Derivative Financial Instruments (Details 2) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Summary of gross asset balances of derivative instruments
Gross amounts of recognized assets	$ 15,568	$ 8,455
Gross amounts netted in the consolidated balance sheet	(8,131)	(3,306)
Net amounts of assets presented in the condensed consolidated balance sheet	7,437	5,149
Amounts subject to master netting arrangements presented on a gross basis	257	82
Counterparty A [Member] | Current Assets [Member]
Summary of gross asset balances of derivative instruments
Gross amounts of recognized assets	3,682	6,445
Gross amounts netted in the consolidated balance sheet	(1,823)	(2,373)
Net amounts of assets presented in the condensed consolidated balance sheet	1,859	4,072
Counterparty A [Member] | Other assets [Member]
Summary of gross asset balances of derivative instruments
Gross amounts of recognized assets	2,858	1,096
Gross amounts netted in the consolidated balance sheet	(1,467)	(370)
Net amounts of assets presented in the condensed consolidated balance sheet	1,391	726
Counterparty B [Member] | Current Assets [Member]
Summary of gross asset balances of derivative instruments
Gross amounts of recognized assets	2,239	530
Gross amounts netted in the consolidated balance sheet	(1,264)	(224)
Net amounts of assets presented in the condensed consolidated balance sheet	975	306
Amounts subject to master netting arrangements presented on a gross basis	257	82
Counterparty B [Member] | Other assets [Member]
Summary of gross asset balances of derivative instruments
Gross amounts of recognized assets	2,859	384
Gross amounts netted in the consolidated balance sheet	(1,510)	(339)
Net amounts of assets presented in the condensed consolidated balance sheet	1,349	45
Counterparty C [Member] | Current Assets [Member]
Summary of gross asset balances of derivative instruments
Gross amounts of recognized assets	2,231
Gross amounts netted in the consolidated balance sheet	(1,087)
Net amounts of assets presented in the condensed consolidated balance sheet	1,144
Counterparty C [Member] | Other assets [Member]
Summary of gross asset balances of derivative instruments
Gross amounts of recognized assets	1,699
Gross amounts netted in the consolidated balance sheet	(980)
Net amounts of assets presented in the condensed consolidated balance sheet	$ 719