Risk Management Activities (Details) (USD $)	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012	Dec. 31, 2012
Derivatives, Fair Value [Line Items]
Derivatives Assets, Current	$ 105,667,000		$ 105,667,000		$ 166,135,000
Derivative Liabilities, Current	5,482,000		5,482,000		7,617,000
Foreign Currency Exchange Rate Derivative [Abstract]
After-Tax Net Impact Other Comprehensive Income Increase (Decrease) From The Foreign Currency Swap	(700,000)	(900,000)	1,000,000	700,000
Interest Rate Derivative [Abstract]
Interest Rate Swap, Notional Amount	350,000,000		350,000,000
After-Tax Net Impact Other Comprehensive Income Increase (Decrease) From Interest Rate Swap	400,000	100,000	800,000	(200,000)
Derivative Collateral [Abstract]
Collateral Held on Derivative	5,000,000		5,000,000		6,000,000
Crude Oil and Natural Gas Derivative Contracts [Member] | Assets From Price Risk Management Activities [Member]
Derivatives, Fair Value [Line Items]
Derivatives Assets, Current	106,000,000		106,000,000		166,000,000
Derivative Assets, Current, Gross	163,000,000		163,000,000		271,000,000
Derivative Liabilities, Current, Gross	57,000,000		57,000,000		105,000,000
Crude Oil and Natural Gas Derivative Contracts [Member] | Liabilities From Price Risk Management Activities [Member]
Derivatives, Fair Value [Line Items]
Derivative Liabilities, Current	5,000,000		5,000,000		8,000,000
Derivative Assets, Current, Gross	57,000,000		57,000,000		105,000,000
Derivative Assets, Noncurrent, Gross	3,000,000		3,000,000
Derivative Liabilities, Current, Gross	62,000,000		62,000,000		113,000,000
Derivative Liabilities, Noncurrent, Gross	4,000,000		4,000,000		13,000,000
Crude Oil and Natural Gas Derivative Contracts [Member] | Other Assets [Member]
Derivatives, Fair Value [Line Items]
Other Assets	7,000,000		7,000,000		0
Derivative Assets, Noncurrent, Gross	10,000,000		10,000,000
Derivative Liabilities, Noncurrent, Gross	3,000,000		3,000,000
Crude Oil and Natural Gas Derivative Contracts [Member] | Other Liabilities [Member]
Derivatives, Fair Value [Line Items]
Other Liabilities	1,000,000		1,000,000		13,000,000
Foreign Currency Swap [Member] | Other Current Liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative Liabilities, Current	43,000,000		43,000,000		0
Foreign Currency Swap [Member] | Other Liabilities [Member]
Derivatives, Fair Value [Line Items]
Other Liabilities	0		0		55,000,000
Interest Rate Swap [Member] | Other Current Liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative Liabilities, Current	2,000,000		2,000,000		0
Interest Rate Swap [Member] | Other Liabilities [Member]
Derivatives, Fair Value [Line Items]
Other Liabilities	$ 0		$ 0		$ 4,000,000
Crude Oil Derivative Contracts January (closed) [Member]
Derivative [Line Items]
Volume (Bbld)			101,000
Derivative Weighted Average Price Crude Oil ($/Bbl)			99.29
Crude Oil Derivative Contracts February - April (closed) [Member]
Derivative [Line Items]
Volume (Bbld)			109,000
Derivative Weighted Average Price Crude Oil ($/Bbl)			99.17
Crude Oil Derivative Contracts May - June (closed) [Member]
Derivative [Line Items]
Volume (Bbld)			101,000
Derivative Weighted Average Price Crude Oil ($/Bbl)			99.29
Crude Oil Derivative Contracts July - September [Member]
Derivative [Line Items]
Volume (Bbld)			111,000
Derivative Weighted Average Price Crude Oil ($/Bbl)			98.25
Crude Oil Derivative Contracts October - December [Member]
Derivative [Line Items]
Volume (Bbld)			103,000
Derivative Weighted Average Price Crude Oil ($/Bbl)			98.26
Volumes (Bbld) - Derivative Option Contracts			8,000
Average Price ($/Bbl) - Derivative Option Contracts			98.11
Crude Oil Derivative Contracts Year Two January - March [Member]
Derivative [Line Items]
Volume (Bbld)			85,000
Derivative Weighted Average Price Crude Oil ($/Bbl)			96.24
Crude Oil Derivative Contracts Year Two January - June [Member]
Derivative [Line Items]
Volumes (Bbld) - Derivative Option Contracts			54,000
Average Price ($/Bbl) - Derivative Option Contracts			98.91
Crude Oil Derivative Contracts Year Two April - June [Member]
Derivative [Line Items]
Volume (Bbld)			75,000
Derivative Weighted Average Price Crude Oil ($/Bbl)			96.20
Crude Oil Derivative Contracts Year Two July - December [Member]
Derivative [Line Items]
Volumes (Bbld) - Derivative Option Contracts			75,000
Average Price ($/Bbl) - Derivative Option Contracts			96.20
Crude Oil Derivative Contracts Year Two April - December [Member]
Derivative [Line Items]
Volumes (Bbld) - Derivative Option Contracts			10,000
Average Price ($/Bbl) - Derivative Option Contracts			96.60
Natural Gas Derivative Contracts January - April (closed) [Member]
Derivative [Line Items]
Volume (MMBtud)			150,000
Derivative Weighted Average Price Natural Gas ($/MMBtu)			4.79
Natural Gas Derivative Contracts May - July (closed) [Member]
Derivative [Line Items]
Volume (MMBtud)			200,000
Derivative Weighted Average Price Natural Gas ($/MMBtu)			4.72
Natural Gas Derivative Contracts August - October [Member]
Derivative [Line Items]
Volume (MMBtud)			200,000
Derivative Weighted Average Price Natural Gas ($/MMBtu)			4.72
Volumes (MMBtud) - Derivative Option Contracts			200,000
Average Price ($/MMBtu) - Derivative Option Contracts			4.72
Natural Gas Derivative Contracts November - December [Member]
Derivative [Line Items]
Volume (MMBtud)			150,000
Derivative Weighted Average Price Natural Gas ($/MMBtu)			4.79
Volumes (MMBtud) - Derivative Option Contracts			150,000
Average Price ($/MMBtu) - Derivative Option Contracts			4.79
Natural Gas Derivative Contracts Year Two January - December [Member]
Derivative [Line Items]
Volume (MMBtud)			170,000
Derivative Weighted Average Price Natural Gas ($/MMBtu)			4.54
Volumes (MMBtud) - Derivative Option Contracts			320,000
Average Price ($/MMBtu) - Derivative Option Contracts			4.66