Fair value measurements - Summary of valuation by the fair value hierarchy (Details) - Recurring basis - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Assets at fair value:
Derivative Asset	$ 1,348	$ 0
Total assets	1,348	0
Liabilities at fair value:
Derivative Liability	(430)	(3,364)
Total net assets	918	(3,364)
Level 1
Assets at fair value:
Derivative Asset	0	0
Total assets	0	0
Liabilities at fair value:
Derivative Liability	0	0
Total net assets	0	0
Level 2
Assets at fair value:
Derivative Asset	1,348	0
Total assets	1,348	0
Liabilities at fair value:
Derivative Liability	(430)	(3,364)
Total net assets	918	(3,364)
Level 3
Assets at fair value:
Derivative Asset	0	0
Total assets	0	0
Liabilities at fair value:
Derivative Liability	0	0
Total net assets	$ 0	$ 0