License Agreements and Services Agreement with Ionis - Summary of Development Expenses Related to TTR Agreement (Details) - USD ($) $ in Thousands	3 Months Ended
Mar. 31, 2020	Mar. 31, 2019
Related Party Transaction [Line Items]
Total development expense incurred by the collaboration related to development activities under the TTR License Agreement	$ (18,331)	$ (14,380)
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	$ (8,190)	$ (6,133)

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