Derivative Financial Instruments Fair Value (Details) (USD $)	Sep. 30, 2013	Dec. 31, 2012	Sep. 30, 2012
Fair value of derivative assets and liabilities
Derivative asset	$ 43,305,000	$ 37,873,000
Derivative asset	28,659,000	16,466,000
Derivative liability	22,648,000	8,999,000
Derivatives liability	6,873,000	6,645,000
Derivative Asset, Fair Value, Amount Offset Against Collateral	71,964,000	54,339,000
Derivative Asset, Fair Value, Amount Not Offset Against Collateral	(29,225,000)	(13,400,000)
Derivative Asset, Fair Value, Amount Offset Against Collateral, Net	42,739,000	40,939,000
Derivative Liability, Fair Value, Amount Offset Against Collateral	(29,521,000)	(15,644,000)
Derivative Liability, Fair Value, Amount Not Offset Against Collateral	29,225,000	13,400,000
Derivative Liability, Fair Value, Amount Offset Against Collateral, Net	(296,000)	(2,244,000)
Fair value of oil, natural gas, NGL commodity derivative markets	42,400,000		38,700,000
Derivatives not designated as hedging instruments
Fair value of derivative assets and liabilities
Derivative asset	43,305,000	37,873,000
Derivative asset	28,659,000	16,466,000
Derivative liability	22,648,000	8,999,000
Derivatives liability	6,873,000	6,645,000
Level 2 | Derivatives not designated as hedging instruments | Fair Value, Measurements, Recurring [Member]
Fair value of derivative assets and liabilities
Derivative assets not designated as hedging instruments	71,964,000	54,339,000
Derivative liabilities not designated as hedging instruments	$ 29,521,000	$ 15,644,000