License Agreements and Services Agreement with Ionis - Summary of Development Expenses Related to TTR Agreement (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Related Party Transaction [Line Items]
Total development expense incurred by the collaboration related to development activities under the TTR License Agreement	$ (18,185)	$ (12,032)	$ (36,516)	$ (26,412)
Akcea [Member]
Related Party Transaction [Line Items]
Akcea's share of TTR development expense reflected in research and development expense in our condensed consolidated statement of operations	$ (8,290)	$ (5,265)	$ (16,480)	$ (11,398)