Fair value measurements - Summary of valuation by the fair value hierarchy (Details) - Recurring basis - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Assets at fair value:
Derivative Asset	$ 3,253	$ 1,348
Total assets	3,253	1,348
Liabilities at fair value:
Derivative Liability	(14,498)	(430)
Total net liabilities	(11,245)	918
Level 1
Assets at fair value:
Derivative Asset	0	0
Total assets	0	0
Liabilities at fair value:
Derivative Liability	0	0
Total net liabilities	0	0
Level 2
Assets at fair value:
Derivative Asset	3,253	1,348
Total assets	3,253	1,348
Liabilities at fair value:
Derivative Liability	(1,498)	(430)
Total net liabilities	1,755	918
Level 3
Assets at fair value:
Derivative Asset	0	0
Total assets	0	0
Liabilities at fair value:
Derivative Liability	(13,000)	0
Total net liabilities	$ (13,000)	$ 0