Derivative Financial Instruments (Details 3) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Summary of gross liability balances of derivative instruments
Gross amounts of recognized liabilities	$ 8,388	$ 3,976
Gross amounts netted in the consolidated balance sheet	(8,131)	(3,306)
Net amounts of liabilities presented in the condensed consolidated balance sheet	257	670
Amounts subject to master netting arrangements presented on a gross basis	257	82
Counterparty A [Member] | Current liabilities [Member]
Summary of gross liability balances of derivative instruments
Gross amounts of recognized liabilities	1,823	2,373
Gross amounts netted in the consolidated balance sheet	(1,823)	(2,373)
Net amounts of liabilities presented in the condensed consolidated balance sheet	0	0
Counterparty A [Member] | Long-term liabilities [Member]
Summary of gross liability balances of derivative instruments
Gross amounts of recognized liabilities	1,467	370
Gross amounts netted in the consolidated balance sheet	(1,467)	(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	1,521	894
Gross amounts netted in the consolidated balance sheet	(1,264)	(224)
Net amounts of liabilities presented in the condensed consolidated balance sheet	257	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	257	82
Counterparty B [Member] | Long-term liabilities [Member]
Summary of gross liability balances of derivative instruments
Gross amounts of recognized liabilities	1,510	339
Gross amounts netted in the consolidated balance sheet	(1,510)	(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	1,087
Gross amounts netted in the consolidated balance sheet	(1,087)
Net amounts of liabilities presented in the condensed consolidated balance sheet	0
Counterparty C [Member] | Long-term liabilities [Member]
Summary of gross liability balances of derivative instruments
Gross amounts of recognized liabilities	980
Gross amounts netted in the consolidated balance sheet	(980)
Net amounts of liabilities presented in the condensed consolidated balance sheet	$ 0