Commitments and Contingencies - Additional Information (Details)	1 Months Ended			3 Months Ended
	May 31, 2018 USD ($)	Apr. 30, 2018 USD ($) ft²	Jul. 31, 2016 USD ($) ft²	Mar. 31, 2019 USD ($) ft² Extension_Option	Dec. 12, 2018 USD ($)	Mar. 31, 2018 USD ($)
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
Operating lease, payments				1,700,000
Total remaining contract costs				143,100,000
Contract cost incurred in research and development activities				$ 71,900,000
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				$ 167,500,000
Preliminary fair value of the off-market element					$ 29,500,000
Accretion expense				200,000
Siegfried Evionnaz SA
Commitments And Contingencies [Line Items]
Cost of purchased minimum quantity product				$ 77,100,000
Agreement expiration date				Dec. 31, 2021
Agreement renewal term termination				1 year
Agreement early termination				Dec. 31, 2021
Minimum
Commitments And Contingencies [Line Items]
Operating lease, incremental borrowing rates				5.91%
Operating lease, remaining lease term				2 years 10 months 28 days
Maximum
Commitments And Contingencies [Line Items]
Operating lease, incremental borrowing rates				6.94%
Operating lease, remaining lease term				7 years 8 months 8 days
Letter of Credit | Other Current Assets
Commitments And Contingencies [Line Items]
Increase (decrease) in security deposit in connection with lease	$ (200,000)	$ 500,000
Total security deposit in connection with lease		1,800,000		$ 1,600,000		$ 1,300,000
Increase (decrease) in security deposit in connection with lease	$ (200,000)	$ 500,000
Office Space
Commitments And Contingencies [Line Items]
Lease expiration date				Sep. 11, 2026
Lease period, number of extension options | Extension_Option				1
Lease extension period				5 years
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
Cambridge Lease Amendment
Commitments And Contingencies [Line Items]
Area of property leased | ft²		65,167
Third Amendment to the Cambridge Lease
Commitments And Contingencies [Line Items]
Area of property leased | ft²			45,362
Monthly lease payments			$ 242,000
Operating lease payments commencement date			Jan. 01, 2017
Lease Monthly Payments On Or About January 1 2017
Commitments And Contingencies [Line Items]
Monthly lease payments			$ 22,000
Fifth Amendment to the Cambridge Lease | 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
Boston Lease | 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
Keryx Biopharmaceuticals, Inc. | Letter of Credit | Other Current Assets
Commitments And Contingencies [Line Items]
Total security deposit in connection with lease				$ 800,000
Keryx Biopharmaceuticals, Inc. | Office Space
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