Fair value measurements - Summary of valuation by the fair value hierarchy (Details) - Recurring basis - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Assets at fair value:
Derivative Asset	$ 0	$ 4
Total assets	0	4
Liabilities at fair value:
Derivative Liability	(3,364)	(4,464)
Total net liabilities	(3,364)	(4,460)
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	0	4
Total assets	0	4
Liabilities at fair value:
Derivative Liability	(3,364)	(4,464)
Total net liabilities	(3,364)	(4,460)
Level 3
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