Derivative Financial Instruments (Details 3) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Summary of gross liability balances of derivative instruments
Gross amounts of recognized liabilities	$ 12,931	$ 3,976
Gross amounts netted in the consolidated balance sheet	(10,300)	(3,306)
Net amounts of liabilities presented in the condensed consolidated balance sheet	2,631	670
Counterparty A [Member] | Current liabilities [Member]
Summary of gross liability balances of derivative instruments
Gross amounts of recognized liabilities	2,055	2,373
Gross amounts netted in the consolidated balance sheet	(1,992)	(2,373)
Net amounts of liabilities presented in the condensed consolidated balance sheet	63
Counterparty A [Member] | Long-term liabilities [Member]
Summary of gross liability balances of derivative instruments
Gross amounts of recognized liabilities	3,265	370
Gross amounts netted in the consolidated balance sheet	(3,265)	(370)
Counterparty B [Member] | Current liabilities [Member]
Summary of gross liability balances of derivative instruments
Gross amounts of recognized liabilities	1,724	894
Gross amounts netted in the consolidated balance sheet	(402)	(224)
Net amounts of liabilities presented in the condensed consolidated balance sheet	1,322	670
Counterparty B [Member] | Long-term liabilities [Member]
Summary of gross liability balances of derivative instruments
Gross amounts of recognized liabilities	1,952	339
Gross amounts netted in the consolidated balance sheet	(1,815)	(339)
Net amounts of liabilities presented in the condensed consolidated balance sheet	137
Counterparty C [Member] | Current liabilities [Member]
Summary of gross liability balances of derivative instruments
Gross amounts of recognized liabilities	1,758
Gross amounts netted in the consolidated balance sheet	(928)
Net amounts of liabilities presented in the condensed consolidated balance sheet	830
Counterparty C [Member] | Long-term liabilities [Member]
Summary of gross liability balances of derivative instruments
Gross amounts of recognized liabilities	2,177
Gross amounts netted in the consolidated balance sheet	(1,898)
Net amounts of liabilities presented in the condensed consolidated balance sheet	$ 279