Fair value measurement - Derivatives and hedging activities - Fair value measurements (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Valuation of financial assets and liabilities by the fair value hierarchy
Credit value adjustment, percentage of fair value of derivatives for Level 3 classificatiom greater than	20.00%
Recurring basis | Fair Value
Assets at fair value:
Derivative financial instruments	$ 6	$ 60
Total assets	6	60
Liabilities at fair value:
Derivative financial instruments	(8,879)	(10,954)
Total	(8,873)	(10,894)
Recurring basis | 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	0	0
Recurring basis | Level 2
Assets at fair value:
Derivative financial instruments	6	60
Total assets	6	60
Liabilities at fair value:
Derivative financial instruments	(8,879)	(10,954)
Total	(8,873)	(10,894)
Recurring basis | 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	$ 0	$ 0