Income tax (expense) benefit - Components (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Loss before income taxes attributable to domestic and international operations
Domestic									$ (66,109)	$ (54,509)	$ (46,300)
Foreign									(6,892)	(1,053)	(1,109)
Loss before income taxes									(73,001)	(55,562)	(47,409)
Current tax
Domestic										(4)	(3)
Foreign									55	(118)	(172)
Deferred tax
Foreign									(1,410)	794	620
Total income tax (expense) benefit	$ (101)	$ (1,872)	$ 967	$ (349)	$ 671	$ (28)	$ 12	$ 17	$ (1,355)	$ 672	$ 445
Effective income tax reconciliation
Statutory income tax rate									12.50%	25.00%	25.00%
Non-deductible expenses									(0.80%)	(0.20%)	(0.10%)
Income not subject to tax									0.90%	2.80%	1.90%
Impairment									1.40%
Tax credits									0.20%	0.40%
Foreign rate differential									21.00%	(2.50%)	(5.00%)
Other									(1.40%)	0.20%	0.20%
Valuation allowance									(35.60%)	(24.50%)	(21.10%)
Effective income tax rate									(1.80%)	1.20%	0.90%
Austria
Effective income tax reconciliation
Statutory income tax rate										25.00%	25.00%
Ireland
Effective income tax reconciliation
Statutory income tax rate									12.50%