Collaboration Agreements - Alnylam Narrative (Details) - USD ($) $ in Millions	1 Months Ended	3 Months Ended
	Apr. 30, 2019	Jun. 30, 2019
Alnylam
Research Collaboration and Licensing Arrangements [Line Items]
Upfront payment made		$ 400.0
Additional payment eligible to be made	$ 200.0
Initial research term	5 years
Potential extension of research term	5 years
Alnylam Stock Purchase Agreement
Research Collaboration and Licensing Arrangements [Line Items]
Cash consideration	$ 400.0
Minimum | Alnylam
Research Collaboration and Licensing Arrangements [Line Items]
Extension of research term fee	$ 200.0
Maximum | Alnylam
Research Collaboration and Licensing Arrangements [Line Items]
Initial research term	7 years
Extension of research term fee	$ 400.0