Summary of valuation by the fair value hierarchy (Details) (Recurring basis, USD $) In Thousands, unless otherwise specified	Dec. 31, 2013	Dec. 31, 2012
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	2	6
Total assets	2	6
Liabilities at fair value:
Derivative financial instruments	(4,890)	(8,879)
Total net liabilities	(4,888)	(8,873)
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
Fair Value
Assets at fair value:
Derivative financial instruments	2	6
Total assets	2	6
Liabilities at fair value:
Derivative financial instruments	(4,890)	(8,879)
Total net liabilities	$ (4,888)	$ (8,873)