Derivative Financial Instruments (Details 2) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Summary of gross asset balances of derivative instruments
Gross amounts of recognized assets	$ 10,784	$ 8,455
Gross amounts netted in the consolidated balance sheet	(8,752)	(3,306)
Net amounts of assets presented in the condensed consolidated balance sheet	2,032	5,149
Amounts subject to master netting arrangements presented on a gross basis	439	82
Counterparty A [Member] | Current Assets [Member]
Summary of gross asset balances of derivative instruments
Gross amounts of recognized assets	2,104	6,445
Gross amounts netted in the consolidated balance sheet	(2,104)	(2,373)
Net amounts of assets presented in the condensed consolidated balance sheet	0	4,072
Counterparty A [Member] | Other assets [Member]
Summary of gross asset balances of derivative instruments
Gross amounts of recognized assets	1,034	1,096
Gross amounts netted in the consolidated balance sheet	(909)	(370)
Net amounts of assets presented in the condensed consolidated balance sheet	125	726
Counterparty B [Member] | Current Assets [Member]
Summary of gross asset balances of derivative instruments
Gross amounts of recognized assets	1,741	530
Gross amounts netted in the consolidated balance sheet	(1,289)	(224)
Net amounts of assets presented in the condensed consolidated balance sheet	452	306
Amounts subject to master netting arrangements presented on a gross basis	138	82
Counterparty B [Member] | Other assets [Member]
Summary of gross asset balances of derivative instruments
Gross amounts of recognized assets	1,553	384
Gross amounts netted in the consolidated balance sheet	(1,039)	(339)
Net amounts of assets presented in the condensed consolidated balance sheet	514	45
Counterparty C [Member] | Current Assets [Member]
Summary of gross asset balances of derivative instruments
Gross amounts of recognized assets	2,874
Gross amounts netted in the consolidated balance sheet	(2,289)
Net amounts of assets presented in the condensed consolidated balance sheet	585
Amounts subject to master netting arrangements presented on a gross basis	301
Counterparty C [Member] | Other assets [Member]
Summary of gross asset balances of derivative instruments
Gross amounts of recognized assets	1,478
Gross amounts netted in the consolidated balance sheet	(1,122)
Net amounts of assets presented in the condensed consolidated balance sheet	$ 356