Revenue - Deferred revenue (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Change in contract with customer, liability
Amounts invoiced in the period			$ (3,217)
Revenue in the period	$ (237)	$ (40,792)	(394)	$ (58,026)
Foreign exchange arising on consolidation			(140)
Deferred revenue at end of the period	$ 2,683		$ 2,683