Derivative Financial Instruments Fair Value (Details) (USD $)	Dec. 31, 2012	Dec. 31, 2011
Fair value of derivative assets and liabilities
Price Risk Derivatives, at Fair Value, Net	$ 38,700,000	$ 31,200,000
Derivative Assets, Current	37,873,000	55,813,000
Derivative Liabilities, Current	8,999,000	42,806,000
Derivative Assets, Noncurrent	16,466,000	31,062,000
Derivative Liabilities, Noncurrent	6,645,000	12,875,000
Not Designated as Hedging Instrument [Member]
Fair value of derivative assets and liabilities
Derivative Assets, Current	37,873,000	55,813,000
Derivative Liabilities, Current	8,999,000	42,806,000
Derivative Assets, Noncurrent	16,466,000	31,062,000
Derivative Liabilities, Noncurrent	6,645,000	12,875,000
Fair Value, Inputs, Level 2 [Member] | Fair Value, Measurements, Recurring [Member] | Not Designated as Hedging Instrument [Member]
Fair value of derivative assets and liabilities
Fair Value: Derivative Assets	54,339,000	86,875,000
Fair Value: Derivative Liabilities	$ 15,644,000	$ 55,681,000