Derivative Instruments (Details) (USD $)	3 Months Ended
	Mar. 31, 2012	Mar. 31, 2011	Dec. 31, 2011
Accumulated Other Comprehensive Loss
Accumulated Other Comprehensive Income (Losses) Before Tax	$ (6,900,000)		$ (8,900,000)
Accumulated other comprehensive loss	(4,250,000)		(5,517,000)
Reclassified unrealized losses, before tax from AOCL into earnings	(2,000,000)
Reclassified unrealized losses, after tax from AOCL into earnings	(1,267,000)	(9,523,000)
Net losses after-tax expected to be reclassified into earnings from AOCL during the next 12 months	(4,200,000)
Termination of Natural Gas Derivatives
Cash settlement upon termination of natural gas derivatives	14,700,000
Non-cash fair value loss related to natural gas derivative contracts that were terminated during the period	16,600,000
Net loss resulting from termination of natural gas derivatives	$ 1,900,000
2012 | Oil Bbl/D
Notional amount of derivative instruments
Notional amount of derivatives (Bbl/MMBtu)	21,000
2012 | Natural Gas MMBtu/D
Notional amount of derivative instruments
Notional amount of derivatives (Bbl/MMBtu)	0
2013 | Oil Bbl/D
Notional amount of derivative instruments
Notional amount of derivatives (Bbl/MMBtu)	15,000
2013 | Natural Gas MMBtu/D
Notional amount of derivative instruments
Notional amount of derivatives (Bbl/MMBtu)	0
2014 | Oil Bbl/D
Notional amount of derivative instruments
Notional amount of derivatives (Bbl/MMBtu)	4,000
2014 | Natural Gas MMBtu/D
Notional amount of derivative instruments
Notional amount of derivatives (Bbl/MMBtu)	0
Crude oil three-way collars - Derivative 1 | 2014
Notional amount of derivative instruments
Floor prices (in dollars per barrel)	70
Swap prices (in dollars per barrel)	90
Ceiling prices (in dollars per barrel)	120
Crude oil three-way collars - Derivative 1 | 2014 | Derivative Floor Price 70.00 | Derivative Swap Price 90.00 | Derivative Ceiling Price 120.00
Notional amount of derivative instruments
Notional amount of derivatives (Bbl/MMBtu)	1,000
Crude oil three-way collars - Derivative 2 | 2014
Notional amount of derivative instruments
Floor prices (in dollars per barrel)	70
Swap prices (in dollars per barrel)	90
Ceiling prices (in dollars per barrel)	121.80
Crude oil three-way collars - Derivative 2 | 2014 | Derivative Floor Price 70.00 | Derivative Swap Price 90.00 | Derivative Ceiling Price 121.80
Notional amount of derivative instruments
Notional amount of derivatives (Bbl/MMBtu)	1,000