Collaboration, License, and Other Agreements - Alnylam Narratives (Details) $ in Millions	12 Months Ended
	Dec. 31, 2019 USD ($) payment
Alnylam Pharmaceuticals, Inc. Collaboration Agreement
Disaggregation of Revenue [Line Items]
Upfront payment made	$ 400.0
Number of milestones | payment	2
Collaborative arrangement additional payment eligible	$ 200.0
Alnylam Pharmaceuticals, Inc. Collaboration Agreement | Alnylam Pharmaceuticals, Inc. Stock Purchase Agreement
Disaggregation of Revenue [Line Items]
Cash consideration for Stock Purchase Agreement	400.0
Alnylam Pharmaceuticals, Inc. Collaboration Agreement, Eye Program
Disaggregation of Revenue [Line Items]
Collaborative arrangement additional payment eligible	100.0
Alnylam Pharmaceuticals, Inc. Collaboration Agreement, CNS Program
Disaggregation of Revenue [Line Items]
Collaborative arrangement additional payment eligible	$ 100.0