Fair value measurements - Financial instruments recorded at fair value (Details) (Recurring basis, USD $) In Thousands, unless otherwise specified	Mar. 31, 2014	Dec. 31, 2013
Assets at fair value:
Derivative financial instruments		$ 2
Total assets		2
Liabilities at fair value:
Derivative financial instruments	(825)	(4,890)
Total net liabilities	(825)	(4,888)
Level 1
Assets at fair value:
Derivative financial instruments		0
Total assets		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
Total assets		2
Liabilities at fair value:
Derivative financial instruments	(825)	(4,890)
Total net liabilities	(825)	(4,888)
Level 3
Assets at fair value:
Derivative financial instruments		0
Total assets		0
Liabilities at fair value:
Derivative financial instruments	0	0
Total net liabilities	$ 0	$ 0