413,000	2,761,000	3,045,000	$ 15,822,000
Prepaid expenses and other current assets
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Accounts receivable		3,442,000	1,248,000	3,442,000	1,248,000	1,722,000	0
Accounts payable
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Deferred revenue		0	4,446,000	0	4,446,000	7,227,000	$ 0
License, collaboration and other revenue
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Recognized revenue		12,003,000	25,596,000	$ 53,853,000	144,311,000
Otsuka Pharmaceutical Company. Ltd. | Otsuka U.S. Agreement
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Number of performance obligations | performance_obligation				3
Collaborative arrangements additional cost sharing payments amount				$ 106,700,000
Transaction price				504,200,000
Reimbursement of costs	$ 33,800,000
Collaborative arrangements cost sharing payments	345,400,000
Deferred revenue		22,972,000		22,972,000
Short-term deferred revenue		6,308,000		6,308,000
Deferred revenue, net of current portion		16,664,000		16,664,000
Cost share costs incurred		3,600,000	1,200,000
Reimbursable costs		1,800,000	500,000
Cost share costs by partner for license agreement		0	500,000
Costs reimbursed			300,000
Otsuka Pharmaceutical Company. Ltd. | Otsuka U.S. Agreement | Global Development Plan
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Collaborative arrangements additional cost sharing payments expected amount				$ 139,800,000
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%
Otsuka Pharmaceutical Company. Ltd. | Otsuka U.S. Agreement | Maximum
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Percentage of remaining creditable amount applied to future payments				50.00%
Otsuka Pharmaceutical Company. Ltd. | Otsuka U.S. Agreement | Accounts receivable
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Accounts receivable		2,400,000		$ 2,400,000
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		2,400,000		2,400,000		1,200,000
Otsuka Pharmaceutical Company. Ltd. | Otsuka U.S. Agreement | Accounts payable
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Deferred revenue						$ 5,000,000
Otsuka Pharmaceutical Company. Ltd. | Otsuka U.S. Agreement | License, collaboration and other revenue
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Recognized revenue		6,144,000	$ 16,256,000	28,988,000	$ 80,721,000
Otsuka Pharmaceutical Company. Ltd. | Otsuka U.S. Agreement | HIF Product
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Eligible milestone payments (up to)		65,000,000		65,000,000
Otsuka Pharmaceutical Company. Ltd. | Otsuka U.S. Agreement | Licensed Products
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Eligible milestone payments (up to)		$ 575,000,000		575,000,000
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
Otsuka Pharmaceutical Company. Ltd. | Otsuka U.S. Agreement | Development And Regulatory Milestones
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Milestone revenue				$ 0