Commitments and Contingencies - Narrative (Details) - USD ($) $ in Thousands			1 Months Ended									110 Months Ended
	Jul. 22, 2019	Oct. 30, 2018	Oct. 31, 2020	Mar. 31, 2020	Sep. 30, 2019	Jul. 31, 2019	May 31, 2019	Apr. 30, 2019	Oct. 31, 2018	Jun. 30, 2018	Sep. 30, 2011	Oct. 31, 2020	Dec. 31, 2018
Cano v. Guyer, et al., No. 650601/2018
Commitments and contingencies
Litigation settlement awared to other party								$ 300
License Agreements | Archemix | C5 Licensed Product | Achievement of specified clinical and regulatory milestones
Commitments and contingencies
Payments for license fees			$ 6,000	$ 1,000							$ 1,000	$ 9,000
License Agreements | Archemix | C5 Licensed Product | Achievement of specified clinical and regulatory milestones | Maximum
Commitments and contingencies
Amount to be paid on achievement of milestone											50,500
License Agreements | Archemix | C5 Licensed Product | First indication
Commitments and contingencies
Amount to be paid on achievement of milestone											24,500
License Agreements | Archemix | C5 Licensed Product | First indication | Maximum
Commitments and contingencies
Amount to be paid on achievement of milestone											24,500
License Agreements | Archemix | C5 Licensed Product | Second and third indication
Commitments and contingencies
Amount to be paid on achievement of milestone											23,500
License Agreements | Archemix | C5 Licensed Product | Second and third indication | Maximum
Commitments and contingencies
Amount to be paid on achievement of milestone											23,500
License Agreements | Archemix | C5 Licensed Product | Sustained delivery applications
Commitments and contingencies
Amount to be paid on achievement of milestone											2,500
License Agreements | Archemix | C5 Licensed Product | Sustained delivery applications | Maximum
Commitments and contingencies
Amount to be paid on achievement of milestone											2,500
License Agreements | Archemix | C5 Licensed Product | Achievement of specified commercial milestones | Maximum
Commitments and contingencies
Amount to be paid on achievement of milestone											$ 22,500
License Agreements | University Of Massachusetts (UMass) | miniCEP290
Commitments and contingencies
Shares issued for license fees (in shares)	75,000					75,000
RHO-adRP License Agreement | University of Florida Research Foundation (UFRF) | Specified Clinical, Marketing Approval and Reimbursement Approval Milestones Related to a Licensed Product
Commitments and contingencies
Amount to be paid on achievement of milestone													$ 23,500
RHO-adRP License Agreement | University of Florida Research Foundation (UFRF) | Specified Commercial Sales Milestones Related to a Licensed Product
Commitments and contingencies
Amount to be paid on achievement of milestone													$ 70,000
Inception 4
Commitments and contingencies
Contingent consideration		$ 105,000							$ 105,000
Contingently issuable shares, asset acquisition (in shares)		7,200,000							7,200,000
Contingently issuable shares, asset acquisition, as a percentage of total shares outstanding		19.90%							19.90%
Inception 4 | GA
Commitments and contingencies
Clinical and marketing approval milestones									$ 45,000
Inception 4 | Wet AMD
Commitments and contingencies
Clinical and marketing approval milestones									$ 60,000
Research and development | License Agreements | University Of Massachusetts (UMass) | miniCEP290
Commitments and contingencies
Payments for license fees					$ 400
Research and development | License Agreements | University Of Massachusetts (UMass) | miniCEP290 | Achievement of specified clinical and regulatory milestones
Commitments and contingencies
Payments for license fees						$ 14,750	$ 14,750
Shares issued for license fees (in shares)						75,000	75,000
Research and development | License Agreements | University Of Massachusetts (UMass) | miniCEP290 | Specified Commercial Sales Milestones Related to a Licensed Product
Commitments and contingencies
Payments for license fees						$ 48,000	$ 48,000
Research and development | RHO-adRP License Agreement | University of Florida Research Foundation (UFRF)
Commitments and contingencies
Payments for license fees										$ 500
Research and development | RHO-adRP License Agreement | University of Florida Research Foundation (UFRF) | Specified Clinical, Marketing Approval and Reimbursement Approval Milestones Related to a Licensed Product
Commitments and contingencies
Payments for license fees										23,500
Research and development | RHO-adRP License Agreement | University of Florida Research Foundation (UFRF) | Specified Commercial Sales Milestones Related to a Licensed Product
Commitments and contingencies
Payments for license fees										$ 70,000
Research and development | BEST1 License Agreement | University Of Florida Research Foundation (UFRF) And University Of Pennsylvania (Penn) | Primary Licensed Product | Specified Clinical, Marketing Approval and Reimbursement Approval Milestones Related to a Licensed Product
Commitments and contingencies
Payments for license fees							15,700
Research and development | BEST1 License Agreement | University Of Florida Research Foundation (UFRF) And University Of Pennsylvania (Penn) | Primary Licensed Product | Specified Commercial Sales Milestones Related to a Licensed Product
Commitments and contingencies
Payments for license fees							48,000
Research and development | BEST1 License Agreement | University Of Florida Research Foundation (UFRF) And University Of Pennsylvania (Penn) | Other Licensed Product | Specified Clinical, Marketing Approval and Reimbursement Approval Milestones Related to a Licensed Product
Commitments and contingencies
Payments for license fees							3,100
Research and development | BEST1 License Agreement | University Of Florida Research Foundation (UFRF) And University Of Pennsylvania (Penn) | Other Licensed Product | Specified Commercial Sales Milestones Related to a Licensed Product
Commitments and contingencies
Payments for license fees							$ 9,600