Commitments and Contingencies - Additional Information (Details)			1 Months Ended			3 Months Ended
	Apr. 09, 2019	Jan. 01, 2017 USD ($)	Sep. 30, 2019	Apr. 30, 2018 USD ($) ft²	Jul. 31, 2016 USD ($) ft²	Mar. 31, 2020 USD ($) ft² Extension_Option	Mar. 31, 2019 USD ($)	Dec. 12, 2018 USD ($)
Commitments And Contingencies [Line Items]
Lessee, operating lease, residual value guarantee, description						The lease agreements do not contain residual value guarantees.
Lessee, operating lease, residual value guarantee						false
Operating lease, cost						$ 1,700,000	$ 1,700,000
Operating lease, payments						$ 1,700,000	$ 1,700,000
Supply agreement description						The Esteve Agreement has an initial term of four years, beginning April 9, 2019 and ending April 9, 2023
Initial term of supply agreement	four years
Total remaining contract costs						$ 41,100,000
Contract cost incurred in research and development activities						67,400,000
Supply Agreement [Member]
Commitments And Contingencies [Line Items]
Minimum Commitment through the first quarter of 2021						$ 0
BioVectra Inc
Commitments And Contingencies [Line Items]
Agreement expiration year						2026
Description of termination of agreement prior to contract term						The Company may terminate the BioVectra Agreement prior to the expiration of the contract term, which could result in early termination fee.
Cost of purchased minimum quantity product						$ 135,300,000
Preliminary fair value of the off-market element								$ 29,500,000
Accretion expense						900,000
Siegfried Evionnaz SA
Commitments And Contingencies [Line Items]
Cost of purchased minimum quantity product						$ 53,300,000
Agreement expiration date						Dec. 31, 2021
Agreement renewal term termination						1 year
Agreement early termination						Dec. 31, 2021
Esteve Química, S.A
Commitments And Contingencies [Line Items]
Minimum Commitment through the first quarter of 2021						$ 6,000.0
Minimum Commitment through the second quarter of 2021						$ 13,600
Minimum
Commitments And Contingencies [Line Items]
Operating lease, incremental borrowing rates based on remaining lease term						5.91%
Operating lease, remaining lease term						1 year 8 months 1 day
Maximum
Commitments And Contingencies [Line Items]
Operating lease, incremental borrowing rates based on remaining lease term						6.94%
Operating lease, remaining lease term						6 years 5 months 12 days
Keryx Biopharmaceuticals, Inc.
Commitments And Contingencies [Line Items]
Preliminary fair value of the off-market element								$ 29,510,000
Cambridge
Commitments And Contingencies [Line Items]
Area of property leased | ft²				65,167	45,362
Monthly lease payments		$ 22,000			$ 242,000
Operating lease payments commencement date					Jan. 01, 2017
Cambridge | Letter of Credit | Other Current Assets
Commitments And Contingencies [Line Items]
Total security deposit in connection with lease						$ 1,600,000
Cambridge | Office Space
Commitments And Contingencies [Line Items]
Area of property leased | ft²				19,805
Monthly lease payments				$ 135,000
Operating lease payments commencement date				Feb. 28, 2019
Leased property delivered, date				2018-09
Leased property subject to annual rent escalations, commencement date				2019-09
Lease expiration date						Sep. 11, 2026
Lease period, number of extension options | Extension_Option						1
Lease extension period						5 years
Cambridge | Lab Space
Commitments And Contingencies [Line Items]
Lease expiration date						Nov. 30, 2021
Lease period, number of extension options | Extension_Option						1
Lease extension period						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
Boston | Office Space
Commitments And Contingencies [Line Items]
Area of property leased | ft²						27,300
Monthly lease payments						$ 136,000
Lease expiration month year						2023-02
Boston | Office Space | Keryx Biopharmaceuticals, Inc.
Commitments And Contingencies [Line Items]
Lease expiration date						Feb. 28, 2023
Lease period, number of extension options | Extension_Option						1
Lease extension period						5 years
Operating sublease commencement date			Oct. 16, 2019
Sublease expiration date			Feb. 28, 2023
Sublease rental income						$ 400,000