Commitments and Contingencies - Narrative (Details) $ in Thousands				1 Months Ended
	Jul. 22, 2019 shares	Mar. 12, 2019 USD ($)	Oct. 30, 2018 USD ($) shares	Sep. 30, 2019 USD ($)	Jul. 31, 2019 shares	May 31, 2019 USD ($) licensed_product shares	Oct. 31, 2018 USD ($) shares	Jun. 30, 2018 USD ($)	Sep. 30, 2011 USD ($)	Dec. 31, 2019 USD ($)	Sep. 30, 2018 USD ($)
Commitments and contingencies
Number of licensed products | licensed_product						1
Litigation settlement awared to other party		$ 300
License Agreements | Archemix | C5 Licensed Product | Achievement of specified clinical and regulatory milestones
Commitments and contingencies
Amount to be paid on achievement of milestone										$ 1,000
Payments for license fees									$ 1,000
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									56,500
License Agreements | Archemix | C5 Licensed Product | First indication
Commitments and contingencies
Amount to be paid on achievement of milestone									30,500
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
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) | 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) | 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
Shares issued for license fees (in shares) | shares						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
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