Derivative Financial Instruments (Details 3) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Summary of gross liability balances of derivative instruments
Gross amounts of recognized liabilities	$ 12,930	$ 3,976
Gross amounts netted in the consolidated balance sheet	(8,752)	(3,306)
Net amounts of liabilities presented in the condensed consolidated balance sheet	4,178	670
Amounts subject to master netting arrangements presented on a gross basis	439	82
Counterparty A [Member] | Current liabilities [Member]
Summary of gross liability balances of derivative instruments
Gross amounts of recognized liabilities	3,329	2,373
Gross amounts netted in the consolidated balance sheet	(2,104)	(2,373)
Net amounts of liabilities presented in the condensed consolidated balance sheet	1,225	0
Counterparty A [Member] | Long-term liabilities [Member]
Summary of gross liability balances of derivative instruments
Gross amounts of recognized liabilities	909	370
Gross amounts netted in the consolidated balance sheet	(909)	(370)
Net amounts of liabilities presented in the condensed consolidated balance sheet	0	0
Counterparty B [Member] | Current liabilities [Member]
Summary of gross liability balances of derivative instruments
Gross amounts of recognized liabilities	2,749	894
Gross amounts netted in the consolidated balance sheet	(1,289)	(224)
Net amounts of liabilities presented in the condensed consolidated balance sheet	1,460	670
Counterparty B [Member] | Current Assets [Member]
Summary of gross liability balances of derivative instruments
Amounts subject to master netting arrangements presented on a gross basis	138	82
Counterparty B [Member] | Long-term liabilities [Member]
Summary of gross liability balances of derivative instruments
Gross amounts of recognized liabilities	1,039	339
Gross amounts netted in the consolidated balance sheet	(1,039)	(339)
Net amounts of liabilities presented in the condensed consolidated balance sheet	0	0
Counterparty C [Member] | Current liabilities [Member]
Summary of gross liability balances of derivative instruments
Gross amounts of recognized liabilities	3,782
Gross amounts netted in the consolidated balance sheet	(2,289)
Net amounts of liabilities presented in the condensed consolidated balance sheet	1,493
Counterparty C [Member] | Current Assets [Member]
Summary of gross liability balances of derivative instruments
Amounts subject to master netting arrangements presented on a gross basis	301
Counterparty C [Member] | Long-term liabilities [Member]
Summary of gross liability balances of derivative instruments
Gross amounts of recognized liabilities	1,122
Gross amounts netted in the consolidated balance sheet	(1,122)
Net amounts of liabilities presented in the condensed consolidated balance sheet	$ 0