License Agreements and Services Agreement with Ionis - Summary of Development Expenses Related to TTR Agreement (Details) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
Sep. 30, 2019	Sep. 30, 2019
Related Party Transaction [Line Items]
Total development expense incurred by the collaboration related to development activities under the TTR Agreement	$ (17,243)	$ (43,655)
Akcea [Member]
Related Party Transaction [Line Items]
Akcea's share of TTR development expense reflected in research and development expense in our condensed consolidated statements of operations	$ (7,525)	$ (18,923)

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