Income tax expense - Components (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Loss before income taxes attributable to domestic and international operations
Domestic	$ (71,344)	$ (78,761)	$ (113,699)
Foreign	1,999	(3,902)	(1,032)
Loss before income taxes	(69,345)	(82,663)	(114,731)
Current tax
Foreign	(139)	(101)	(49)
Total income tax expense	$ (139)	$ (101)	$ (49)
Effective income tax reconciliation
Statutory income tax rate	12.50%	12.50%	12.50%
Non-deductible expenses		(0.10%)	(0.10%)
Income not subject to tax	0.20%	0.30%	0.30%
Tax credits	0.60%	0.40%	0.10%
Foreign rate differential	3.00%	0.60%	(3.10%)
In-process research and development			(3.50%)
Tax audit assessments		(11.80%)
Other	2.30%	0.80%	0.20%
Valuation allowance	(18.80%)	(2.80%)	(6.50%)
Effective income tax rate	(0.20%)	(0.10%)	(0.10%)
Ireland
Effective income tax reconciliation
Statutory income tax rate	12.50%	12.50%	12.50%