Commitments and contingencies (Details) $ in Millions	9 Months Ended
Commitments and contingencies (Details) $ in Millions	Sep. 30, 2018 USD ($) claim	Aug. 23, 2018 USD ($)	Jun. 30, 2016 USD ($)
Other Commitments [Abstract]
Development milestone payments which the entity is obligated to pay	$ 4.5
Claims settled and dismissed \| claim	2
Akcea
Other Commitments [Abstract]
Upfront licensing fee	$ 12.0
Development and regulatory milestone payment obligations, period	30 days
Milestone, potential achievements, regulatory approval	$ 4.0
Akcea \| Maximum
Other Commitments [Abstract]
Milestone, potential achievements, regulatory approval	8.0
Funding agreement \| Wellcome trust
Other Commitments [Abstract]
Development milestone payments which the entity is obligated to pay			$ 0.8
Funding agreement \| Wellcome trust \| Maximum
Other Commitments [Abstract]
Development milestone payments which the entity is obligated to pay	22.4
Non-collaborative Arrangement Transactions \| Akcea
Other Commitments [Abstract]
Development milestone payments which the entity is obligated to pay	6.0
Agilis
Other Commitments [Abstract]
Development milestone payments which the entity is obligated to pay	40.0	$ 40.0
Agilis \| Maximum
Other Commitments [Abstract]
Development milestone payments which the entity is obligated to pay	60.0
Agilis \| Non-collaborative Arrangement Transactions
Other Commitments [Abstract]
Development milestone payments which the entity is obligated to pay	$ 40.0	$ 40.0