Collaboration Agreements - Alnylam Narrative (Details) - USD ($)		1 Months Ended	3 Months Ended
	Aug. 31, 2019	Apr. 30, 2019	Jun. 30, 2019
Alnylam
Research Collaboration and Licensing Arrangements [Line Items]
Upfront payment made			$ 400,000,000.0
Additional payment eligible to be made		$ 200,000,000.0
Initial research term		5 years
Potential extension of research term		5 years
Maximum amount of sales milestone payments if total sales achieve specific levels	$ 325,000,000.0
Alnylam Stock Purchase Agreement
Research Collaboration and Licensing Arrangements [Line Items]
Cash consideration		$ 400,000,000.0
Minimum | Alnylam
Research Collaboration and Licensing Arrangements [Line Items]
Extension of research term fee		$ 200,000,000.0
Maximum | Alnylam
Research Collaboration and Licensing Arrangements [Line Items]
Initial research term		7 years
Extension of research term fee		$ 400,000,000.0