Commitments and Contingencies - Narrative (Details) $ in Thousands		1 Months Ended
	Oct. 30, 2018 USD ($) shares	May 31, 2019 USD ($) licensed_product shares	Dec. 31, 2018 USD ($)	Sep. 30, 2011 USD ($)
Commitments and contingencies
Number of licensed products | licensed_product		1
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				$ 56,500
License Agreements | Archemix | C5 Licensed Product | First indication | Maximum
Commitments and contingencies
Amount to be paid on achievement of milestone				30,500
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 | 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
RHO-adRP License Agreement | University of Florida Research Foundation (UFRF) | Specified clinical, marketing approval and reimbursement approval milestones with respect 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 with respect to a licensed product
Commitments and contingencies
Amount to be paid on achievement of milestone			$ 70,000
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%
Inception 4 | GA Product
Commitments and contingencies
Clinical and marketing approval milestones	$ 45,000
Inception 4 | Wet AMD Product
Commitments and contingencies
Clinical and marketing approval milestones	$ 60,000
Research and Development Expense | License Agreements | University Of Massachusetts (UMass) | miniCEP290 | Achievement of specified clinical and regulatory milestones
Commitments and contingencies
Payments for license fees		$ 14,750
Shares issued for license fees (in shares) | shares		75,000
Research and Development Expense | License Agreements | University Of Massachusetts (UMass) | miniCEP290 | Specified commercial sales milestones with respect to a licensed product
Commitments and contingencies
Payments for license fees		$ 48,000
Research and Development Expense | 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 with respect to a licensed product
Commitments and contingencies
Payments for license fees		15,700
Research and Development Expense | BEST1 License Agreement | University Of Florida Research Foundation (UFRF) And University Of Pennsylvania (Penn) | Primary Licensed Product | Specified commercial sales milestones with respect to a licensed product
Commitments and contingencies
Payments for license fees		48,000
Research and Development Expense | 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 with respect to a licensed product
Commitments and contingencies
Payments for license fees		3,100
Research and Development Expense | BEST1 License Agreement | University Of Florida Research Foundation (UFRF) And University Of Pennsylvania (Penn) | Other Licensed Product | Specified commercial sales milestones with respect to a licensed product
Commitments and contingencies
Payments for license fees		$ 9,600