Impairment (Details) (USD $)	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2012 Rate	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011	Dec. 31, 2011
Impairment [Abstract]
Pretax noncash impairment charges	$ 444,880,000	$ 0	$ 444,880,000	$ 0	$ 354,400,000
Management oil price outlook (BBL)	87.09
Management gas price outlook (MCF)	4.64
Discount rate used in impairment calculation	10.00%
Raton Shale Field [Member]
Impaired Long-Lived Assets Held and Used [Line Items]
Carrying value of asset group	$ 2,300,000,000		$ 2,300,000,000