Derivative Financial Instruments (Details 2) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Summary of gross asset balances of derivative instruments
Gross amounts of recognized assets	$ 12,585	$ 8,455
Gross amounts netted in the consolidated balance sheet	(10,300)	(3,306)
Net amounts of assets presented in the condensed consolidated balance sheet	2,285	5,149
Counterparty A [Member] | Current Assets [Member]
Summary of gross asset balances of derivative instruments
Gross amounts of recognized assets	2,677	6,445
Gross amounts netted in the consolidated balance sheet	(1,992)	(2,373)
Net amounts of assets presented in the condensed consolidated balance sheet	685	4,072
Counterparty A [Member] | Other assets [Member]
Summary of gross asset balances of derivative instruments
Gross amounts of recognized assets	3,990	1,096
Gross amounts netted in the consolidated balance sheet	(3,265)	(370)
Net amounts of assets presented in the condensed consolidated balance sheet	725	726
Counterparty B [Member] | Current Assets [Member]
Summary of gross asset balances of derivative instruments
Gross amounts of recognized assets	615	530
Gross amounts netted in the consolidated balance sheet	(402)	(224)
Net amounts of assets presented in the condensed consolidated balance sheet	213	306
Counterparty B [Member] | Other assets [Member]
Summary of gross asset balances of derivative instruments
Gross amounts of recognized assets	2,132	384
Gross amounts netted in the consolidated balance sheet	(1,815)	(339)
Net amounts of assets presented in the condensed consolidated balance sheet	317	45
Counterparty C [Member] | Current Assets [Member]
Summary of gross asset balances of derivative instruments
Gross amounts of recognized assets	1,172
Gross amounts netted in the consolidated balance sheet	(928)
Net amounts of assets presented in the condensed consolidated balance sheet	244
Counterparty C [Member] | Other assets [Member]
Summary of gross asset balances of derivative instruments
Gross amounts of recognized assets	1,999
Gross amounts netted in the consolidated balance sheet	(1,898)
Net amounts of assets presented in the condensed consolidated balance sheet	$ 101