License Agreements and Services Agreement with Ionis - Summary of Development Expenses Related to TTR Agreement (Details) $ in Thousands	3 Months Ended
Mar. 31, 2019 USD ($)
Related Party Transaction [Line Items]
Total development expense incurred by the collaboration related to development activities under the TTR Agreement	$ (14,380)
Akcea [Member]
Related Party Transaction [Line Items]
Akcea's share of TTR development expense reflected in research and development expense in our consolidated statements of operations	$ (6,133)

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