Risk Management Activities and Derivatives Transactions (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2011 gal mmbtu	Sep. 30, 2010	Sep. 30, 2011 years gal mmbtu	Sep. 30, 2010	Dec. 31, 2010
Economic derivatives
Cash collateral posted	$ 112		$ 112		$ 164
Notional MMBtu of natural gas	339,400,000		339,400,000
Notional gallons of oil	12,300,000		12,300,000
Interest rate cash flow hedges
Open interest rate hedges mandatory termination dates			2
After-tax (gains) losses in accumulated other comprehensive income	140		140		63
Next 12 months amount of (gains) losses to reclass to interest expense			12
Notional amount of interest rate cash flow hedges	500		500		1,050
Contingent features
Aggregate fair value of instruments in liability position	377		377
Additional collateral to post	265		265
Derivatives Fair Value [Line Items]
Derivative asset fair value	7		7		19
Derivative liability fair value	632		632		552
Derivatives Designated as Hedging Instruments
Amount of Gain or (Loss) Recognized in OCI, Net of Tax on Derivatives	(69)	(30)	(83)	(80)
Amount of Gain or (Loss), Net of Tax Reclassified from Accumulated OCI into Income	(2)	(1)	(5)	(4)
Amount of Pre-tax Gain or (Loss) Recognized in Income on Derivatives	(1)		(3)
Derivatives Not Designated As Hedging Instruments
Amount of Gain or (Loss) Recognized in Income on Derivatives	(62)	1	(57)	1
Fair Value Of Derivative Instruments Parenthetical Information
CVO's in connection with the acquisition of Florida Progress during 2000	98.6		98.6
Original fair value loss transition adjustment, pre-tax, recorded in 2003	38		38
Original fair value loss transition adjustment, after-tax, recorded in 2003	23		23
Interest rate derivatives
Derivatives Designated as Hedging Instruments
Amount of Gain or (Loss) Recognized in OCI, Net of Tax on Derivatives	(68)	(30)	(82)	(80)
Amount of Gain or (Loss), Net of Tax Reclassified from Accumulated OCI into Income	(2)	(1)	(5)	(4)
Amount of Pre-tax Gain or (Loss) Recognized in Income on Derivatives	(1)		(3)
Interest rate derivatives | Prepayments and other current assets | Designated as Hedging Instrument [Member]
Derivatives Fair Value [Line Items]
Derivative asset fair value					1
Interest rate derivatives | Other assets and deferred debits | Designated as Hedging Instrument [Member]
Derivatives Fair Value [Line Items]
Derivative asset fair value					3
Interest rate derivatives | Derivative liabilities, current | Designated as Hedging Instrument [Member]
Derivatives Fair Value [Line Items]
Derivative liability fair value	70		70		32
Interest rate derivatives | Derivative liabilities, long-term | Designated as Hedging Instrument [Member]
Derivatives Fair Value [Line Items]
Derivative liability fair value	16		16		7
Commodity derivatives
Derivatives Designated as Hedging Instruments
Amount of Gain or (Loss) Recognized in OCI, Net of Tax on Derivatives	(1)		(1)
Derivatives Not Designated As Hedging Instruments
Realized Gain or (Loss)	(91)	(114)	(219)	(264)
Unrealized Gain or (Loss)	(157)	(181)	(201)	(417)
Amount of Gain or (Loss) Recognized in Income on Derivatives			1
Commodity derivatives | Prepayments and other current assets | Not Designated as Hedging Instrument [Member]
Derivatives Fair Value [Line Items]
Derivative asset fair value	6		6		11
Commodity derivatives | Other assets and deferred debits | Not Designated as Hedging Instrument [Member]
Derivatives Fair Value [Line Items]
Derivative asset fair value	1		1		4
Commodity derivatives | Derivative liabilities, current | Designated as Hedging Instrument [Member]
Derivatives Fair Value [Line Items]
Derivative liability fair value	1		1
Commodity derivatives | Derivative liabilities, current | Not Designated as Hedging Instrument [Member]
Derivatives Fair Value [Line Items]
Derivative liability fair value	231		231		226
Commodity derivatives | Derivative liabilities, long-term | Not Designated as Hedging Instrument [Member]
Derivatives Fair Value [Line Items]
Derivative liability fair value	237		237		268
Other liabilities and deferred credits | CVOs | Not Designated as Hedging Instrument [Member]
Derivatives Fair Value [Line Items]
Derivative liability fair value					15
CVOs
Derivatives Not Designated As Hedging Instruments
Amount of Gain or (Loss) Recognized in Income on Derivatives	(63)		(59)
CVOs | Other liabilities, current | Not Designated as Hedging Instrument [Member]
Derivatives Fair Value [Line Items]
Derivative liability fair value	74		74
Fair value loss transition adjustment
Derivatives Not Designated As Hedging Instruments
Amount of Gain or (Loss) Recognized in Income on Derivatives	1	1	1	1
Fair value loss transition adjustment | Derivative liabilities, current | Not Designated as Hedging Instrument [Member]
Derivatives Fair Value [Line Items]
Derivative liability fair value	1		1		1
Fair value loss transition adjustment | Derivative liabilities, long-term | Not Designated as Hedging Instrument [Member]
Derivatives Fair Value [Line Items]
Derivative liability fair value	2		2		3
Fair value of derivatives not designated as hedging instruments | Not Designated as Hedging Instrument [Member]
Derivatives Fair Value [Line Items]
Derivative asset fair value	7		7		15
Derivative liability fair value	542		542		509
Designated as Hedging Instrument [Member]
Derivatives Fair Value [Line Items]
Derivative asset fair value					4
Derivative liability fair value	87		87		39
Not Designated as Hedging Instrument [Member]
Derivatives Fair Value [Line Items]
Derivative asset fair value	7		7		15
Derivative liability fair value	$ 545		$ 545		$ 513