Fair value measurements - Financial instruments recorded at fair value (Details) - Recurring basis - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Assets at fair value:
Derivative financial instruments	$ 0	$ 0
Total assets	0	0
Liabilities at fair value:
Derivative financial instruments	(2,388)	(3,364)
Total net liabilities	(2,388)	(3,364)
Level 1
Assets at fair value:
Derivative financial instruments	0	0
Total assets	0	0
Liabilities at fair value:
Derivative financial instruments	0	0
Total net liabilities	0	0
Level 2
Assets at fair value:
Derivative financial instruments	0	0
Total assets	0	0
Liabilities at fair value:
Derivative financial instruments	(2,388)	(3,364)
Total net liabilities	(2,388)	(3,364)
Level 3
Assets at fair value:
Derivative financial instruments	0	0
Total assets	0	0
Liabilities at fair value:
Derivative financial instruments	0	0
Total net liabilities	$ 0	$ 0