Derivative Instruments (Details 3) (USD $)	12 Months Ended			3 Months Ended	12 Months Ended
	Dec. 31, 2011 counterparty	Dec. 31, 2010	Dec. 31, 2009	Dec. 31, 2010 Interest rate contract	Dec. 31, 2011 Designated as Hedging Instrument Accumulated other comprehensive loss Commodity contract	Dec. 31, 2010 Designated as Hedging Instrument Accumulated other comprehensive loss Commodity contract	Dec. 31, 2009 Designated as Hedging Instrument Accumulated other comprehensive loss Commodity contract	Dec. 31, 2011 Designated as Hedging Instrument Accumulated other comprehensive loss Interest rate contract	Dec. 31, 2010 Designated as Hedging Instrument Accumulated other comprehensive loss Interest rate contract	Dec. 31, 2009 Designated as Hedging Instrument Accumulated other comprehensive loss Interest rate contract	Dec. 31, 2011 Designated as Hedging Instrument Sales of oil and natural gas Commodity contract	Dec. 31, 2010 Designated as Hedging Instrument Sales of oil and natural gas Commodity contract	Dec. 31, 2009 Designated as Hedging Instrument Sales of oil and natural gas Commodity contract	Dec. 31, 2011 Designated as Hedging Instrument Earnings from discontinued operations Commodity contract	Dec. 31, 2010 Designated as Hedging Instrument Earnings from discontinued operations Commodity contract	Dec. 31, 2009 Designated as Hedging Instrument Earnings from discontinued operations Commodity contract	Dec. 31, 2011 Designated as Hedging Instrument Interest Interest rate contract	Dec. 31, 2010 Designated as Hedging Instrument Interest Interest rate contract	Dec. 31, 2009 Designated as Hedging Instrument Interest Interest rate contract	Dec. 31, 2011 Designated as Hedging Instrument Realized and unrealized (gain) loss on derivatives, net Commodity contract	Dec. 31, 2010 Designated as Hedging Instrument Realized and unrealized (gain) loss on derivatives, net Commodity contract	Dec. 31, 2009 Designated as Hedging Instrument Realized and unrealized (gain) loss on derivatives, net Commodity contract
Location and amount of derivative instrument (gains) losses before income taxes recognized in the condensed statements of income
Derivative Instruments, (Gain) Loss Reclassified from AOCL into Income, Effective Portion, Net											$ 0	$ 0	$ (58,800,000)	$ 0	$ 0	$ (20,500,000)	$ 0	$ 0	$ 7,000,000
Derivative Instruments, (Gain) Loss Reclassified from AOCL into Income, Effective Portion, Amortization of Frozen Amounts											60,900,000	18,400,000	0				800,000	8,300,000	0
Derivative Instruments, Loss Recognized in Income, Ineffective Portion and Amount Excluded from Effectiveness Testing																				0	0	600,000
(Gain) loss recognized in earnings	(13,908,000)	31,847,000	7,756,000	1,900,000	0	0	206,400,000	0	0	2,700,000
Credit risk
Maximum loss if all counterparties to derivative contracts failed to perform	7,500,000
Number of counterparties to the Company's commodity derivative contracts (in counterparties)	7
Number of counterparties with net liability position (in counterparties)	4
Derivative instruments, net liability position	$ 30,200,000
Derivative instruments, number of largest counterparties (in counterparties)	2
Derivative value held by largest counterparties (as a percent)	84.00%