Risk Management Activities and Derivatives Transactions (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2011 mmbtu gal	Jun. 30, 2010	Jun. 30, 2011 years mmbtu gal	Jun. 30, 2010	Dec. 31, 2010
Economic derivatives
Cash collateral posted	$ 122		$ 122		$ 164
Notional MMBtu of natural gas	291,300,000		291,300,000
Notional gallons of oil	17,600,000		17,600,000
Interest rate cash flow hedges
Open interest rate hedges mandatory termination dates			2
After-tax (gains) losses in accumulated other comprehensive income	74		74		63
Next 12 months amount of (gains) losses to reclass to interest expense			7
Notional amount of interest rate cash flow hedges	925		925		1,050
Contingent features
Aggregate fair value of instruments in liability position	362		362
Additional collateral to post	240		240
Derivative asset fair value
Total derivatives designated as hedging instruments	1		1		4
Total derivatives not designated as hedging instruments	18		18		15
Total derivatives	19		19		19
Derivative liability fair value
Total derivatives designated as hedging instruments	35		35		39
Total derivatives not designated as hedging instruments	427		427		513
Total derivatives	462		462		552
Derivatives Designated as Hedging Instruments
Amount of Gain or (Loss) Recognized in OCI, Net of Tax on Derivatives	(16)	(44)	(14)	(50)
Amount of Gain or (Loss), Net of Tax Reclassified from Accumulated OCI into Income	(2)	(2)	(3)	(3)
Derivatives Not Designated As Hedging Instruments
Amount of Gain or (Loss) Recognized in Income on Derivatives	5	1	5	0
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	(16)	(44)	(14)	(50)
Amount of Gain or (Loss), Net of Tax Reclassified from Accumulated OCI into Income	(2)	(2)	(3)	(3)
Amount of Pre-tax Gain or (Loss) Recognized in Income on Derivatives			(2)	0
Interest rate derivatives | Prepayments and other current assets
Derivative asset fair value
Total derivatives designated as hedging instruments	0		0		1
Interest rate derivatives | Other assets and deferred debits
Derivative asset fair value
Total derivatives designated as hedging instruments	1		1		3
Interest rate derivatives | Derivative liabilities, current
Derivative liability fair value
Total derivatives designated as hedging instruments	24		24		32
Interest rate derivatives | Derivative liabilities, long-term
Derivative liability fair value
Total derivatives designated as hedging instruments	11		11		7
Commodity derivatives
Derivatives Designated as Hedging Instruments
Amount of Gain or (Loss) Recognized in OCI, Net of Tax on Derivatives			0	0
Derivatives Not Designated As Hedging Instruments
Realized Gain or (Loss)	(76)	(91)	(128)	(150)
Unrealized Gain or (Loss)	(68)	(2)	(44)	(236)
Amount of Gain or (Loss) Recognized in Income on Derivatives	1	1	1	0
Commodity derivatives | Prepayments and other current assets
Derivative asset fair value
Total derivatives not designated as hedging instruments	15		15		11
Commodity derivatives | Other assets and deferred debits
Derivative asset fair value
Total derivatives not designated as hedging instruments	3		3		4
Commodity derivatives | Derivative liabilities, current
Derivative liability fair value
Total derivatives not designated as hedging instruments	189		189		226
Commodity derivatives | Derivative liabilities, long-term
Derivative liability fair value
Total derivatives not designated as hedging instruments	223		223		268
Other liabilities and deferred credits | CVOs
Derivative liability fair value
Total derivatives not designated as hedging instruments	11		11		15
CVOs
Derivatives Not Designated As Hedging Instruments
Amount of Gain or (Loss) Recognized in Income on Derivatives	4	0	4	0
Fair value loss transition adjustment
Derivatives Not Designated As Hedging Instruments
Amount of Gain or (Loss) Recognized in Income on Derivatives	0	0	0	0
Fair value loss transition adjustment | Derivative liabilities, current
Derivative liability fair value
Total derivatives not designated as hedging instruments	1		1		1
Fair value loss transition adjustment | Derivative liabilities, long-term
Derivative liability fair value
Total derivatives not designated as hedging instruments	3		3		3
Fair value of derivatives not designated as hedging instruments
Derivative asset fair value
Total derivatives not designated as hedging instruments	18		18		15
Derivative liability fair value
Total derivatives not designated as hedging instruments	$ 423		$ 423		$ 509