Commitments and Contingencies - Narrative (Details) $ in Thousands		1 Months Ended	3 Months Ended		9 Months Ended
	Oct. 30, 2018 USD ($) shares	May 31, 2019 USD ($) licensed_product shares	Sep. 30, 2020 USD ($)	Sep. 30, 2019 USD ($)	Sep. 30, 2020 USD ($)	Sep. 30, 2019 USD ($)	Dec. 31, 2018 USD ($)	Sep. 30, 2011 USD ($)
Commitments and contingencies
Research and development			$ 18,841	$ 10,383	$ 45,311	$ 28,077
Number of licensed products | licensed_product		1
Inception 4
Commitments and contingencies
Contingent consideration arrangements, maximum amount	$ 105,000
Contingent consideration arrangements, maximum amount (in shares) | shares	7,200,000
Contingent consideration arrangements, contingently issuable shares as a percentage of shares outstanding	19.90%
Clinical Milestone
Commitments and contingencies
Research and development			$ 6,000		$ 6,000
GA Product | Inception 4
Commitments and contingencies
Clinical and marketing approval milestones	$ 45,000
Wet AMD Product | Inception 4
Commitments and contingencies
Clinical and marketing approval milestones	$ 60,000
Archemix | C5 Licensed Product | Achievement of specified clinical and regulatory milestones | Maximum | License Agreements
Commitments and contingencies
Amount to be paid on achievement of milestone								$ 50,500
Archemix | C5 Licensed Product | First indication | Maximum | License Agreements
Commitments and contingencies
Amount to be paid on achievement of milestone								24,500
Archemix | C5 Licensed Product | Second and third indication | Maximum | License Agreements
Commitments and contingencies
Amount to be paid on achievement of milestone								23,500
Archemix | C5 Licensed Product | Sustained delivery applications | Maximum | License Agreements
Commitments and contingencies
Amount to be paid on achievement of milestone								2,500
Archemix | C5 Licensed Product | Achievement of specified commercial milestones | Maximum | License Agreements
Commitments and contingencies
Amount to be paid on achievement of milestone								$ 22,500
University of Florida Research Foundation (UFRF) | Specified clinical, marketing approval and reimbursement approval milestones with respect to a licensed product | RHO-adRP License Agreement
Commitments and contingencies
Amount to be paid on achievement of milestone							$ 23,500
University of Florida Research Foundation (UFRF) | Specified commercial sales milestones with respect to a licensed product | RHO-adRP License Agreement
Commitments and contingencies
Amount to be paid on achievement of milestone							$ 70,000
University Of Florida Research Foundation (UFRF) And University Of Pennsylvania (Penn) | Primary Licensed Product | Specified clinical, marketing approval and reimbursement approval milestones with respect to a licensed product | Research and Development Expense | BEST1 License Agreement
Commitments and contingencies
Payments for license fees		$ 15,700
University Of Florida Research Foundation (UFRF) And University Of Pennsylvania (Penn) | Primary Licensed Product | Specified commercial sales milestones with respect to a licensed product | Research and Development Expense | BEST1 License Agreement
Commitments and contingencies
Payments for license fees		48,000
University Of Florida Research Foundation (UFRF) And University Of Pennsylvania (Penn) | Other Licensed Product | Specified clinical, marketing approval and reimbursement approval milestones with respect to a licensed product | Research and Development Expense | BEST1 License Agreement
Commitments and contingencies
Payments for license fees		3,100
University Of Florida Research Foundation (UFRF) And University Of Pennsylvania (Penn) | Other Licensed Product | Specified commercial sales milestones with respect to a licensed product | Research and Development Expense | BEST1 License Agreement
Commitments and contingencies
Payments for license fees		9,600
University Of Massachusetts (UMass) | miniCEP290 | Achievement of specified clinical and regulatory milestones | License Agreements | Research and Development Expense
Commitments and contingencies
Payments for license fees		$ 14,750
Shares issued for license fees (in shares) | shares		75,000
University Of Massachusetts (UMass) | miniCEP290 | Specified commercial sales milestones with respect to a licensed product | License Agreements | Research and Development Expense
Commitments and contingencies
Payments for license fees		$ 48,000