Commitments and Contingencies - Additional Information (Details)					1 Months Ended		3 Months Ended		9 Months Ended
	Sep. 04, 2020	Apr. 02, 2020	Apr. 09, 2019	Jan. 01, 2017 USD ($)	Feb. 28, 2019 USD ($)	Jul. 31, 2016 USD ($) ft²	Sep. 30, 2020 USD ($) ft² extensionOption	Sep. 30, 2019 USD ($)	Sep. 30, 2020 USD ($) ft² extensionOption	Sep. 30, 2019 USD ($)	Jun. 30, 2020 USD ($)	Dec. 31, 2019 USD ($)	Dec. 12, 2018 USD ($)	Apr. 30, 2018 ft²
Commitments And Contingencies [Line Items]
Operating lease, cost							$ 1,700,000	$ 1,700,000	$ 5,000,000.0	$ 5,000,000.0
Operating lease, payments							1,800,000	$ 1,700,000	5,300,000	$ 5,200,000
Purchase commitment, period increase (decrease)									10,800,000
Total remaining contract costs							17,600,000		17,600,000
Contract cost incurred in research and development activities									119,800,000
Supply Agreement, Patheon Inc.
Commitments And Contingencies [Line Items]
Minimum purchase commitment							1,300,000		1,300,000
BioVectra Inc
Commitments And Contingencies [Line Items]
Long-term minimum purchase commitment renewal term	1 year
Cost of purchased minimum quantity product							96,200,000		96,200,000
Siegfried Evionnaz SA
Commitments And Contingencies [Line Items]
Cost of purchased minimum quantity product							$ 51,400,000		$ 51,400,000
Agreement renewal term termination									1 year
Esteve Química, S.A
Commitments And Contingencies [Line Items]
Initial term of supply agreement			4 years
Long-term minimum purchase commitment									$ 26,000,000.0
STA Pharmaceutical Hong Kong Limited
Commitments And Contingencies [Line Items]
Long-term minimum purchase commitment									$ 44,700,000
Minimum purchase commitment, initial term		4 years
Minimum
Commitments And Contingencies [Line Items]
Operating lease, incremental borrowing rates based on remaining lease term							5.91%		5.91%
Operating lease, remaining lease term							1 year 2 months 1 day		1 year 2 months 1 day
Maximum
Commitments And Contingencies [Line Items]
Operating lease, incremental borrowing rates based on remaining lease term							6.94%		6.94%
Operating lease, remaining lease term							5 years 11 months 12 days		5 years 11 months 12 days
Keryx Biopharmaceuticals, Inc.
Commitments And Contingencies [Line Items]
Preliminary fair value of the off-market element							$ 41,000,000.0		$ 41,000,000.0		$ 41,500,000	$ 30,200,000	$ 29,510,000
Cambridge
Commitments And Contingencies [Line Items]
Area of property leased (in square feet) | ft²						45,362								65,167
Annual lease expense						$ 242,000
Monthly lease expense				$ 22,000
Cambridge | Letter of Credit | Other current assets
Commitments And Contingencies [Line Items]
Total security deposit in connection with lease							$ 1,600,000		$ 1,600,000
Cambridge | Office Space
Commitments And Contingencies [Line Items]
Area of property leased (in square feet) | ft²														19,805
Monthly lease expense					$ 135,000
Lease period, number of extension options | extensionOption							1		1
Lease extension period							5 years		5 years
Cambridge | Lab Space
Commitments And Contingencies [Line Items]
Lease period, number of extension options | extensionOption							1		1
Lease extension period							2 years		2 years
Boston | Keryx Biopharmaceuticals, Inc. | Letter of Credit | Other current assets
Commitments And Contingencies [Line Items]
Total security deposit in connection with lease							$ 800,000		$ 800,000
Boston | Office Space
Commitments And Contingencies [Line Items]
Area of property leased (in square feet) | ft²							27,300		27,300
Monthly lease expense									$ 136,000
Boston | Office Space | Keryx Biopharmaceuticals, Inc.
Commitments And Contingencies [Line Items]
Lease period, number of extension options | extensionOption							1		1
Lease extension period							5 years		5 years
Sublease rental income							$ 400,000		$ 1,300,000