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Reference 1: http://www.xbrl.org/2003/role/presentationRef

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-Name Regulation S-X (SX)

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Reference 2: http://www.xbrl.org/2003/role/presentationRef

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-Name Statement of Financial Accounting Standard (FAS)

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Risk Management (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended			9 Months Ended
	Sep. 30, 2011	Mar. 31, 2011	Sep. 30, 2010	Sep. 30, 2011	Sep. 30, 2010	Dec. 31, 2010
Energy Commodity Price Risk Management [Abstract]
Amount of gain/(loss) recognized in income on derivative (ineffective portion and amount excluded from effectiveness testing)	$ 8.5		$ (9.5)	$ 10.4	$ 4.6
Amount of gain/(loss) reclassified from Accumulated OCI into income (effective portion)	(49.0)		(47.6)	(188.8)	(134.6)
Accumulated other comprehensive loss	(172.6)			(172.6)		186.4
Amount included in accumulated other comprehensive income associated with energy commodity price risk management activities				165.1	(307.1)
Amount of unrealized loss associated with energy commodity price risk management expected to be reclassified to earnings in next twelve months	56.7			56.7
Maximum length of time of exposure in commodity hedges				December 2015
Asset derivatives [Abstract]
Fair value of asset derivatives designated as hedging contracts	833.2			833.2		280.8
Fair value of asset derivatives not designated as hedging contracts	5.4			5.4		3.9
Total asset derivatives	838.6			838.6		284.7
Liability derivatives [Abstract]
Fair value of liability derivatives designated as hedging contracts						448.1
Fair value of liability derivatives not designated as hedging contracts	4.9			4.9		5.6
Total liability derivatives	93.3			93.3		453.7
Unamortized premium from early termination of interest rate swap agreements	494.7			494.7		456.5
Approximate weighted average amortization period on unamortized premium				17.9Y
Derivative Instruments, Gain (Loss) by Hedging Relationship, by Income Statement Location, by Derivative Instrument Risk [Line Items]
Amount of gain recognized in income on derivative and on related hedged items	436.8		219.9	501.1	634.1
Amount of loss recognized in income on derivative and on related hedged items	(436.8)		(219.9)	(501.1)	(634.1)
Amount of gain/(loss) recognized in income on derivative and on related hedged items	(0.1)		0.2	0.1	1.0
Amount of gain/(loss) recognized in OCI on derivative (effective portion)	386.6		(83.3)	288.8	84.4
Amount of gain/(loss) reclassified from Accumulated OCI into income (effective portion)	(49.0)		(47.6)	(188.8)	(134.6)
Amount of gain/(loss) recognized in income on derivative (ineffective portion and amount excluded from effectiveness testing)	8.5		(9.5)	10.4	4.6
Interest Rate Risk Management [Abstract]
Notional amount of interest rate swap agreements	5,325			5,325		4,775
Maximum length of time of exposure in interest rate hedges	2035-03-15			2035-03-15
Number of additional fixed-to-variable interest rate swap agreements	2	4
Notional amount of additional interest rate swap agreements	250	500
Number of swaps terminated during period				2
Notional principal of terminated swaps				200
Proceeds from termination of swaps				73.0
Crude oil (MMBbl) [Member]
Derivative [Line Items]
Net open position long/(short)	(21.8)			(21.8)
Natural gas fixed price (Bcf) [Member]
Derivative [Line Items]
Net open position long/(short)	(3.6)			(3.6)
Natural gas basis designated as hedge (Bcf) [Member]
Derivative [Line Items]
Net open position long/(short)	(4.2)			(4.2)
Natural gas basis not designated as hedge (Bcf) [Member]
Derivative [Line Items]
Net open position long/(short)	2.3			2.3
Natural gas fixed price not designated as hedge (Bcf) [Member]
Derivative [Line Items]
Net open position long/(short)	0.2			0.2
Energy commodity derivative contracts [Member] \| Cash Flow Hedging [Member] \| Other comprehensive income [Member]
Derivative Instruments, Gain (Loss) by Hedging Relationship, by Income Statement Location, by Derivative Instrument Risk [Line Items]
Amount of gain/(loss) recognized in OCI on derivative (effective portion)	386.6		(83.3)	288.8	84.4
Energy commodity derivative contracts [Member] \| Cash Flow Hedging [Member] \| Revenues-natural gas sales [Member]
Energy Commodity Price Risk Management [Abstract]
Amount of gain/(loss) reclassified from Accumulated OCI into income (effective portion)			3.6	1.0	5.3
Derivative Instruments, Gain (Loss) by Hedging Relationship, by Income Statement Location, by Derivative Instrument Risk [Line Items]
Amount of gain/(loss) reclassified from Accumulated OCI into income (effective portion)			3.6	1.0	5.3
Energy commodity derivative contracts [Member] \| Cash Flow Hedging [Member] \| Revenues-product sales and other [Member]
Energy Commodity Price Risk Management [Abstract]
