Risk Management Activities (Details) (USD $)	3 Months Ended									3 Months Ended			12 Months Ended		3 Months Ended					12 Months Ended
	Mar. 31, 2012	Mar. 31, 2011	Dec. 31, 2011	Mar. 31, 2012 Crude Oil and Natural Gas Derivative Contracts [Member]	Dec. 31, 2011 Crude Oil and Natural Gas Derivative Contracts [Member]	Mar. 31, 2012 Interest Rate Swap [Member]	Dec. 31, 2011 Interest Rate Swap [Member]	Mar. 31, 2012 Foreign Currency Swap [Member]	Dec. 31, 2011 Foreign Currency Swap [Member]	Mar. 31, 2012 Crude Oil Derivative Contracts January - February [Member]	Mar. 31, 2012 Natural Gas Derivative Contracts January - April [Member]	Mar. 31, 2012 Natural Gas Derivative Contracts May - December [Member]	Dec. 31, 2014 Natural Gas Derivative Contracts January - December [Member]	Dec. 31, 2013 Natural Gas Derivative Contracts January - December [Member]	Mar. 31, 2012 Crude Oil Derivative Contracts March [Member]	Mar. 31, 2012 Crude Oil Derivative Contracts April - June [Member]	Mar. 31, 2012 Crude Oil Derivative Contracts July - August [Member]	Mar. 31, 2012 Crude Oil Derivative Contracts September - December [Member]	Mar. 31, 2012 Crude Oil Derivative Contracts July - December [Member]	Dec. 31, 2013 Crude Oil Derivative Contracts September - February [Member]
Risk Management Activities [Abstract]
Gains (Losses) on Mark-to-Market Commodity Derivative Contracts	$ 134,208,000	$ (66,746,000)
Realized Gains (Losses)	133,601,000	24,937,000
Assets from Price Risk Management Activities	451,399,000		450,730,000	441,000,000	451,000,000
Liabilities from Price Risk Management Activities	25,787,000		0	26,000,000	0
Derivative [Line Items]
Volume (MMBtud)											525,000	525,000	150,000	150,000
Volume (Bbld)										34000					52000	52,000	35,000	17,000
Volumes (MMBtud) - Derivative Option Contracts												425,000	150000	150000
Average Price ($/MMBtu) - Derivative Option Contracts												5.44	4.79	4.79
Derivative Weighted Average Price Crude Oil ($/Bbl)										104.95					105.80	105.80	105.56	103.59
Derivative Weighted Average Price Natural Gas ($/MMBtu)											5.44	5.44	4.79	4.79
Volumes (Bbld) - Derivative Contacts																			17,000	18,000
Average Price ($/Bbl) - Derivative Contracts																			106.31	107.42
Derivatives, Fair Value [Line Items]
Derivatives Assets, Current	451,399,000		450,730,000	441,000,000	451,000,000
Other Assets				71,000,000	35,000,000
Derivative Liabilities, Current	25,787,000		0	26,000,000	0
Other Liabilities						4,000,000	3,000,000	54,000,000	52,000,000
Foreign Currency Exchange Rate Derivative [Abstract]
Debt Instrument Issuance Face Value	150,000,000
After-Tax Net Impact Other Comprehensive Income Increase (Decrease) From The Foreign Currency Swap	1,500,000	500,000
Interest Rate Cash Flow Hedges [Abstract]
Interest Rate Swap, Notional Amount	350,000,000
After-tax impact from interest rate swap	400,000	1,000,000
Derivative Collateral [Abstract]
Collateral Held on Derivative	$ 78,000,000		$ 67,000,000