Derivative Financial Instruments (Details 2) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Assets
Gross Amounts of Assets in the Consolidated Balance Sheet	$ 26,995	$ 30,382
Gross Amounts Not Offset in the Consolidated Balance Sheet	(9,759)	(12,178)
Net Amount	17,236	18,204
Liabilities
Gross Amounts of Liabilities in the Consolidated Balance Sheet	(46,998)	(59,419)
Gross Amounts Not Offset in the Consolidated Balance Sheet	9,759	12,178
Net Amount	(37,239)	(47,241)
Current Assets
Assets
Gross Amounts of Assets in the Consolidated Balance Sheet	16,814	19,504
Gross Amounts Not Offset in the Consolidated Balance Sheet	(8,039)	(9,541)
Net Amount	8,775	9,963
Non-current Assets
Assets
Gross Amounts of Assets in the Consolidated Balance Sheet	10,181	10,878
Gross Amounts Not Offset in the Consolidated Balance Sheet	(1,720)	(2,637)
Net Amount	8,461	8,241
Current Liabilities
Liabilities
Gross Amounts of Liabilities in the Consolidated Balance Sheet	(23,060)	(27,229)
Gross Amounts Not Offset in the Consolidated Balance Sheet	8,039	9,541
Net Amount	(15,021)	(17,688)
Non-current Liabilities
Liabilities
Gross Amounts of Liabilities in the Consolidated Balance Sheet	(23,938)	(32,190)
Gross Amounts Not Offset in the Consolidated Balance Sheet	1,720	2,637
Net Amount	(22,218)	(29,553)
Commodity contracts | Current Assets
Assets
Gross Amounts of Assets in the Consolidated Balance Sheet	13,514	16,438
Gross Amounts Not Offset in the Consolidated Balance Sheet	(8,039)	(9,541)
Net Amount	5,475	6,897
Commodity contracts | Non-current Assets
Assets
Gross Amounts of Assets in the Consolidated Balance Sheet	7,003	7,798
Gross Amounts Not Offset in the Consolidated Balance Sheet	(1,720)	(2,637)
Net Amount	5,283	5,161
Commodity contracts | Current Liabilities
Liabilities
Gross Amounts of Liabilities in the Consolidated Balance Sheet	(13,765)	(16,679)
Gross Amounts Not Offset in the Consolidated Balance Sheet	8,039	9,541
Net Amount	(5,726)	(7,138)
Commodity contracts | Non-current Liabilities
Liabilities
Gross Amounts of Liabilities in the Consolidated Balance Sheet	(1,874)	(2,637)
Gross Amounts Not Offset in the Consolidated Balance Sheet	1,720	2,637
Net Amount	(154)
Embedded derivatives in commodity contracts | Current Assets
Assets
Gross Amounts of Assets in the Consolidated Balance Sheet	3,300	3,066
Net Amount	3,300	3,066
Embedded derivatives in commodity contracts | Non-current Assets
Assets
Gross Amounts of Assets in the Consolidated Balance Sheet	3,178	3,080
Net Amount	3,178	3,080
Embedded derivatives in commodity contracts | Current Liabilities
Liabilities
Gross Amounts of Liabilities in the Consolidated Balance Sheet	(9,295)	(10,550)
Net Amount	(9,295)	(10,550)
Embedded derivatives in commodity contracts | Non-current Liabilities
Liabilities
Gross Amounts of Liabilities in the Consolidated Balance Sheet	(22,064)	(29,553)
Net Amount	$ (22,064)	$ (29,553)