Fair value measurements - Financial instruments recorded at fair value (Details) - Recurring basis - USD ($) $ in Thousands	Jun. 30, 2015	Dec. 31, 2014
Assets at fair value:
Derivative financial instruments	$ 4	$ 13
Total assets	4	13
Liabilities at fair value:
Derivative financial instruments	(4,084)	(3,602)
Total net liabilities	(4,080)	(3,589)
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	4	13
Total assets	4	13
Liabilities at fair value:
Derivative financial instruments	(4,084)	(3,602)
Total net liabilities	(4,080)	(3,589)
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