Risk Management Activities (Details) (USD $)	3 Months Ended		9 Months Ended
	Sep. 30, 2011	Sep. 30, 2010	Sep. 30, 2011	Sep. 30, 2010	Dec. 31, 2010
Notes To Financial Statements [Abstract]
Gains on Mark-to-Market Commodity Derivative Contracts	$ 357,664,000	$ 60,998,000	$ 480,539,000	$ 105,816,000
Realized Gains (Losses)			83,765,000	25,180,000
Assets from Price Risk Management Activities	364,991,000		364,991,000		48,153,000
Liabilities from Price Risk Management Activities	0		0		28,339,000
Derivative [Line Items]
Principal amount of notes issued by Canadian subsidiary, whose exchange rate is hedged by foreign currency swap transaction	150,000,000		150,000,000
After-tax impact on OCI as a result of the foreign currency swap transaction.	500,000	(500,000)	300,000	3,500,000
Derivatives, Fair Value [Line Items]
Derivatives Assets, Current	364,991,000		364,991,000		48,153,000
Derivative Liabilities, Current	0		0		28,339,000
Interest Rate Cash Flow Hedges [Abstract]
Principal amount of Floating Rate Senior notes, whose interest rate is hedged by an interest rate swap transaction	350,000,000		350,000,000
After-tax impact on OCI as a result of the interest rate swap transaction.	(1,600,000)		(4,200,000)
Natural Gas Financial Price Swap Contracts Year Two [Member]
Derivative [Line Items]
Volume (MMBtud)			525,000
Volumes (MMBtud) - Swaption Contracts			425,000
Average Price ($/MMBtu) - Swaption Contracts			5.44
Derivative Swap Type Weighted Average Fixed Price			5.44
Crude Oil Financial Price Swap Contracts Year Two [Member]
Derivative [Line Items]
Volume (Bbld)			11,000
Derivative Weighted Average Price Crude Oil			106.37
Natural Gas Financial Price Swap Contract Current Year Period One [Member]
Derivative [Line Items]
Volume (MMBtud)			275,000
Volumes (MMBtud) - Swaption Contracts			500,000
Average Price ($/MMBtu) - Swaption Contracts			4.73
Derivative Swap Type Weighted Average Fixed Price			5.19
Natural Gas Financial Price Swap Contracts Current Year Period Two [Member]
Derivative [Line Items]
Volume (MMBtud)			425,000
Derivative Swap Type Weighted Average Fixed Price			5.09
Natural Gas Financial Price Swap Contracts Current Year Period Three [Member]
Derivative [Line Items]
Volume (MMBtud)			425,000
Derivative Swap Type Weighted Average Fixed Price			5.09
Natural Gas Financial Price Swap Contracts Current Year Period Four [Member]
Derivative [Line Items]
Volume (MMBtud)			475,000
Derivative Swap Type Weighted Average Fixed Price			5.03
Natural Gas Financial Price Swap Contracts Current Year Period Five [Member]
Derivative [Line Items]
Volume (MMBtud)			650,000
Derivative Swap Type Weighted Average Fixed Price			4.90
Crude Oil Financial Price Swap Contracts Current Year Period One [Member]
Derivative [Line Items]
Volume (Bbld)			17,000
Derivative Weighted Average Price Crude Oil			90.44
Crude Oil Financial Price Swap ContractsCurrent Year Period Two [Member]
Derivative [Line Items]
Volume (Bbld)			18,000
Derivative Weighted Average Price Crude Oil			90.69
Crude Oil Financial Price Swap Contracts Current Year Period Three [Member]
Derivative [Line Items]
Volume (Bbld)			20,000
Derivative Weighted Average Price Crude Oil			91.82
Crude Oil Financial Price Swap Contracts Current Year Period Four [Member]
Derivative [Line Items]
Volume (Bbld)			24,000
Derivative Weighted Average Price Crude Oil			93.61
Crude Oil Financial Price Swap Contract Current Year Period Six [Member]
Derivative [Line Items]
Volume (Bbld)			30,000
Derivative Weighted Average Price Crude Oil			97.02
Crude Oil Financial Price Swap Current Year Period Seven [Member]
Derivative [Line Items]
Volume (Bbld)			30,000
Derivative Weighted Average Price Crude Oil			97.02
Natural Gas Financial Price Swap Contracts Current Year Period Six [Member]
Derivative [Line Items]
Volume (MMBtud)			650,000
Derivative Swap Type Weighted Average Fixed Price			4.90
Natural Gas Financial Price Swap Contracts Current Year Period Seven [Member]
Derivative [Line Items]
Volume (MMBtud)			650,000
Derivative Swap Type Weighted Average Fixed Price			4.90
Crude Oil Financial Price Swap Current Year Period Five [Member]
Derivative [Line Items]
Volume (Bbld)			24,000
Derivative Weighted Average Price Crude Oil			93.61
Foreign currency swap - Noncurrent Portion [Member]
Derivatives, Fair Value [Line Items]
Other Liabilities	46,000,000		46,000,000		55,000,000
Crude oil and natural gas price swaps and natural gas swaptions [Member]
Notes To Financial Statements [Abstract]
Assets from Price Risk Management Activities	365,000,000		365,000,000		51,000,000
Derivatives, Fair Value [Line Items]
Derivatives Assets, Current	365,000,000		365,000,000		51,000,000
Other Assets	61,000,000		61,000,000		18,000,000
Crude oil price swaps, natural gas price and basis swaps and natural gas swaptions [Member]
Notes To Financial Statements [Abstract]
Liabilities from Price Risk Management Activities					30,000,000
Derivatives, Fair Value [Line Items]
Derivative Liabilities, Current					30,000,000
Interest rate swap - Noncurrent Portion [Member]
Derivatives, Fair Value [Line Items]
Other Assets					2,000,000
Other Liabilities	$ 5,000,000		$ 5,000,000