Amount of gain/(loss) reclassified from Accumulated OCI into income (effective portion)	(50.5)		(44.2)	(202.7)	(142.6)
Derivative Instruments, Gain (Loss) by Hedging Relationship, by Income Statement Location, by Derivative Instrument Risk [Line Items]
Amount of gain/(loss) reclassified from Accumulated OCI into income (effective portion)	(50.5)		(44.2)	(202.7)	(142.6)
Energy commodity derivative contracts [Member] \| Cash Flow Hedging [Member] \| Gas purchases and other costs of sales [Member]
Energy Commodity Price Risk Management [Abstract]
Amount of gain/(loss) recognized in income on derivative (ineffective portion and amount excluded from effectiveness testing)			(1.6)		(0.8)
Amount of gain/(loss) reclassified from Accumulated OCI into income (effective portion)	1.5		(7.0)	12.9	2.7
Derivative Instruments, Gain (Loss) by Hedging Relationship, by Income Statement Location, by Derivative Instrument Risk [Line Items]
Amount of gain/(loss) reclassified from Accumulated OCI into income (effective portion)	1.5		(7.0)	12.9	2.7
Amount of gain/(loss) recognized in income on derivative (ineffective portion and amount excluded from effectiveness testing)			(1.6)		(0.8)
Energy commodity derivative contracts [Member] \| Cash Flow Hedging [Member] \| Revenues [Member]
Energy Commodity Price Risk Management [Abstract]
Amount of gain/(loss) recognized in income on derivative (ineffective portion and amount excluded from effectiveness testing)	8.5		(7.9)	10.4	5.4
Derivative Instruments, Gain (Loss) by Hedging Relationship, by Income Statement Location, by Derivative Instrument Risk [Line Items]
Amount of gain/(loss) recognized in income on derivative (ineffective portion and amount excluded from effectiveness testing)	8.5		(7.9)	10.4	5.4
Energy commodity derivative contracts [Member] \| Gas purchases and other costs of sales [Member]
Derivative Instruments, Gain (Loss) by Hedging Relationship, by Income Statement Location, by Derivative Instrument Risk [Line Items]
Amount of gain/(loss) recognized in income on derivative and on related hedged items	(0.1)		0.2	0.1	1.0
Energy commodity derivative contracts [Member] \| Current [Member]
Asset derivatives [Abstract]
Fair value of asset derivatives designated as hedging contracts	123.7			123.7		20.1
Fair value of asset derivatives not designated as hedging contracts	5.4			5.4		3.9
Liability derivatives [Abstract]
Fair value of liability derivatives designated as hedging contracts	67.0			67.0		275.9
Fair value of liability derivatives not designated as hedging contracts	4.9			4.9		5.6
Energy commodity derivative contracts [Member] \| Noncurrent [Member]
Asset derivatives [Abstract]
Fair value of asset derivatives designated as hedging contracts	133.0			133.0		43.1
Liability derivatives [Abstract]
Fair value of liability derivatives designated as hedging contracts	21.4			21.4		103.0
Energy commodity derivative contracts [Member] \| Subtotal derivative [Member]
Asset derivatives [Abstract]
Fair value of asset derivatives designated as hedging contracts	256.7			256.7		63.2
Fair value of asset derivatives not designated as hedging contracts	5.4			5.4		3.9
Liability derivatives [Abstract]
Fair value of liability derivatives designated as hedging contracts	88.4			88.4		378.9
Fair value of liability derivatives not designated as hedging contracts	4.9			4.9		5.6
Interest rate swap agreements [Member] \| Fair Value Hedging [Member] \| Interest, net-income/expense [Member]
Derivative Instruments, Gain (Loss) by Hedging Relationship, by Income Statement Location, by Derivative Instrument Risk [Line Items]
Amount of gain recognized in income on derivative and on related hedged items	436.8		219.9	501.1	634.1
Amount of loss recognized in income on derivative and on related hedged items	(436.8)		(219.9)	(501.1)	(634.1)
Interest rate swap agreements [Member] \| Current [Member]
Asset derivatives [Abstract]
Fair value of asset derivatives designated as hedging contracts	6.1			6.1
Interest rate swap agreements [Member] \| Noncurrent [Member]
Asset derivatives [Abstract]
Fair value of asset derivatives designated as hedging contracts	570.4			570.4		217.6
Liability derivatives [Abstract]
Fair value of liability derivatives designated as hedging contracts						69.2
Interest rate swap agreements [Member] \| Subtotal derivative [Member]
Asset derivatives [Abstract]
Fair value of asset derivatives designated as hedging contracts	576.5			576.5		217.6
Liability derivatives [Abstract]
Fair value of liability derivatives designated as hedging contracts						69.2
Asset Position [Member]
Derivative Credit Risk [Line Items]
Interest rate swap agreements	576.5			576.5
Energy commodity derivative contracts	262.1			262.1
Gross exposure	838.6			838.6
Netting agreement impact	(78.5)			(78.5)
Net exposure	760.1			760.1
Margin Deposits Given To Held From Counterparties	8.9			8.9		2.4
Two Notches Decrease In Credit Rating [Member]
Additional Collateral Obligations [Line Items]
Incremental obligations	12.8			12.8
Cumulative obligations	$ 12.8			$ 12.8