Inception 4 Acquisition (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions		1 Months Ended	12 Months Ended
	Oct. 30, 2018	Oct. 31, 2018	Dec. 31, 2018
Schedule Of Asset Acquisition [Line Items]
Share price (USD per share)		$ 2.26
Non-cash charge on acquired in-process research and development			$ 6.9
Transaction costs			$ 1.3
Inception 4
Schedule Of Asset Acquisition [Line Items]
Number of shares issued in asset acquisition (in shares)		5.2
Value of shares issued in asset acquisition		$ 11.7
Contingent consideration	$ 105.0	$ 105.0
Contingently issuable shares, asset acquisition (in shares)	7.2	7.2
Contingently issuable shares, asset acquisition, as a percentage of total shares outstanding	19.90%	19.90%
Cash acquired in asset acquisition		$ 6.1
GA | Inception 4
Schedule Of Asset Acquisition [Line Items]
Clinical and marketing approval milestones		45.0
Wet AMD | Inception 4
Schedule Of Asset Acquisition [Line Items]
Clinical and marketing approval milestones		$ 60.0